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Processing of spodumene concentrates in fluidized-bed systems
Minerals Engineering 148 (2020) 106205
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Processing of spodumene concentrates in fluidized-bed systems Edgar Gasafi , Robert Pardemann ⁎
T
Outotec GmbH & Co. KG, 61440 Oberursel, Germany
ARTICLE INFO
ABSTRACT
Keywords: Calcination Decrepitation Fluidized bed Lithium Spodumene
The production of lithium carbonate or lithium hydroxide out of lithium bearing ores requires a thermal activation for the phase transition in the mineral to enable an acid or soda leaching in the downstream hydrometallurgical section. In this paper, traditional processing of lithium ores in the lithium industry is reviewed and opportunities to improve product quality and recovery rate by simultaneously reducing energy consumption will be presented. The conventional process for pyro-processing is still based on rotary kiln, a technology in use since the early days of lithium ore processing, albeit not significantly further developed since. A new technology in this field is fluidized-bed calcination. A study of the major process parameters (temperature and residence time) was performed on laboratory and larger bench scale aiming for optimal product quality for subsequent processing. When converting α- to both β- and γ-spodumene, high conversion rates were achieved for all investigated materials. The conversion of α- to only β-spodumene varies significantly by concentrate. Preferential conditions were temperatures in the range of 1050–1070 °C with retention times of typically 25–40 min. Additional results include the characterization of granulometric properties, in particular the effect of defragmentation by decrepitation, elemental and mineral phase composition, and further aspects specifically related to the application of fluidized bed technology. After identifying optimal process conditions for calcination and proving general feasibility at laboratory scale, the technical feasibility was confirmed for a wide range of spodumene concentrates at bench and pilot scale (20 up to 500 kg/h feed) providing the basis for industrial process design. Thus, spodumene decrepitation through applying fluidized-bed technology is technically feasible for non-sticking material and high and homogenous product quality can be achieved.
1. Introduction THERE is an increasing demand worldwide for lithium and lithium compounds, which is mainly driven by the expected electric and hybrid vehicle boom in the near future. Lithium is used in rechargeable batteries as a cathode material. Due to its low density and the high electrochemical standard potential it is for the time being the most promising and preferred material to push the electro mobilization (Julien et al., 2016). There are various projections in the increase of lithium ion battery production for the next decades. The common view is that lithium demand will significantly increase while the current supply will not meet the expected requirement in the future and production capacity has to be adapted to fill this gap. Due to the high relevance for the future mobilization and the importance for the economy, lithium is therefore considered a strategic metal by the US Department of Energy (Anonymous, 2010). Two main routes for the production of lithium carbonate or lithium hydroxide can be distinguished: either directly from a lithium enriched
⁎
brine or via the thermal activation and leaching of lithium bearing ores, including spodumene, petalite, lepidolite, zinnwaldite and others (Garrett, 2004; Nogueira et al., 2014; Ziemann et al., 2012; Yan, et al., 2012). The first industrial production of lithium carbonate started in 1923 in Langelsheim, Germany out of the mineral zinnwaldite (Wietelmann and Steinbild, 2013). With the exploration of the brine deposits, mainly in South-America but also in China and in the U.S., the extraction from brine was gaining a bigger share. However, lithium production from brine is limited regarding proven reserves with only a number of locations worldwide and with regards towards flexibility in adapting to market request. Today 55% of the lithium production is based on brines, while 45% is based on hard rock with increasing tendency for lithium ores. The most important lithium rock source besides lepidolite and petalite is spodumene ore (Deutsche Bank, 2016; Anonymus, 2016). At the first glance, extraction from brines seems to be more economic as it includes less process steps because the lithium is already
Corresponding author. E-mail address: [email protected] (E. Gasafi).
https://doi.org/10.1016/j.mineng.2020.106205 Received 30 April 2019; Received in revised form 5 December 2019; Accepted 12 January 2020 0892-6875/ © 2020 Elsevier Ltd. All rights reserved.
Minerals Engineering 148 (2020) 106205
E. Gasafi and R. Pardemann
dissolved and thus requires less capital investment. Extraction from lithium ores requires a thermal processing step at temperatures usually beyond 1000 °C leading to higher fuel energy consumption. It facilitates the phase transition in the lithium bearing mineral enabling an acid or soda leaching in the downstream hydro-metallurgical section. On the other hand, using lithium minerals as a source for lithium production is attractive from a strategic political point of view due to their wider geographic distribution and the larger reserve potential (Kesler et al., 2012). Generally, the production from minerals may still be improved (in particular with regard to the traditional processing of lithium ores) to increase competitiveness on the market. This paper addresses the processing of lithium bearing spodumene ore concentrates. It focuses on the thermal decrepitation by applying state-of-the-art fluidized-bed processes being widely used for alumina calcination and in the roasting of pyrite concentrates, today. Besides comparing the traditional rotary kiln with fluidized-bed processing, it introduces experimental results from fluidized-bed test work and characterization of a variety of spodumene concentrates. The impact of process parameters like temperature or retention time on product quality and certain aspects of importance for fluidized-bed application are highlighted. Hydrometallurgical processes or additional acid baking which may be required after decrepitation are not discussed in the paper.
These investigations are helping to understand the material behavior and to establish a baseline for optimizing the calcination process. There are however no projects reported which are going beyond smallscale laboratory research. Compared to other metallurgical industries the installed capacity and the number of built plants is relatively low limiting the know-how and experience transfer from one operating plant to another and putting challenges to the construction of new plants. Mainly driven by the high time and cost pressure in developing and executing new construction projects for processing lithium ores, there is a trend to applying traditional technologies rather than spending some effort to further develop and improve the calcination process. Thereby, significant potential for reducing operating cost in the long run at almost equal initial capital investment is disregarded. A change from traditional rotary kiln to fluidized bed may result in less downtime and lower maintenance cost as explained in Section 2.3. The potential for further development of thermal processing shall be illustrated in the example of decrepitation of spodumene ore. Whereas spodumene ore is treated without adding additional reactants, lepidolite, zinnwaldite and lithium bearing clays require sulfur containing reactants for deliberation of lithium sulfate at higher temperature (Choubey et al., 2016). The objective of spodumene calcination is to convert the spodumene from monoclinic α-spodumene to tetragonal βspodumene or to the intermediate phase of hexagonal γ spodumene, see Fig. 1. (adapted from (Salakjani et al., 2016)). This process is strongly dependent on temperature and retention time in the furnace and normally starts at 950 °C. Below 950 °C the residence time would be too high to detect remarkable phase changes. With rising temperature and residence time the conversion rates increase as well. At a temperature of 1100 °C the α-phase can be completely converted to β-and γ-spodumene (Salakjani et al., 2016). However, first sintering may be observed dependent on mineralogy at such high temperature negatively impacting later processing. Hence, there is only a small operation window for optimization between 1050 °C and 1100 °C. There is also a change of physical properties with phase transition. It has been noted in the literature, that the apparent surface area will increase from 0.1 to 1 m2/g while the specific gravity drops from about 3.2 g/cm3 to around 2 g/cm3, if α-spodumene is completely converted to β-spodumene (Salakjani et al., 2016). This is in line with own experimental findings, where a decrease of raw density from 3.1 to 2.5 g/cm3 (bulk density changed from 1.5 to 0.9 g/cm3) was observed during calcination at 1.060 °C. Simultaneously, the apparent surface area increased from 345 to 1066 cm2/g. This change in properties needs to be considered in the design of a calcination system.
2. Technology review 2.1. Thermal processing in the production of lithium from minerals The lithium content in the run of mine ore is generally low at a level of about 1–3 wt%. Therefore, the mined ore is usually concentrated next to the deposit and then used either directly as marketable commodity or as feedstock for chemical processing. One of these applications is the production of an intermediate, either Li-carbonate or Lihydroxide being ultimately used for the lithium battery (LiB) production. Thermal treatment of lithium ores is necessary to deliberate the lithium for further downstream extraction. This process is highly endothermic and consumes significant amounts of heat at a high furnace temperature. The thermal energy is mainly needed for drying the concentrate and for heating up the dry material to the reaction temperature. It is to a minor extent necessary for the conversion reaction itself. The required reactor temperature depends on the mineralogy of the lithium ore and ranges between 900 °C and 1100 °C. Substantial energy losses may result by heat transfer over equipment surfaces, heat losses through the released hot off-gas or during product cooling if no additional measures for improving energy efficiency are taken. Today, spodumene calcination is still based on rotary kiln, a technology in use since the early days of lithium ore processing without any significant further development since (Garrett, 2004). The focus of research and development activities is finding alternatives for direct extraction of lithium form the lithium bearing ore (Martin et al., 2017). However, there are no examples of commercially exploited projects existing so far. Understanding and improving the decrepitation step has been considered only in rare number of research projects. Mechanical activation of α-spodumene for further processing into lithium compounds was investigated by Boldyrev (2010). Technical applicability of microwaves for the conversion of spodumene were studied on laboratory scale (Peltosaari et al., 2015). Phase transformation mechanism of spodumene during its calcination was investigated by Abdullah et al. (2019). It was found that α- spodumene transforms into β-spodumene at high temperatures along various reaction pathways including transition phases of γ-spodumene. The stability of γ-spodumene strongly depends on the mechanical treatment of the sample, and the heating rate of the calcination process. Particle size has also an impact on the conversion rate. Generally finer ground samples result in faster and more-complete conversion of α-spodumene compared to a coarser specimen.
2.2. Fluidized-bed system for calcination The feedstock from the concentration plant will be calcined having a typical initial moisture content of 4–10 wt%. This moisture content is largely depending on concentrator design and performance. The conventional calcination takes place in rotary kilns with limited or without use of waste-heat for pre-heating of solid or gaseous streams. In contrast, the concept of the fluidized-bed calcination incorporates the basic principles of energy-efficient processing and relies on recovering as much heat as possible within the system. Dependent on fluidization
Fig. 1. Phase transition from monoclinic α-spodumene to hexagonal γ-spodumene and tetragonal β-spodumene. 2
Minerals Engineering 148 (2020) 106205
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fuel oil are most common in the industry. The gas velocity in the furnace is higher than the fluidization velocity of the particles. Dependent on the particle size, most of the particles are entrained with the gas flow. A sufficient residence time of the solid material is set by the design of the furnace and it is achieved by circulation of the material through the recycle cyclone. During operation it can be adjusted within a certain range by varying the differential pressure of the fluidized bed. The differential pressure is directly linked to the gas velocity and to the amount of material contained in the bed. The calcined material will be discharged from the furnace through a specially-designed discharge device.
Fig. 2. Block diagram of a spodumene calcination plant.
conditions, different types of fluidized-bed reactors can be distinguished, i.e. stationary fluidized bed, circulating fluidized bed etc. The circulating fluidized bed was selected to meet requirements to residence time and temperature of the decrepitation process. High energy efficiency is achieved by using the sensible heat from the off-gas for drying and heating up the feedstock. Heat carried by the solid product discharged from the reactor is transferred to combustion air during cooling. The need for energy for heating up all educts in the furnace is significantly reduced (see Fig. 2). The process for lithium calcination with the major process steps is outlined in the following.
2.2.3. Cooling of product The hot product leaving the reactor is conveyed to the cooling section of the system, including one or more cooling stages and a fluidized-bed cooler, where the final cooling to a target temperature takes place. The energy of the hot product is recovered by pre-heating primary and secondary air. The cooling stages upstream of the fluidized bed cooler work according the same principle as the preheating stages. The only difference is that the hot solid material transfers its heat to cooler air. The fluidized bed cooler consists of a cascade of fluidized beds working at rather low gas velocity. As the upstream cooling stages, it is characterized by direct heat transfer from the solid to the air. The exit temperature is adjusted by the final indirectly-cooled stage applying cooling water. This temperature is determined by the downstream process, either for sulfate or soda leaching.
2.2.1. Drying and pre-heating of concentrate Wet spodumene concentrate is delivered to a feed bin of the calcination plant. It is then discharged to the venturi preheater of the first preheating stage, where solids are mixed with the off-gas leaving the cyclone of the second preheating stage. The spodumene entering the venturi pre-heater of the second preheating stage is mixed with the hot off-gas leaving the circulating fluidized-bed (CFB) furnace and is further heated up. Each preheating stage is a combination of a suspension preheater (venturi preheater) and a cyclone. The sensible heat contained in the off-gas evaporates the entire surface moisture of the concentrate. The tubular suspension preheater consists of pipe with multiple sections, one for feeding solids into a gas flow and one for enhanced mixing of gas and solids. It does not require any additional mechanical equipment as it relies on the venturi principle for entrainment of the solid particles by the gas flow. The intensive mixing ensures efficient and direct heat transfer. The exit temperature from the preheating stage normally corresponds to the weight-based mixed temperature of gas and solids. Because solids and gas are sent to different directions (solids to the furnace, off-gas to the stack) a cyclone is installed after each preheater. The cyclone mechanically separates the bulk of the solids from the gas stream before it enters a dust filter. Dust separation can be accomplished either by a fabric filter or an electrostatic precipitator. The precipitated solids are recirculated into the furnace or mixed with the product if they already meet product quality specifications. The number of pre-heating stages mainly depends on the reduction of fuel consumption vs. capital costs for installing the equipment. Two to maximum three stages are typical for large plants.
2.3. Comparison of fluidized-bed system with traditional rotary kiln technology The described fluidization technology shows differences to rotary kiln systems. This refers to energy consumption, process performance, and product quality, as well as plant footprint. A more illustrative flow sheet of the fluidized-bed process is shown in Fig. 3. The following process stages are included in Fig. 3: 1. 2. 3. 4.
2.2.2. Calcination of spodumene ores The final and complete conversion of the Li-mineral takes place in the CFB furnace. It consists of a rather slim but high tubular furnace connected to a recycle cyclone. The furnace is refractory-lined and characterized by the injection of two air flows. Primary air, a pre-heated portion of the combustion air, is injected via a nozzle bottom to ensure sufficient fluidization of the solids in the furnace. A remaining secondary air flow released from the fluid-bed cooler is fed directly into the fluidized bed reactor to maintain complete combustion of the fuel. Moreover, this staged injection of combustion air enables minimum formation of NOx. The required process heat for heating-up and decrepitating the spodumene at a temperature of 1050–1100 °C is generated by flameless fuel combustion directly in the CFB furnace. Thereby, the fuel is injected into the fluidized bed of solid material. The fuel type can vary and depends on price, local availability, but also on the restrictions from the downstream process. Natural gas or low-sulfur
feed bin; feeding screw; venturi pre-heater 1; cyclone pre-heating stage 1;
Fig. 3. Schematic of a CFB-based calcination process. 3
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3. Test work Calcination of lithium ore in fluidized-bed system for a variety of lithium bearing ores was investigated for many years at Outotec’s Research Centre located in Frankfurt, Germany. The performed studies commonly aimed for investigating the influence of major process parameters including particle size, temperature and residence time. They were performed at laboratory, bench, and pilot scale. Whereas laboratory testing is mainly applied for scoping studies, bench-scale and pilot scale testing is performed to provide the basis for industrial process design. Test work results obtained for five selected spodumenebased ore concentrates will be introduced in this article. Besides providing characteristic material data, results obtained at differing scale will be presented. Details of laboratory and pilot-scale testing will be discussed with regard to one of the ores (Ore A). The raw mineral, either run-of-mine ores or concentrates, was normally pre-treated by drying and if needed by grinding or screening depending on the test facility in which it was processed (with grain size ranging between < 1 mm for laboratory-scale testing and < 3 mm grain size for pilot testing). 3.1. Characteristics of investigated spodumene ores
Fig. 4. Schematic of a rotary kiln-based calcination process.
Some major characteristics of the spodumene-based lithium bearing ores are provided in Tables 2, 3 and 4. Moisture content was determined by drying in a heating cabinet for 12 h at 105 °C. The loss on ignition (LOI) was determined as mass difference of the dry material after glowing the material in a muffle furnace at 1000 °C. Mineral phase composition was determined through semi-quantitative XRD analysis. The absolute mineral phase composition values determined through XRD might differ from reality, if amorphous components are present in the material. However, it allows for the comparison of the material before and after testing implying a constant amorphous content. The elemental composition was analyzed by X-ray fluorescence (XRF) and it is typically presented in oxidic form. The lithium content of the lithiumbearing ores was determined by ICP-MS or ICP-OES because lithium is too light to be determined by XRF. Because the sample materials were investigated over a longer period, not all materials were characterized in the same way and to the same extent. Minimum fluidization velocities of spodumene ore concentrates and calcined material are summarized in Table 5.
5. 6. 7. 8.
electrostatic precipitator or filter; venturi pre-heater 2; cyclone pre-heating stage 2; furnace with recycle cyclone (sold product is discharged from recycle cyclone underflow); 9. cooling cyclone 1; 10. cooling cyclone 2; 11. fluidized-bed final product cooler. The schematic of an advanced rotary kiln-based process with solids pre-heating is shown in Fig. 4. It includes: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
feed bin including feeding device; venturi pre-heating stage 1; cyclone of pre-heating stage 1; venturi pre-heating stage 2; cyclone pre-heating stage 2; electrostatic precipitator or filter; stack; mixing bin; rotary kiln furnace; rotary sectional drum cooler.
3.2. Optimal process conditions The objective for all investigated ores was to identify preferential conditions for the decrepitation process. For this purpose, test work was performed at different scales:
• laboratory scale: batch-type 50 mm fluidized bed facility, 100–150 g of feed material per run, • bench scale: continuously operated 200–360 mm circulating fluidized-bed reactors, 20–75 kg/h material input, • pilot scale: continuously operated 700 mm circulating fluidized-bed
There are many differences between the rotary kiln and a fluidized bed furnace, which make them suitable for different applications. Table 1 summarizes the characteristics and differences between the two furnace types. The lower consumption of energy of CFB-based processes as compared to rotary kiln is known from other industries. Fig. 5 illustrates the progressive reduction of the specific fuel energy requirement for alumina calcination plants, starting from traditional rotary kiln in the 1970 s and ending at the current benchmark of less than 2.7 GJ per ton alumina marked by an Outotec calciner. This low specific fuel consumption is achieved on a continuous basis (long term average). It should be noted that this was achieved in a 40-year-old plant by retrofitting and upgrading the plant to a more modern configuration including both a hydrate bypass and a hydrate dryer. Because of the process similarities, decrepitation of Lithium spodumene minerals can significantly benefit from the technological experience gathered in the field of alumina calcination with regards to process efficiency and product quality.
reactor, max. 500 kg/h material input.
Not all test work stages were performed for every material because of the increasing costs and increasing amount of test material required for larger-scale testing. Besides presenting results for optimal process conditions (temperature and retention time) for the seven ores, results are discussed in more detail on the example of Ore A. The relevance of results for commercial applications increases from lab- to pilot-scale. In general, at least benchscale testing is required by engineering companies before granting process guarantees. The overall conversion of α-spodumene (see Eq. (1)) and in particular conversion of α to β-spodumene (see Eq. (2)) can be defined as 4
Minerals Engineering 148 (2020) 106205
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Table 1 Comparison of fluidized-bed system and rotary kiln. Rotary Kiln
Fluidized Bed
Feedstock
Low effort for feed preparation because of less stringent requirement to feed PSD
Temperature distribution
Inhomogeneous axial and radial temperature distribution (peaks of up to 1800 °C close to the flame formed by the burner) Predominantly radiant heat transfer over convective and conductive transfer Heterogeneous and potentially insufficient product quality resulting from uneven temperature and residence time distribution High fuel and electricity demand because of high maximum process temperature and different heat transfer regime (the electricity demand is linked with the larger size of the ID fan or air compressor)No use of sensible heat of solids if additional effort for a dust recovery system shall be avoided (cooling applying)
Higher effort for feed preparation caused by: ▪ Need for sufficient fluidity (PSD, moisture) ▪ Upper particle size limit of 10 mm Preferentially narrow PSD Uniform axial and radial temperature distribution inside reactor
Heat transfer Product quality Energetic efficiency
Maintenance
Mainly convective and conductive heat transfer Option for adjustment of specific temperature and residence time allowing for provision customer compliant products of constant quality Lower fuel and electricity consumption because of smaller heat requirement (lower average process temperature and smaller overall gas flow) Option for efficient recovery of sensible heat of solids and flue gas by: ▪ Pre-heating and drying of feedstock ▪ Pre-heating of air ▪ Provision of hot boiler feed water for downstream processes No rotating equipment in the furnace system Limited thermal and mechanical refractory stress enabling refractory lifetime of 10 years + x Small because of vertical arrangement and higher specific process intensity
Mechanically highly stressed rotating equipment in the furnace system Need for frequent repairs of refractory causing additional downtimes Refractory replacement at least every two years Large horizontal footprint because of large calcination kiln size and need for additional cooling kiln
Footprint
Table 3 Chemical composition of raw spodumene ore concentrates with regards to major elements. Element
Fe (total) CaO SiO2 MgO Al2O3 TiO2 Mn P K BaO Cu Ni Zn Pb Sn V Cr Ce Na Li
Fig. 5. Energy efficiency for alumina calcination at different installations (modified from (Perander et al., 2017)). Note that the theoretical energy consumption does include 6% surface moisture but does not include any heat losses and assumes all energy is used for the calcination reactions. Table 2 Characteristics of raw spodumene ore concentrates.
Raw moisture Loss on ignition LOI PSD:d80 PSD:d50 Bulk density
wt.% wt.% µm µm kg/l
Ore A
Ore B
Ore C
Ore D
Ore E
Ore F
Ore G
3–13 0.6 406 215 1.48
3.6 n. a. 130 90 n.a.
n. a. n. a. 104 62 n. a.
n. a. 0.5 147 99 1.43
5.1 0.6 129 66 1.29
4.9
n.a.
210 137 1.34
218 144 1.40
Composition in wt.% Ore A
Ore B
Ore C
Ore D
Ore E
Ore F
Ore G
0.82 0.60 65.60 0.16 23.60 0.13 0.07 0.16 0.48 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.40 2.90
0.59 0.13 65.30 0.05 26.10 < 0.05 < 0.05 0.12 0.41 < 0.05 0,06 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 0.19 3.40
1.20 0.79 62.80 0.42 24.70 0.07 0.14 0.10 0.61 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 0.52 2.50
0.50 0.85 63.70 < 0.05 26.30 0.09 0.11 0.06 0.36 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.40 3.40
0.70 0.48 66.40 0,14 24.40 0.09 < 0.05 0.10 0.66 < 0.05 0,06 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.40 4.50
0.58 0.28 63.2 0.06 26.70 0.06 0.23 < 0.03 0.54 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 0.32 6.30
0.16 0.30 64.2 < 0.05 25.90 < 0.05 0.25 < 0.03 0.47 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 0.55 6.44
Table 4 Mineral phase composition of raw spodumene ores determined by semi-quantitative XRD analysis. Mineral phase
n. a. = not available.
α-spodumene Quartz Plagioclase Microcline Muscovite Albite Schorlite Beryl Amphiboles
evaluation criteria. It is calculated from the mass fractions of the corresponding spodumene phases determined through XRD analysis. The suitability of the conversion factors was confirmed with leaching tests of the thermally-treated Ore B. Only β-spodumene (and at extended residence time also γ-spodumene) can be recovered from the decrepitated ore during hydrometallurgical processing after thermal treatment. The minimal target value for total conversion is set at 90% (for Ore B, 5
Composition in wt.% Ore A
Ore B
Ore C
Ore D
Ore E
Ore F
Ore G
64.0 7.1 4.9 9.0 8.9 – – – 6.1
Not available
77.8 3.9 9.6 – 5.4 – 3.3 – –
84.7 6.7 – – 2.7 5.9 – – –
64.0 7.1 4.9 9.0 8.9 – – – 6.1
81.0 2.2 4.6
74.0 5.9 10.6
11.5 – – 0.7 –
8.7 – – 0.8 –
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Table 5 Minimum fluidization velocity in m/s, measured with air at ambient temperature and pressure.
Raw material Calcine
Ore A
Ore B
Ore C
Ore D
Ore E
Ore F
Ore G
0.06* –
– –
– –
0.015 –
0.06 –
0.12 0.06
0.08 0.04
*Value obtained for material < 1 mm.
Fig. 7. Total and relative β-conversion determined for Ore A in laboratory-scale fluidized-bed reactor.
Ore A indicate a significant influence of the plant-scale at which the tests are performed. Moreover, phase change rates determined in muffle furnaces or rotary kilns can also be achieved in fluidized-bed reactors but at significantly shorter retention times. For Ore A, batch-type laboratory tests were performed using a smallscale externally-heated fluidized-bed reactor (50 mm inner reactor diameter) in which 100 g of material was treated at specific temperatures and retention times. 800 l/h of air was used as fluidization medium. Results for conversion of α to β-spodumene and overall αspodumene conversion are shown in Fig. 7. Results are provided as relative values. Values are referred to the conversion determined at lab scale (as shown in Fig. 6). Fig. 7 indicates an increase in total conversion of α-spodumene with increasing temperature. Only a moderate increase is observed at temperature above 1050 °C. Residence time is of significant importance at low temperature, but its relevance diminishes at above 1050 °C. Intermediate formation of γ-spodumene (starting at around 800 °C) and increasing formation of β-spodumene from γ-spodumene at elevated temperature was confirmed through XRD analysis. In contrast to total conversion, β-conversion significantly increases even at > 1050 °C because of conversion of γ to β-spodumene. The observations are in agreement with results of other researchers as reported in literature (Sitando and Crouse, 2012; Salakjani et al., 2016). Despite increased β-spodumene content at higher temperatures, there is a temperature limitation to be considered. Downstream hydrometallurgical treatment benefits from higher β-spodumene content. However decrepitation at temperatures beyond 1100 °C implies the risk of sintering and/or glass formation reactions of the feldspar mineral phases contained in the material. The sintering respectively melting behavior is strongly depending on the mineralogy and the impurities of the lithium ore. In general, an over-calcination needs to be avoided because leachability is reduced due to limiting access of the reagent to the β and γ-spodumene. Optical microscopy can be used visualize the phase changes in the calcination process Sintered phases can be identified as can be seen in the example of over calcined material (see bottom picture in Fig. 8 with sintered feldspar). The change in properties needs to be studies on laboratory scale and can then be used in the design of an industrial calcination system (Gasafi et al., 2018).
Fig. 6. β-conversion and total conversion for spodumene ore concentrates at different test scales.
leaching test results were back-calculated to β-conversion). In general, β-conversion increases with increasing temperature.
conversion = Totalconversion =
x (1)
x +x +x x +x
(2)
x +x + x
Maximum values determined for the different ores are shown in Fig. 6. The corresponding temperature and retention time in the reactor are summarized in Table 6. 4. Results Although most results are available from lab-scale testing, results for Table 6 Optimal temperature and retention time determined at varying test scale. Ore A Laboratory-scale testing Temperature °C 1050 Retention time min 30 Bench-scale testing Temperature °C 1060 Retention time min 30 Pilot-scale testing Temperature °C 1070/80 Retention time min 30–40
Ore B
Ore C
Ore D
Ore E
Ore F
Ore G
– –
1060 30
1060 30
1060 30
1050 20–30
1050 30
1070/80 35
– –
– –
1050 30*
1050 30*
– –
– –
– –
*Retention time not varied during test work. 6
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Fig. 9. Relative total conversion and relative β-conversion determined during pilot-scale testing (T = 1050–1060 °C, retention time: 30–40 min).
retention time of 30–40 min were selected as parameters for a test campaign at pilot scale (4 days, 24 h operation at a feeding rate of 300–400 kg/h). Fig. 9 shows the conversion rate for α- to β-spodumene and the total conversion rate. It indicates almost no variation during the operating period for bed product from the CFB furnace. Moreover, it proves high conversion values for the discharged fines. Data points shown in Fig. 9 are relative values referring to the conversion value determined for the CFB discharge. Because maximizing the recovery of lithium is of high importance to overall process economics, commercial application would target for reclamation and application of fine material to hydrometallurgical processing, too. Hence, conversion rates were determined for the CFB bed product and fines recovered from gas cooling path of the pilot test facility including the filter and a downstream cyclone and heat exchanger. Results are presented in Fig. 9 and indicate high β-and total conversion rates for all selected fractions. Consequently, the discharged fines fractions also fulfill the conversion criterion of 90%. Besides proofing process performance at pilot scale, the purpose was to determine important figures for plant scale-up such as tendency for sticking and sintering and gaseous emissions to be expected from the process (which are not discussed in more detail in this article).
Fig. 8. Results of optical microscopy investigation with glassy feldspar regions marked with dotted line.
Hence, the selection of the optimal process temperature strongly depends on the mineral feedstock (in particular its tendency for sintering). It requires careful consideration of maximum conversion values at high temperature vs. deteriorating effects on the actually recoverable amount of lithium, which commonly decreases with increasing temperature. Considering previous experience and results obtained from laboratory-scale test work, a temperature of 1050 °C to 1060 °C and a
5. Conclusion Today, 55% of lithium production is based on brines while 45% is based on hard rock, with an increasing trend towards lithium ores. Due to the growing demand for lithium, the number of projects and the plant capacity is increasing. The conventional process for calcination is 7
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still based on rotary kiln, a technology in use since the early days and not significantly further developed since. Fluidized-bed calcination was introduced as an alternative technology in this article. A general advantage of fluidized-bed over rotary kiln calcination is the establishment of a homogenous temperature profile in the reactor whereas rotary kiln is characterized by a temperature gradient along the reactor length with the highest temperature in the flame zone of the burner. Moreover, residence time in a CFB reactor can be accurately controlled. Based on experience of existing industrial fluidized-bed calcination plants, thermal processing of lithium ore was investigated applying test facilities ranging from small lab scale to pilot scale. The objective for bench- and pilot-scale testing was to provide both engineering and design data. Optimal process conditions were determined through the tests. A retention time in the range of 25–40 min was identified as sufficient for extensive conversion of α- to β- and γ-spodumene. Depending on the spodumene concentrate, the β-spodumene content varies. A temperate between 1050 and 1070 °C was sufficient to achieve ≥90% total conversion of α-spodumene. Lower temperature may result in lower overall conversion and lower β-spodumene content dependent on the raw material. The temperature effect can be partially compensated by prolonged retention time. An upper temperature limitation is given by the material softening or sintering causing sticking. A common observation was an improved conversion with larger reactor scale. Typical total conversion was in the range of 92–95 % with maximum β-conversion of 67–84%. Furthermore, the raw concentrates and calcined products were granulometrically characterized. All calcined materials show a significant reduction in bulk density from about 1.3–1.5 kg/l for the raw material to 0.6–0.8 kg/l for the calcine. Sticking tendency is strongly impacted by mineralogy and grain size of the feed material. Only one concentrate was excluded from further testing after performing lab scale tests. Hence, fluidized bed proved to be a technically feasible process for the vast majority of investigated concentrates and for spodumene ore in general. Because of process related advantages such as significantly reduced energy consumption and high product quality compared to the tradition rotary kiln process, fluidized-bed is an attractive process to be considered for future decrepitation plants.
Declaration of Competing Interest We declare that we have no conflict of interest. References Abdullah, A.A., Oskierski, H.C., Altarawneh, M., Senanayake, G., Lumpkin, G., Dlugogo, B.Z., 2019. Phase transformation mechanism of spodumene during its calcination. Miner. Eng. Anonymous, 2010. Critical Materials Strategy. US Department of Energy, Washington. Anonymus, 2016. Lithium. In Mineral commodity summaries 2016. United States Geological Survey, Reston. Boldyrev, N. P., T., K., D., M., V., R., 2010. Mechanical activation of α-spodumene for further processing into lithium compounds. Theoret. Found. Chem. Eng., vol. 503–507. Choubey, P.K., Kim, M.-S., Srivastava, R.R., Lee, J.-C., Lee, J.-Y., 2016. Advance review on the exploitation of the prominent energy-storage element: Lithium. Part I: From mineral and brine resources. Miner. Eng. 119–137. Deutsche Bank, 2016. FITT report Welcome to the Lithium-ion Age. Deutsche Bank, Frankfurt. Martin, G., Schneider, A., Voigt, W., Bertau, M., 2017. Lithium extraction from the mineral zinnwaldite: Part II: Lithium carbonate recovery by direct carbonation of sintered zinnwaldite concentrate. Miner. Eng. 75–81. Garrett, D.E., 2004. Handbook of lithium and natural calcium chloride: their deposits, processing, uses and properties. Amsterdam [u.a.]. Elsevier Acad. Press. Gasafi, E., Perander, L., Beisheim, T., 2018. Calcining in Circulating Fluidized Bed Systems. In: 23rd International Conference on Fluidized Bed Conversion. Seoul: The Korean Institute of Chemical Engineers (KIChE). Julien, C., Mauger, A., Vijh, A., Zaghib, K., 2016. Lithium Batteries - Science and Technology. Cham [u.a.]. Springer International Publishing. Kesler, S.E., Gruber, P.W., Medina, P.A., Keoleian, G.A., Everson, M.P., Wallington, T.J., 2012. Global lithium resources: relative importance of pegmatite, brine and other deposits. Ore Geol. Rev. 48, 55–69. Nogueira, C., Margarido, F., Vieceli, N., Durão, F., Guimarães, C., 2014. Comparison of Processes for Lithium Recovery from. Lepidolite by H2SO4 Digestion or HCl Leaching. In: Proceedings of the International Conference on Mining, Material and Metallurgical Engineering, (p. Paper No. 72). Prague. Peltosaari, O., Tanskanen, P., Heikkinen, E.-P., Fabritius, T., 2015. α → γ → β-phase transformation of spodumene with hybrid microwave and conventional furnaces. Miner. Eng. 54–60. Perander, L., Scarsella, A., Gasafi, E., Schmidt, H.-W. (2017). CFB Alumina Calciners / New and Future Generation. TMS 2017 146th Annual Meeting and Exhibition. TMS. Salakjani, N.K., Singh, P., Nikoloski, A.N., 2016. Mineralogical transformations of spodumene concentrate from Greenbushes, Western Australia. Part 1: Conventional heating”. Miner. Eng. 98, 71–79. Sitando, O., Crouse, P., 2012. Processing of a Zimbabwean petalite to obtain lithium carbonate. Int. J. Miner. Process. 45–50. Wietelmann, U., Steinbild, M., 2013. Lithium and Lithium Compounds. In Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH Verlag, Weinheim. Yan, Q., Li, X., Wang, Z., Wu, X., Wang, J., Guo, H., Peng, W., 2012. Extraction of lithium from lepidolite by sulfation roasting and water leaching. Int. J. Miner. Process. 110–111, 1–5. Ziemann, S., Weil, M., Schebek, L., 2012. Tracing the fate of lithium––The development of a material flow model. Resour., Conserv. Recycl., pp. 26-34.
CRediT authorship contribution statement Edgar Gasafi: Conceptualization, Methodology, Investigation. Robert Pardemann: Investigation, Validation, Writing - review & editing.
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Processing of spodumene concentrates in fluidized-bed systems Edgar Gasafi , Robert Pardemann ⁎
T
Outotec GmbH & Co. KG, 61440 Oberursel, Germany
ARTICLE INFO
ABSTRACT
Keywords: Calcination Decrepitation Fluidized bed Lithium Spodumene
The production of lithium carbonate or lithium hydroxide out of lithium bearing ores requires a thermal activation for the phase transition in the mineral to enable an acid or soda leaching in the downstream hydrometallurgical section. In this paper, traditional processing of lithium ores in the lithium industry is reviewed and opportunities to improve product quality and recovery rate by simultaneously reducing energy consumption will be presented. The conventional process for pyro-processing is still based on rotary kiln, a technology in use since the early days of lithium ore processing, albeit not significantly further developed since. A new technology in this field is fluidized-bed calcination. A study of the major process parameters (temperature and residence time) was performed on laboratory and larger bench scale aiming for optimal product quality for subsequent processing. When converting α- to both β- and γ-spodumene, high conversion rates were achieved for all investigated materials. The conversion of α- to only β-spodumene varies significantly by concentrate. Preferential conditions were temperatures in the range of 1050–1070 °C with retention times of typically 25–40 min. Additional results include the characterization of granulometric properties, in particular the effect of defragmentation by decrepitation, elemental and mineral phase composition, and further aspects specifically related to the application of fluidized bed technology. After identifying optimal process conditions for calcination and proving general feasibility at laboratory scale, the technical feasibility was confirmed for a wide range of spodumene concentrates at bench and pilot scale (20 up to 500 kg/h feed) providing the basis for industrial process design. Thus, spodumene decrepitation through applying fluidized-bed technology is technically feasible for non-sticking material and high and homogenous product quality can be achieved.
1. Introduction THERE is an increasing demand worldwide for lithium and lithium compounds, which is mainly driven by the expected electric and hybrid vehicle boom in the near future. Lithium is used in rechargeable batteries as a cathode material. Due to its low density and the high electrochemical standard potential it is for the time being the most promising and preferred material to push the electro mobilization (Julien et al., 2016). There are various projections in the increase of lithium ion battery production for the next decades. The common view is that lithium demand will significantly increase while the current supply will not meet the expected requirement in the future and production capacity has to be adapted to fill this gap. Due to the high relevance for the future mobilization and the importance for the economy, lithium is therefore considered a strategic metal by the US Department of Energy (Anonymous, 2010). Two main routes for the production of lithium carbonate or lithium hydroxide can be distinguished: either directly from a lithium enriched
⁎
brine or via the thermal activation and leaching of lithium bearing ores, including spodumene, petalite, lepidolite, zinnwaldite and others (Garrett, 2004; Nogueira et al., 2014; Ziemann et al., 2012; Yan, et al., 2012). The first industrial production of lithium carbonate started in 1923 in Langelsheim, Germany out of the mineral zinnwaldite (Wietelmann and Steinbild, 2013). With the exploration of the brine deposits, mainly in South-America but also in China and in the U.S., the extraction from brine was gaining a bigger share. However, lithium production from brine is limited regarding proven reserves with only a number of locations worldwide and with regards towards flexibility in adapting to market request. Today 55% of the lithium production is based on brines, while 45% is based on hard rock with increasing tendency for lithium ores. The most important lithium rock source besides lepidolite and petalite is spodumene ore (Deutsche Bank, 2016; Anonymus, 2016). At the first glance, extraction from brines seems to be more economic as it includes less process steps because the lithium is already
Corresponding author. E-mail address: [email protected] (E. Gasafi).
https://doi.org/10.1016/j.mineng.2020.106205 Received 30 April 2019; Received in revised form 5 December 2019; Accepted 12 January 2020 0892-6875/ © 2020 Elsevier Ltd. All rights reserved.
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dissolved and thus requires less capital investment. Extraction from lithium ores requires a thermal processing step at temperatures usually beyond 1000 °C leading to higher fuel energy consumption. It facilitates the phase transition in the lithium bearing mineral enabling an acid or soda leaching in the downstream hydro-metallurgical section. On the other hand, using lithium minerals as a source for lithium production is attractive from a strategic political point of view due to their wider geographic distribution and the larger reserve potential (Kesler et al., 2012). Generally, the production from minerals may still be improved (in particular with regard to the traditional processing of lithium ores) to increase competitiveness on the market. This paper addresses the processing of lithium bearing spodumene ore concentrates. It focuses on the thermal decrepitation by applying state-of-the-art fluidized-bed processes being widely used for alumina calcination and in the roasting of pyrite concentrates, today. Besides comparing the traditional rotary kiln with fluidized-bed processing, it introduces experimental results from fluidized-bed test work and characterization of a variety of spodumene concentrates. The impact of process parameters like temperature or retention time on product quality and certain aspects of importance for fluidized-bed application are highlighted. Hydrometallurgical processes or additional acid baking which may be required after decrepitation are not discussed in the paper.
These investigations are helping to understand the material behavior and to establish a baseline for optimizing the calcination process. There are however no projects reported which are going beyond smallscale laboratory research. Compared to other metallurgical industries the installed capacity and the number of built plants is relatively low limiting the know-how and experience transfer from one operating plant to another and putting challenges to the construction of new plants. Mainly driven by the high time and cost pressure in developing and executing new construction projects for processing lithium ores, there is a trend to applying traditional technologies rather than spending some effort to further develop and improve the calcination process. Thereby, significant potential for reducing operating cost in the long run at almost equal initial capital investment is disregarded. A change from traditional rotary kiln to fluidized bed may result in less downtime and lower maintenance cost as explained in Section 2.3. The potential for further development of thermal processing shall be illustrated in the example of decrepitation of spodumene ore. Whereas spodumene ore is treated without adding additional reactants, lepidolite, zinnwaldite and lithium bearing clays require sulfur containing reactants for deliberation of lithium sulfate at higher temperature (Choubey et al., 2016). The objective of spodumene calcination is to convert the spodumene from monoclinic α-spodumene to tetragonal βspodumene or to the intermediate phase of hexagonal γ spodumene, see Fig. 1. (adapted from (Salakjani et al., 2016)). This process is strongly dependent on temperature and retention time in the furnace and normally starts at 950 °C. Below 950 °C the residence time would be too high to detect remarkable phase changes. With rising temperature and residence time the conversion rates increase as well. At a temperature of 1100 °C the α-phase can be completely converted to β-and γ-spodumene (Salakjani et al., 2016). However, first sintering may be observed dependent on mineralogy at such high temperature negatively impacting later processing. Hence, there is only a small operation window for optimization between 1050 °C and 1100 °C. There is also a change of physical properties with phase transition. It has been noted in the literature, that the apparent surface area will increase from 0.1 to 1 m2/g while the specific gravity drops from about 3.2 g/cm3 to around 2 g/cm3, if α-spodumene is completely converted to β-spodumene (Salakjani et al., 2016). This is in line with own experimental findings, where a decrease of raw density from 3.1 to 2.5 g/cm3 (bulk density changed from 1.5 to 0.9 g/cm3) was observed during calcination at 1.060 °C. Simultaneously, the apparent surface area increased from 345 to 1066 cm2/g. This change in properties needs to be considered in the design of a calcination system.
2. Technology review 2.1. Thermal processing in the production of lithium from minerals The lithium content in the run of mine ore is generally low at a level of about 1–3 wt%. Therefore, the mined ore is usually concentrated next to the deposit and then used either directly as marketable commodity or as feedstock for chemical processing. One of these applications is the production of an intermediate, either Li-carbonate or Lihydroxide being ultimately used for the lithium battery (LiB) production. Thermal treatment of lithium ores is necessary to deliberate the lithium for further downstream extraction. This process is highly endothermic and consumes significant amounts of heat at a high furnace temperature. The thermal energy is mainly needed for drying the concentrate and for heating up the dry material to the reaction temperature. It is to a minor extent necessary for the conversion reaction itself. The required reactor temperature depends on the mineralogy of the lithium ore and ranges between 900 °C and 1100 °C. Substantial energy losses may result by heat transfer over equipment surfaces, heat losses through the released hot off-gas or during product cooling if no additional measures for improving energy efficiency are taken. Today, spodumene calcination is still based on rotary kiln, a technology in use since the early days of lithium ore processing without any significant further development since (Garrett, 2004). The focus of research and development activities is finding alternatives for direct extraction of lithium form the lithium bearing ore (Martin et al., 2017). However, there are no examples of commercially exploited projects existing so far. Understanding and improving the decrepitation step has been considered only in rare number of research projects. Mechanical activation of α-spodumene for further processing into lithium compounds was investigated by Boldyrev (2010). Technical applicability of microwaves for the conversion of spodumene were studied on laboratory scale (Peltosaari et al., 2015). Phase transformation mechanism of spodumene during its calcination was investigated by Abdullah et al. (2019). It was found that α- spodumene transforms into β-spodumene at high temperatures along various reaction pathways including transition phases of γ-spodumene. The stability of γ-spodumene strongly depends on the mechanical treatment of the sample, and the heating rate of the calcination process. Particle size has also an impact on the conversion rate. Generally finer ground samples result in faster and more-complete conversion of α-spodumene compared to a coarser specimen.
2.2. Fluidized-bed system for calcination The feedstock from the concentration plant will be calcined having a typical initial moisture content of 4–10 wt%. This moisture content is largely depending on concentrator design and performance. The conventional calcination takes place in rotary kilns with limited or without use of waste-heat for pre-heating of solid or gaseous streams. In contrast, the concept of the fluidized-bed calcination incorporates the basic principles of energy-efficient processing and relies on recovering as much heat as possible within the system. Dependent on fluidization
Fig. 1. Phase transition from monoclinic α-spodumene to hexagonal γ-spodumene and tetragonal β-spodumene. 2
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fuel oil are most common in the industry. The gas velocity in the furnace is higher than the fluidization velocity of the particles. Dependent on the particle size, most of the particles are entrained with the gas flow. A sufficient residence time of the solid material is set by the design of the furnace and it is achieved by circulation of the material through the recycle cyclone. During operation it can be adjusted within a certain range by varying the differential pressure of the fluidized bed. The differential pressure is directly linked to the gas velocity and to the amount of material contained in the bed. The calcined material will be discharged from the furnace through a specially-designed discharge device.
Fig. 2. Block diagram of a spodumene calcination plant.
conditions, different types of fluidized-bed reactors can be distinguished, i.e. stationary fluidized bed, circulating fluidized bed etc. The circulating fluidized bed was selected to meet requirements to residence time and temperature of the decrepitation process. High energy efficiency is achieved by using the sensible heat from the off-gas for drying and heating up the feedstock. Heat carried by the solid product discharged from the reactor is transferred to combustion air during cooling. The need for energy for heating up all educts in the furnace is significantly reduced (see Fig. 2). The process for lithium calcination with the major process steps is outlined in the following.
2.2.3. Cooling of product The hot product leaving the reactor is conveyed to the cooling section of the system, including one or more cooling stages and a fluidized-bed cooler, where the final cooling to a target temperature takes place. The energy of the hot product is recovered by pre-heating primary and secondary air. The cooling stages upstream of the fluidized bed cooler work according the same principle as the preheating stages. The only difference is that the hot solid material transfers its heat to cooler air. The fluidized bed cooler consists of a cascade of fluidized beds working at rather low gas velocity. As the upstream cooling stages, it is characterized by direct heat transfer from the solid to the air. The exit temperature is adjusted by the final indirectly-cooled stage applying cooling water. This temperature is determined by the downstream process, either for sulfate or soda leaching.
2.2.1. Drying and pre-heating of concentrate Wet spodumene concentrate is delivered to a feed bin of the calcination plant. It is then discharged to the venturi preheater of the first preheating stage, where solids are mixed with the off-gas leaving the cyclone of the second preheating stage. The spodumene entering the venturi pre-heater of the second preheating stage is mixed with the hot off-gas leaving the circulating fluidized-bed (CFB) furnace and is further heated up. Each preheating stage is a combination of a suspension preheater (venturi preheater) and a cyclone. The sensible heat contained in the off-gas evaporates the entire surface moisture of the concentrate. The tubular suspension preheater consists of pipe with multiple sections, one for feeding solids into a gas flow and one for enhanced mixing of gas and solids. It does not require any additional mechanical equipment as it relies on the venturi principle for entrainment of the solid particles by the gas flow. The intensive mixing ensures efficient and direct heat transfer. The exit temperature from the preheating stage normally corresponds to the weight-based mixed temperature of gas and solids. Because solids and gas are sent to different directions (solids to the furnace, off-gas to the stack) a cyclone is installed after each preheater. The cyclone mechanically separates the bulk of the solids from the gas stream before it enters a dust filter. Dust separation can be accomplished either by a fabric filter or an electrostatic precipitator. The precipitated solids are recirculated into the furnace or mixed with the product if they already meet product quality specifications. The number of pre-heating stages mainly depends on the reduction of fuel consumption vs. capital costs for installing the equipment. Two to maximum three stages are typical for large plants.
2.3. Comparison of fluidized-bed system with traditional rotary kiln technology The described fluidization technology shows differences to rotary kiln systems. This refers to energy consumption, process performance, and product quality, as well as plant footprint. A more illustrative flow sheet of the fluidized-bed process is shown in Fig. 3. The following process stages are included in Fig. 3: 1. 2. 3. 4.
2.2.2. Calcination of spodumene ores The final and complete conversion of the Li-mineral takes place in the CFB furnace. It consists of a rather slim but high tubular furnace connected to a recycle cyclone. The furnace is refractory-lined and characterized by the injection of two air flows. Primary air, a pre-heated portion of the combustion air, is injected via a nozzle bottom to ensure sufficient fluidization of the solids in the furnace. A remaining secondary air flow released from the fluid-bed cooler is fed directly into the fluidized bed reactor to maintain complete combustion of the fuel. Moreover, this staged injection of combustion air enables minimum formation of NOx. The required process heat for heating-up and decrepitating the spodumene at a temperature of 1050–1100 °C is generated by flameless fuel combustion directly in the CFB furnace. Thereby, the fuel is injected into the fluidized bed of solid material. The fuel type can vary and depends on price, local availability, but also on the restrictions from the downstream process. Natural gas or low-sulfur
feed bin; feeding screw; venturi pre-heater 1; cyclone pre-heating stage 1;
Fig. 3. Schematic of a CFB-based calcination process. 3
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3. Test work Calcination of lithium ore in fluidized-bed system for a variety of lithium bearing ores was investigated for many years at Outotec’s Research Centre located in Frankfurt, Germany. The performed studies commonly aimed for investigating the influence of major process parameters including particle size, temperature and residence time. They were performed at laboratory, bench, and pilot scale. Whereas laboratory testing is mainly applied for scoping studies, bench-scale and pilot scale testing is performed to provide the basis for industrial process design. Test work results obtained for five selected spodumenebased ore concentrates will be introduced in this article. Besides providing characteristic material data, results obtained at differing scale will be presented. Details of laboratory and pilot-scale testing will be discussed with regard to one of the ores (Ore A). The raw mineral, either run-of-mine ores or concentrates, was normally pre-treated by drying and if needed by grinding or screening depending on the test facility in which it was processed (with grain size ranging between < 1 mm for laboratory-scale testing and < 3 mm grain size for pilot testing). 3.1. Characteristics of investigated spodumene ores
Fig. 4. Schematic of a rotary kiln-based calcination process.
Some major characteristics of the spodumene-based lithium bearing ores are provided in Tables 2, 3 and 4. Moisture content was determined by drying in a heating cabinet for 12 h at 105 °C. The loss on ignition (LOI) was determined as mass difference of the dry material after glowing the material in a muffle furnace at 1000 °C. Mineral phase composition was determined through semi-quantitative XRD analysis. The absolute mineral phase composition values determined through XRD might differ from reality, if amorphous components are present in the material. However, it allows for the comparison of the material before and after testing implying a constant amorphous content. The elemental composition was analyzed by X-ray fluorescence (XRF) and it is typically presented in oxidic form. The lithium content of the lithiumbearing ores was determined by ICP-MS or ICP-OES because lithium is too light to be determined by XRF. Because the sample materials were investigated over a longer period, not all materials were characterized in the same way and to the same extent. Minimum fluidization velocities of spodumene ore concentrates and calcined material are summarized in Table 5.
5. 6. 7. 8.
electrostatic precipitator or filter; venturi pre-heater 2; cyclone pre-heating stage 2; furnace with recycle cyclone (sold product is discharged from recycle cyclone underflow); 9. cooling cyclone 1; 10. cooling cyclone 2; 11. fluidized-bed final product cooler. The schematic of an advanced rotary kiln-based process with solids pre-heating is shown in Fig. 4. It includes: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
feed bin including feeding device; venturi pre-heating stage 1; cyclone of pre-heating stage 1; venturi pre-heating stage 2; cyclone pre-heating stage 2; electrostatic precipitator or filter; stack; mixing bin; rotary kiln furnace; rotary sectional drum cooler.
3.2. Optimal process conditions The objective for all investigated ores was to identify preferential conditions for the decrepitation process. For this purpose, test work was performed at different scales:
• laboratory scale: batch-type 50 mm fluidized bed facility, 100–150 g of feed material per run, • bench scale: continuously operated 200–360 mm circulating fluidized-bed reactors, 20–75 kg/h material input, • pilot scale: continuously operated 700 mm circulating fluidized-bed
There are many differences between the rotary kiln and a fluidized bed furnace, which make them suitable for different applications. Table 1 summarizes the characteristics and differences between the two furnace types. The lower consumption of energy of CFB-based processes as compared to rotary kiln is known from other industries. Fig. 5 illustrates the progressive reduction of the specific fuel energy requirement for alumina calcination plants, starting from traditional rotary kiln in the 1970 s and ending at the current benchmark of less than 2.7 GJ per ton alumina marked by an Outotec calciner. This low specific fuel consumption is achieved on a continuous basis (long term average). It should be noted that this was achieved in a 40-year-old plant by retrofitting and upgrading the plant to a more modern configuration including both a hydrate bypass and a hydrate dryer. Because of the process similarities, decrepitation of Lithium spodumene minerals can significantly benefit from the technological experience gathered in the field of alumina calcination with regards to process efficiency and product quality.
reactor, max. 500 kg/h material input.
Not all test work stages were performed for every material because of the increasing costs and increasing amount of test material required for larger-scale testing. Besides presenting results for optimal process conditions (temperature and retention time) for the seven ores, results are discussed in more detail on the example of Ore A. The relevance of results for commercial applications increases from lab- to pilot-scale. In general, at least benchscale testing is required by engineering companies before granting process guarantees. The overall conversion of α-spodumene (see Eq. (1)) and in particular conversion of α to β-spodumene (see Eq. (2)) can be defined as 4
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Table 1 Comparison of fluidized-bed system and rotary kiln. Rotary Kiln
Fluidized Bed
Feedstock
Low effort for feed preparation because of less stringent requirement to feed PSD
Temperature distribution
Inhomogeneous axial and radial temperature distribution (peaks of up to 1800 °C close to the flame formed by the burner) Predominantly radiant heat transfer over convective and conductive transfer Heterogeneous and potentially insufficient product quality resulting from uneven temperature and residence time distribution High fuel and electricity demand because of high maximum process temperature and different heat transfer regime (the electricity demand is linked with the larger size of the ID fan or air compressor)No use of sensible heat of solids if additional effort for a dust recovery system shall be avoided (cooling applying)
Higher effort for feed preparation caused by: ▪ Need for sufficient fluidity (PSD, moisture) ▪ Upper particle size limit of 10 mm Preferentially narrow PSD Uniform axial and radial temperature distribution inside reactor
Heat transfer Product quality Energetic efficiency
Maintenance
Mainly convective and conductive heat transfer Option for adjustment of specific temperature and residence time allowing for provision customer compliant products of constant quality Lower fuel and electricity consumption because of smaller heat requirement (lower average process temperature and smaller overall gas flow) Option for efficient recovery of sensible heat of solids and flue gas by: ▪ Pre-heating and drying of feedstock ▪ Pre-heating of air ▪ Provision of hot boiler feed water for downstream processes No rotating equipment in the furnace system Limited thermal and mechanical refractory stress enabling refractory lifetime of 10 years + x Small because of vertical arrangement and higher specific process intensity
Mechanically highly stressed rotating equipment in the furnace system Need for frequent repairs of refractory causing additional downtimes Refractory replacement at least every two years Large horizontal footprint because of large calcination kiln size and need for additional cooling kiln
Footprint
Table 3 Chemical composition of raw spodumene ore concentrates with regards to major elements. Element
Fe (total) CaO SiO2 MgO Al2O3 TiO2 Mn P K BaO Cu Ni Zn Pb Sn V Cr Ce Na Li
Fig. 5. Energy efficiency for alumina calcination at different installations (modified from (Perander et al., 2017)). Note that the theoretical energy consumption does include 6% surface moisture but does not include any heat losses and assumes all energy is used for the calcination reactions. Table 2 Characteristics of raw spodumene ore concentrates.
Raw moisture Loss on ignition LOI PSD:d80 PSD:d50 Bulk density
wt.% wt.% µm µm kg/l
Ore A
Ore B
Ore C
Ore D
Ore E
Ore F
Ore G
3–13 0.6 406 215 1.48
3.6 n. a. 130 90 n.a.
n. a. n. a. 104 62 n. a.
n. a. 0.5 147 99 1.43
5.1 0.6 129 66 1.29
4.9
n.a.
210 137 1.34
218 144 1.40
Composition in wt.% Ore A
Ore B
Ore C
Ore D
Ore E
Ore F
Ore G
0.82 0.60 65.60 0.16 23.60 0.13 0.07 0.16 0.48 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.40 2.90
0.59 0.13 65.30 0.05 26.10 < 0.05 < 0.05 0.12 0.41 < 0.05 0,06 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 0.19 3.40
1.20 0.79 62.80 0.42 24.70 0.07 0.14 0.10 0.61 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 0.52 2.50
0.50 0.85 63.70 < 0.05 26.30 0.09 0.11 0.06 0.36 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.40 3.40
0.70 0.48 66.40 0,14 24.40 0.09 < 0.05 0.10 0.66 < 0.05 0,06 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.40 4.50
0.58 0.28 63.2 0.06 26.70 0.06 0.23 < 0.03 0.54 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 0.32 6.30
0.16 0.30 64.2 < 0.05 25.90 < 0.05 0.25 < 0.03 0.47 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 < 0.05 0.55 6.44
Table 4 Mineral phase composition of raw spodumene ores determined by semi-quantitative XRD analysis. Mineral phase
n. a. = not available.
α-spodumene Quartz Plagioclase Microcline Muscovite Albite Schorlite Beryl Amphiboles
evaluation criteria. It is calculated from the mass fractions of the corresponding spodumene phases determined through XRD analysis. The suitability of the conversion factors was confirmed with leaching tests of the thermally-treated Ore B. Only β-spodumene (and at extended residence time also γ-spodumene) can be recovered from the decrepitated ore during hydrometallurgical processing after thermal treatment. The minimal target value for total conversion is set at 90% (for Ore B, 5
Composition in wt.% Ore A
Ore B
Ore C
Ore D
Ore E
Ore F
Ore G
64.0 7.1 4.9 9.0 8.9 – – – 6.1
Not available
77.8 3.9 9.6 – 5.4 – 3.3 – –
84.7 6.7 – – 2.7 5.9 – – –
64.0 7.1 4.9 9.0 8.9 – – – 6.1
81.0 2.2 4.6
74.0 5.9 10.6
11.5 – – 0.7 –
8.7 – – 0.8 –
Minerals Engineering 148 (2020) 106205
E. Gasafi and R. Pardemann
Table 5 Minimum fluidization velocity in m/s, measured with air at ambient temperature and pressure.
Raw material Calcine
Ore A
Ore B
Ore C
Ore D
Ore E
Ore F
Ore G
0.06* –
– –
– –
0.015 –
0.06 –
0.12 0.06
0.08 0.04
*Value obtained for material < 1 mm.
Fig. 7. Total and relative β-conversion determined for Ore A in laboratory-scale fluidized-bed reactor.
Ore A indicate a significant influence of the plant-scale at which the tests are performed. Moreover, phase change rates determined in muffle furnaces or rotary kilns can also be achieved in fluidized-bed reactors but at significantly shorter retention times. For Ore A, batch-type laboratory tests were performed using a smallscale externally-heated fluidized-bed reactor (50 mm inner reactor diameter) in which 100 g of material was treated at specific temperatures and retention times. 800 l/h of air was used as fluidization medium. Results for conversion of α to β-spodumene and overall αspodumene conversion are shown in Fig. 7. Results are provided as relative values. Values are referred to the conversion determined at lab scale (as shown in Fig. 6). Fig. 7 indicates an increase in total conversion of α-spodumene with increasing temperature. Only a moderate increase is observed at temperature above 1050 °C. Residence time is of significant importance at low temperature, but its relevance diminishes at above 1050 °C. Intermediate formation of γ-spodumene (starting at around 800 °C) and increasing formation of β-spodumene from γ-spodumene at elevated temperature was confirmed through XRD analysis. In contrast to total conversion, β-conversion significantly increases even at > 1050 °C because of conversion of γ to β-spodumene. The observations are in agreement with results of other researchers as reported in literature (Sitando and Crouse, 2012; Salakjani et al., 2016). Despite increased β-spodumene content at higher temperatures, there is a temperature limitation to be considered. Downstream hydrometallurgical treatment benefits from higher β-spodumene content. However decrepitation at temperatures beyond 1100 °C implies the risk of sintering and/or glass formation reactions of the feldspar mineral phases contained in the material. The sintering respectively melting behavior is strongly depending on the mineralogy and the impurities of the lithium ore. In general, an over-calcination needs to be avoided because leachability is reduced due to limiting access of the reagent to the β and γ-spodumene. Optical microscopy can be used visualize the phase changes in the calcination process Sintered phases can be identified as can be seen in the example of over calcined material (see bottom picture in Fig. 8 with sintered feldspar). The change in properties needs to be studies on laboratory scale and can then be used in the design of an industrial calcination system (Gasafi et al., 2018).
Fig. 6. β-conversion and total conversion for spodumene ore concentrates at different test scales.
leaching test results were back-calculated to β-conversion). In general, β-conversion increases with increasing temperature.
conversion = Totalconversion =
x (1)
x +x +x x +x
(2)
x +x + x
Maximum values determined for the different ores are shown in Fig. 6. The corresponding temperature and retention time in the reactor are summarized in Table 6. 4. Results Although most results are available from lab-scale testing, results for Table 6 Optimal temperature and retention time determined at varying test scale. Ore A Laboratory-scale testing Temperature °C 1050 Retention time min 30 Bench-scale testing Temperature °C 1060 Retention time min 30 Pilot-scale testing Temperature °C 1070/80 Retention time min 30–40
Ore B
Ore C
Ore D
Ore E
Ore F
Ore G
– –
1060 30
1060 30
1060 30
1050 20–30
1050 30
1070/80 35
– –
– –
1050 30*
1050 30*
– –
– –
– –
*Retention time not varied during test work. 6
Minerals Engineering 148 (2020) 106205
E. Gasafi and R. Pardemann
Fig. 9. Relative total conversion and relative β-conversion determined during pilot-scale testing (T = 1050–1060 °C, retention time: 30–40 min).
retention time of 30–40 min were selected as parameters for a test campaign at pilot scale (4 days, 24 h operation at a feeding rate of 300–400 kg/h). Fig. 9 shows the conversion rate for α- to β-spodumene and the total conversion rate. It indicates almost no variation during the operating period for bed product from the CFB furnace. Moreover, it proves high conversion values for the discharged fines. Data points shown in Fig. 9 are relative values referring to the conversion value determined for the CFB discharge. Because maximizing the recovery of lithium is of high importance to overall process economics, commercial application would target for reclamation and application of fine material to hydrometallurgical processing, too. Hence, conversion rates were determined for the CFB bed product and fines recovered from gas cooling path of the pilot test facility including the filter and a downstream cyclone and heat exchanger. Results are presented in Fig. 9 and indicate high β-and total conversion rates for all selected fractions. Consequently, the discharged fines fractions also fulfill the conversion criterion of 90%. Besides proofing process performance at pilot scale, the purpose was to determine important figures for plant scale-up such as tendency for sticking and sintering and gaseous emissions to be expected from the process (which are not discussed in more detail in this article).
Fig. 8. Results of optical microscopy investigation with glassy feldspar regions marked with dotted line.
Hence, the selection of the optimal process temperature strongly depends on the mineral feedstock (in particular its tendency for sintering). It requires careful consideration of maximum conversion values at high temperature vs. deteriorating effects on the actually recoverable amount of lithium, which commonly decreases with increasing temperature. Considering previous experience and results obtained from laboratory-scale test work, a temperature of 1050 °C to 1060 °C and a
5. Conclusion Today, 55% of lithium production is based on brines while 45% is based on hard rock, with an increasing trend towards lithium ores. Due to the growing demand for lithium, the number of projects and the plant capacity is increasing. The conventional process for calcination is 7
Minerals Engineering 148 (2020) 106205
E. Gasafi and R. Pardemann
still based on rotary kiln, a technology in use since the early days and not significantly further developed since. Fluidized-bed calcination was introduced as an alternative technology in this article. A general advantage of fluidized-bed over rotary kiln calcination is the establishment of a homogenous temperature profile in the reactor whereas rotary kiln is characterized by a temperature gradient along the reactor length with the highest temperature in the flame zone of the burner. Moreover, residence time in a CFB reactor can be accurately controlled. Based on experience of existing industrial fluidized-bed calcination plants, thermal processing of lithium ore was investigated applying test facilities ranging from small lab scale to pilot scale. The objective for bench- and pilot-scale testing was to provide both engineering and design data. Optimal process conditions were determined through the tests. A retention time in the range of 25–40 min was identified as sufficient for extensive conversion of α- to β- and γ-spodumene. Depending on the spodumene concentrate, the β-spodumene content varies. A temperate between 1050 and 1070 °C was sufficient to achieve ≥90% total conversion of α-spodumene. Lower temperature may result in lower overall conversion and lower β-spodumene content dependent on the raw material. The temperature effect can be partially compensated by prolonged retention time. An upper temperature limitation is given by the material softening or sintering causing sticking. A common observation was an improved conversion with larger reactor scale. Typical total conversion was in the range of 92–95 % with maximum β-conversion of 67–84%. Furthermore, the raw concentrates and calcined products were granulometrically characterized. All calcined materials show a significant reduction in bulk density from about 1.3–1.5 kg/l for the raw material to 0.6–0.8 kg/l for the calcine. Sticking tendency is strongly impacted by mineralogy and grain size of the feed material. Only one concentrate was excluded from further testing after performing lab scale tests. Hence, fluidized bed proved to be a technically feasible process for the vast majority of investigated concentrates and for spodumene ore in general. Because of process related advantages such as significantly reduced energy consumption and high product quality compared to the tradition rotary kiln process, fluidized-bed is an attractive process to be considered for future decrepitation plants.
Declaration of Competing Interest We declare that we have no conflict of interest. References Abdullah, A.A., Oskierski, H.C., Altarawneh, M., Senanayake, G., Lumpkin, G., Dlugogo, B.Z., 2019. Phase transformation mechanism of spodumene during its calcination. Miner. Eng. Anonymous, 2010. Critical Materials Strategy. US Department of Energy, Washington. Anonymus, 2016. Lithium. In Mineral commodity summaries 2016. United States Geological Survey, Reston. Boldyrev, N. P., T., K., D., M., V., R., 2010. Mechanical activation of α-spodumene for further processing into lithium compounds. Theoret. Found. Chem. Eng., vol. 503–507. Choubey, P.K., Kim, M.-S., Srivastava, R.R., Lee, J.-C., Lee, J.-Y., 2016. Advance review on the exploitation of the prominent energy-storage element: Lithium. Part I: From mineral and brine resources. Miner. Eng. 119–137. Deutsche Bank, 2016. FITT report Welcome to the Lithium-ion Age. Deutsche Bank, Frankfurt. Martin, G., Schneider, A., Voigt, W., Bertau, M., 2017. Lithium extraction from the mineral zinnwaldite: Part II: Lithium carbonate recovery by direct carbonation of sintered zinnwaldite concentrate. Miner. Eng. 75–81. Garrett, D.E., 2004. Handbook of lithium and natural calcium chloride: their deposits, processing, uses and properties. Amsterdam [u.a.]. Elsevier Acad. Press. Gasafi, E., Perander, L., Beisheim, T., 2018. Calcining in Circulating Fluidized Bed Systems. In: 23rd International Conference on Fluidized Bed Conversion. Seoul: The Korean Institute of Chemical Engineers (KIChE). Julien, C., Mauger, A., Vijh, A., Zaghib, K., 2016. Lithium Batteries - Science and Technology. Cham [u.a.]. Springer International Publishing. Kesler, S.E., Gruber, P.W., Medina, P.A., Keoleian, G.A., Everson, M.P., Wallington, T.J., 2012. Global lithium resources: relative importance of pegmatite, brine and other deposits. Ore Geol. Rev. 48, 55–69. Nogueira, C., Margarido, F., Vieceli, N., Durão, F., Guimarães, C., 2014. Comparison of Processes for Lithium Recovery from. Lepidolite by H2SO4 Digestion or HCl Leaching. In: Proceedings of the International Conference on Mining, Material and Metallurgical Engineering, (p. Paper No. 72). Prague. Peltosaari, O., Tanskanen, P., Heikkinen, E.-P., Fabritius, T., 2015. α → γ → β-phase transformation of spodumene with hybrid microwave and conventional furnaces. Miner. Eng. 54–60. Perander, L., Scarsella, A., Gasafi, E., Schmidt, H.-W. (2017). CFB Alumina Calciners / New and Future Generation. TMS 2017 146th Annual Meeting and Exhibition. TMS. Salakjani, N.K., Singh, P., Nikoloski, A.N., 2016. Mineralogical transformations of spodumene concentrate from Greenbushes, Western Australia. Part 1: Conventional heating”. Miner. Eng. 98, 71–79. Sitando, O., Crouse, P., 2012. Processing of a Zimbabwean petalite to obtain lithium carbonate. Int. J. Miner. Process. 45–50. Wietelmann, U., Steinbild, M., 2013. Lithium and Lithium Compounds. In Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH Verlag, Weinheim. Yan, Q., Li, X., Wang, Z., Wu, X., Wang, J., Guo, H., Peng, W., 2012. Extraction of lithium from lepidolite by sulfation roasting and water leaching. Int. J. Miner. Process. 110–111, 1–5. Ziemann, S., Weil, M., Schebek, L., 2012. Tracing the fate of lithium––The development of a material flow model. Resour., Conserv. Recycl., pp. 26-34.
CRediT authorship contribution statement Edgar Gasafi: Conceptualization, Methodology, Investigation. Robert Pardemann: Investigation, Validation, Writing - review & editing.
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