The selective chlorination of nickel and copper from low-grade nickel-copper sulfide-oxide ore: Mechanism and kinetics

Journal Pre-proofs The selective chlorination of nickel and copper from low-grade nickel-copper sulfide-oxide ore: mechanism and kinetics Fuhui Cui, Wenning Mu, Yuchun Zhai, Xueyi Guo PII: DOI: Reference:

S1383-5866(19)34941-X https://doi.org/10.1016/j.seppur.2020.116577 SEPPUR 116577

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Separation and Purification Technology

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28 October 2019 23 December 2019 14 January 2020

Please cite this article as: F. Cui, W. Mu, Y. Zhai, X. Guo, The selective chlorination of nickel and copper from low-grade nickel-copper sulfide-oxide ore: mechanism and kinetics, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur.2020.116577

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The selective chlorination of nickel and copper from low-grade nickel-copper sulfide-oxide ore: mechanism and kinetics Fuhui Cui1,2,*, Wenning Mu3, Yuchun Zhai3, Xueyi Guo1,2 (1.-School of Metallurgy & Environment, Central South University, Changsha, 410083, China; 2.-Cleaner Metallurgical Engineering Research Center, Nonferrous Metal Industry of China, Changsha, 410083, China; 3.-School of Resources and Materials, Northeastern University at Qinhuangdao, Qinhuangdao 066004, China;)

ABSTRACT On account of the complex occurrence of valuable metals in a large amount of low-grade nickel sulfide ore, obstacles have been caused by using the existing metallurgical method. Therefore, we are reporting a new selective chlorination roasting and water leaching process to treat complex nickel sulfide ore. Anhydrous aluminum chloride was firstly used as the solid chlorination agent to sulfide ore. The chlorination mechanism for metals in nickel-copper sulfide ore were determined by thermodynamic calculation and experiment. The thermodynamic analysis showed that the predominant matter contributing to the chlorination of talc, lizardite, and magnetite were AlCl3 (both solid or gas) and the generated HCl, however, the chlorination of metal sulfide ( pentlandite, chalcopyrite, and pyrite ) were mostly contributed by the generated Cl2. The effect of roasting temperature, the roasting time, the mass ratio of AlCl3 to the ore, the content of O2 in the roasting atmosphere and the particle size of ore were investigated. The orthogonal experiments results showed that the optimal conditions of the chlorination roasting process were the roasting time of 3 h, the roasting temperature of 450°C, the mass ratio of AlCl3 to ore of 1.5:1, the content of O2 of 20%, and particle size of the ore of 96-80 μm, under this condition, an extraction rate of nickel, copper, iron, and magnesium of 91.6%, 88.5%, 28.4%, and 16.4% was obtained. The chlorination mechanism is that, in the chlorination roasting process, nickel and copper were transferred into their corresponding metal chlorides, while most of the iron and magnesium were transferred into ferric oxide and magnesia-alumina silicates, respectively. TAK (thermal analytic kinetics) were used to clarify the chlorination kinetics, the results showed that the apparent activation energy calculated from DSC data was 70.4 kJ·mol-1, while calculated from TG data was 86.5 kJ·mol-1 (average) by using the FWO method,

one of the most probable mechanism function of the chlorination process was

G ( )  0.8701[1  (1   )0.8701 ] (integral form) and

f ( )=(1- )1.8701 (differential form). The chlorination selectivity for

metals could give guidance to the extraction of metals (especially for valuable metals) either in nickel sulfide ore or other minerals with a complex metal occurrence. Keywords: low-grade nickel-copper ore; chlorination; extraction; thermodynamic analysis; kinetics; anhydrous aluminum *

- corresponding author E-mail: [email protected]; [email protected]; (Fuhui Cui) 1

chloride

1. Introduction Since isolated nickel was found in 1751, it has been established as an indispensable metal both in civil and military. According to the survey data of USGS in 2019, the identified land-based resources of nickel greater than 1% (including 1%) contain at least 130 million tons of nickel, with approximately 60% in laterites and 40% in sulfide deposits, and with total reserves of 89 million tons of nickel[1]. However, the continuous mining and utilization of nickel sulfide ore for decades have led to the gradual depletion of rich ore with a single or simple existing form for valuable metals. Major existing sulfide ore has a low content of nickel with a complex occurrence and high content of gangue, which causes many obstacles to using the current pyrometallurgical process[2]. There has nearly 2/3 of the aggregate low-grade nickel sulfide with a high content of MgO and silicate in Jinchuan Group LTD, the third-largest nickel deposit worldwide and the largest in China. The difficulties developed in froth flotation into a sulfide concentrate, smelting and converting in conventionally metallurgical practice results from the elevated content MgO and dense occurrence of nickel. Flotation which yields a relatively high-grade concentrate is a mature technology that has been practiced successfully for more than one century[3]. Variations in the recovery of valuable metals are tantamount to a large extent by changes in mineralogical and textural[4-7]. In the low-grade nickel sulfide ore, serpentine and talc, the dominating MgO-bearing gangue in the ore, often interfere with the concentration of nickel sulfide ore[8]. In the flotation process, serpentine minerals may present as “slime coatings” via composite particles or through attachment to the valuable minerals[8-10], while talc being naturally hydrophobic easily reports to flotation concentrate[11-13], thus concentrate dilution by naturally floating silicates occurs in the flotation process of low-grade nickel sulfide ore with high content MgO-bearing. In the conventional pyrometallurgical process (flash or electric smelting), the concentrate with high content of MgO will cause the problems of furnace nodulation, large viscosity to slag, difficulties in the separation between matte and slag, low recovery of valuable metals, and etc.[14-17]. Furthermore, there commonly produces tons of H2SO4 to mitigate the large quantities of SO2 emission in the smelting of concentrate, while Jinchuan Group Ltd. is located in the deep Gobi of northwest China, while the expenditure on the transportation of H2SO4 exceeds the obtaining sales resulting from the location of Jinchuan Group Ltd., the deep Gobi of northwest China, which induces a “new burden” for H2SO4 because of the huge backlog. In addition, high temperature (~1350°C) requirement for smelting of concentrate calls for high energy consumption and strict equipment requirement. Hydrometallurgical routes for treating nickel sulfide ore have been developed for many years, which are mainly focused on atmospheric acid leaching[18-22], high-pressure acid leaching[23-25], ammonia leaching[2628], heap leaching (bio-leaching)[29-31], bioleaching[32, 33], and etc. However, atmospheric acid leaching often performs 2

as the low recovery of valuable metals, the high burden in following separation process, and the huge wastewater generation[34]; High-pressure acid leaching is generally restricted by its high-quality equipment requirement and incompetence in large-scale and continuous production[34]; Heap leaching and bioleaching are immature future hydrometallurgical methods, restrained by long reaction time and low recovery of valuable metals[32]. Pyrohydrometallurgy is potential route to process nickel sulfide ore, in general, it contains two procedures- roasting and leaching, the roasting process is a reconstruction of targeted refractory mineral phases to easy-leaching phases, such as sulfates and chloride, the advantages in which result from the low roasting temperature (compared with pyrometallurgy) and easy leaching (compared to hydrometallurgy)[35-42]. Therefore, the present work practices a low-temperature chlorinating roasting-water leaching process to achieve the extraction of valuable metals selectively. The thermodynamic calculation was used to clarify the predominant matter in the chlorination of metals. Single-factor and orthogonal experiments were adopted to find the optimal roasting conditions. Another concerning aspect is to reveal the mechanism and kinetics of chlorination of the ore in the roasting process. It could give a reference to the chlorination of metals from both natural and second resources with a low-temperature system.

2. Materials and Methods 2.1 Materials and agents The low-grade nickel-copper sulfide-oxide ore (for short as “the ore” in the following section) adopted in this study was supplied by Jinchuan Group Co., Ltd, China. The rock ore samples were crushed by a jaw crusher and following a doublerolling grinder to pass-through a vibrating screen with different standard sieves, then the ore with different particle sizes was obtained. The characterization of the raw ore was implemented in our previous study[40] ( the details are given in Fig. S1). The content of Ni and Cu were analyzed by ICP-OES (PS-6, Baird Corp, USA), the content of Fe, Mg, Si, and other elements were analyzed by XRF (ARL Optim’x, USA). Table 1 and Fig. S1 gives the chemical composition and XRD patterns of the raw ore, respectively. As seen, the content of Ni and Cu is 4.6 wt % and 2.8 wt %, while the content of MgO is 10.1 wt % in the raw ore. The major mineral phases of the raw ore are talc [Mg3(OH)2Si4O10], lizardite [Mg3Si2O5(OH)4], chalcopyrite (CuFeS2), pyrite (FeS2), magnetite (Fe3O4), and pentlandite (Fe4.5Ni4.5S8). The chemical analysis of the ore indicates that nickel in raw ore exists as four shares, nickel sulfate (2.2 wt %), nickel oxide (32.8 wt %), nickel sulfide (38.2 wt %) and nickel silicates (26.8 wt %) which is the refractory part, while copper in raw ore exists as free oxidized copper (13.2 wt %), combined copper oxide (3.2 wt %), secondary copper sulfide (9.4 wt %) and primary copper sulfide (74.2 wt %) (see in Table S1). The SEM image and EDS 3

mapping of the virgin nickel-copper ore are shown in Fig. S2. Table 1 Chemical composition of the raw ore from Jinchuan Elements

Ni

S

Cu

Al2O3

MgO

Fe

SiO2

CaO

else

(wt %)

4.63

21.5

2.75

0.9

10.13

32.23

14.00

1.22

12.64

Anhydrous aluminum chloride (AlCl3, AR) was used as the chlorination agent in the roasting process. High purity nitrogen gas (N2, >99.999%) and oxygen gas (O2, >99.9%) were used to make a mixed gas with different oxygen content in the roasting atmosphere.

2.2 Procedure and methods The experiment includes two parts, roasting process following by a water leaching process. The detailed is described as follows. 2.2.1 Roasting The rock was crushed by a jaw crusher ( 10060A, Wolin Tech, China ), then the obtained granules were milled in a planetary ball-milling (BS2308, TENCAN POWDER, China) to produce particles with different size. The fine ore particles were dried at 80°C for 24 h. First 20 g of dried ore with certain particle size and amount of AlCl3 were mixed completely in an agate mortar and then placed in a corundum crucible, there all happened in a glovebox to avoid the water absorption of AlCl3 in air atmosphere. Next, the crucible was placed in a tube furnace (DB2, Tianjin Zhonghuan Furnace Corp., China, ~1200°C) with an inert gas and absorption device, and the roasting experiment was conducted at the required temperature for a needed period of time with different oxygen content mixture gas (total rate of 100 ml/min). 2.2.2 Water Leaching The cooled roasted product was leached in hot water with a liquid-solid ratio of 4:1 at 70°C for 2 h. After leaching, the leachate was separated by vacuum filtration. The extraction rate of the metals can be calculated by Eq. (1).

M 

100cM V  100% m0 M

where  M denotes the extraction rate of M, cM is the measured concentration of M in the filtrate, V

(1) is the volume of

filtrate, m0 is the mass of raw ore,  M is the content of M in the raw ore. The concentrations of nickel and copper were determined by UV-VIS method (ultraviolet-visible spectrophotometry ) using a double-beam ultraviolet spectrophotometer (TU-19, Beijing Puxi, China). The concentration of iron was determined by potassium dichromate titration method. the concentration of magnesium was determined by the atomic absorption (AAS) method using an atomic absorption spectrophotometer (AA7000, Shimadzu, Japan). 4

3. Results and discussion 3.1 Thermodynamics estimation The chemistry of AlCl3 chlorination roasting is very intricate because several processes such as oxidation, decomposition, in situ chlorination and evaporation of volatile species occur either simultaneously or sequentially. In order to clarify the predominating matter of AlCl3 in the chlorination reactions, the thermodynamic calculation was used to find the possible reactions contributing to the chlorination reactions. In this paper, the chlorination of ore by anhydrous AlCl3 has three potential possibilities: 1) the direct chlorination of ore by AlCl3, 2) the generated HCl if in the presence of H2O, 3) the generated Cl2 if in the presence of O2. Additionally, the solid AlCl3 has a low melting and boiling point, and strong volatility, so three forms may coexist in the roasting atmosphere. Fig. S5 presents the equilibrium composition of AlCl3 from 0-600°C by using HSC Chemistry software; as seen, ACl3, ACl3(g) and Al2Cl6(g) co-exist ranging the temperature from 100-600°C, it should be pointed out that solid-phase of AlCl3 plays dominating role at low temperature (lower than 200°C), while gas-phases (ACl3(g) and Al2Cl6 (g)) play dominating role at high temperature. Therefore, we calculated the Gibbs free energy change and the logarithm of the equilibrium constant ( Fig. S2) at 1 atm upon elevating the temperature for reactions occurring in the chlorination roasting process between the mineral phases and AlCl3 (three forms), HCl and Cl2. In the calculation process, pentlandite (Ni4.5Fe4.5S8) has the thermodynamic data at room temperature only [43], then we adopted the thermodynamic estimate function to achieve a series of approximate data to carry out the calculation. Furthermore, the reactions between pentlandite (Ni4.5Fe4.5S8) and AlCl3 and HCl cannot be balanced properly, so we use Ni4.5Fe4.5S9 to replace Ni4.5Fe4.5S8, but the thermodynamic data used in the calculation is the approximate data of Ni4.5Fe4.5S8. Hence, the calculations for the reactions being relevant to pentlandite (Ni4.5Fe4.5S8) are approximate values just for reference only. Depending on the calculation results, each potential chlorination possibility (AlCl3, HCl, and Cl2) was assessed to identify the dominating role in the chlorination reactions. The possible reactions between the ore and AlCl3 (three forms) ranging the temperature from 100-600°C are shown in Eq. (2)-(19), similarly, reactions with HCl and Cl2 are given in Eq. (20)-(28), and Eq. (29)-(42), the calculated results are presented in Fig. 1.

CuFeS2  AlCl3  CuCl  FeCl2  0.5Al2S3  0.5S

(2)

CuFeS2  AlCl3 (g)  CuCl  FeCl2  0.5Al2S3  0.5S

(3)

CuFeS2  0.5Al2 Cl6 (g)  CuCl  FeCl2  0.5Al2S3  0.5S

(4)

FeS2 + 2/3AlCl3 = FeCl2 + 1/3Al2S3 + S

(5)

FeS2 + 2/3AlCl3 (g)= FeCl2 + 1/3Al2S3 + S

(6)

FeS2 + 1/3Al2 Cl6 (g)= FeCl2 + 1/3Al2S3 + S

(7)

Fe 4.5 Ni 4.5S9 + 6AlCl3 = 4.5NiCl2 + 4.5FeCl2 + 3Al2S3

(8)

5

Fe 4.5 Ni 4.5S9 + 6AlCl3 (g) = 4.5NiCl2 + 4.5FeCl2 + 3Al2S3

(9)

Fe 4.5 Ni 4.5S9 + 3Al2 Cl6 (g) = 4.5NiCl2 + 4.5FeCl2 + 3Al2S3

(10)

Mg 3Si 4 O10 (OH) 2 + 2AlCl3 = 3MgCl2 + 4SiO 2 + Al2 O3 + H 2 O(g)

(11)

Mg 3Si 4 O10 (OH) 2 + 2AlCl3 (g) = 3MgCl2 + 4SiO 2 + Al2 O3 + H 2 O(g)

(12)

Mg 3Si 4 O10 (OH) 2 + Al2 Cl6 (g) = 3MgCl2 + 4SiO 2 + Al2 O3 + H 2 O(g)

(13)

Mg 3Si 2 O5 (OH) 4 + 2AlCl3 = 3MgCl2 + Al2 O3 + 2SiO 2 + 2H 2 O(g)

(14)

Mg 3Si 2 O5 (OH) 4 + 2AlCl3 (g) = 3MgCl2 + Al2 O3 + 2SiO 2 + 2H 2 O(g)

(15)

Mg 3Si 2 O5 (OH) 4 + Al2 Cl6 (g) = 3MgCl2 + Al2 O3 + 2SiO 2 + 2H 2 O(g)

(16)

Fe3 O 4 + 8/3AlCl3 = 4/3Al2 O3 + 2FeCl3 + FeCl2

(17)

Fe3 O 4 + 8/3AlCl3 (g)= 4/3Al2 O3 + 2FeCl3 + FeCl2

(18)

Fe3 O 4 + 4/3Al2 Cl6 (g)= 4/3Al2 O3 + 2FeCl3 + FeCl2

(19)

Fig. 1 Variation in Gibbs energy change with an increase in temperature for chlorination reactions : a-AlCl3, Eq. (2)-(19); b-HCl(g), Eq. (20)-(28); c-Cl2, Eq. (29)-(43)

As seen in Fig. 1, the chlorination reactions between AlCl3 (three forms) and chalcopyrite (CuFeS2), pyrite (FeS2), and pentlandite (Ni4.5Fe4.5S8) have positive standard Gibbs free energy change, manifesting the reactions occur nonspontaneously. However, the reactions between AlCl3 (three forms) and talc [Mg3Si4O10(OH)2], lizardite [Mg3Si2O5(OH)4] and magnetite [Fe3O4] have negative standard Gibbs free energy change that decreases with the increasing of temperature, 6

indicating that these reactions can occur spontaneously.

AlCl3 + 3H 2 O(g) = Al(OH)3 + 3HCl(g)

(20)

AlCl3 (g) + 3H 2 O(g) = Al(OH)3 + 3HCl(g)

(21)

Al2 Cl6 (g) + 6H 2 O(g) =2Al(OH)3 + 6HCl(g)

(22)

CuFeS2 + 4HCl(g) = CuCl2 + FeCl2 + 2H 2S(g)

(23)

FeS2 + 2HCl(g) = FeCl2 + H 2S(g) + S

(24)

Fe 4.5 Ni 4.5S9 + 18HCl(g) = 4.5FeCl2 + 4.5NiCl2 + 9H 2S(g)

(25)

Mg 3Si 4 O10 (OH) 2 + 6HCl(g) = 3MgCl2 + 4SiO 2 + 4H 2 O(g)

(26)

Mg 3Si 2 O5 (OH) 4 + 6HCl(g) = 3MgCl2 + 2SiO 2 + 5H 2 O(g)

(27)

Fe3 O 4 + 8HCl(g) = FeCl2 + 2FeCl3 + 4H 2 O(g)

(28)

As shown in Fig. 1b, reactions of Eq. (20)-(22) has a negative standard Gibbs free energy change (  G  ) that decrease with the increasing of temperature, which means AlCl3 can generate HCl easily with the existence of H2O, and the gaseous states of AlCl3 [AlCl3(g) and Al2Cl6(g)] perform excellent qualification to generate HCl than the condensed state. The reactions between HCl and pentlandite (Ni4.5Fe4.5S8), talc [Mg3Si4O10(OH)2], lizardite [Mg3Si2O5(OH)4] and magnetite [Fe3O4] have negative standard Gibbs free energy change at low temperature but positive at high temperature, suggesting that these chlorination reactions occur spontaneously at low temperature. The chlorination reactions of chalcopyrite (CuFeS2) and pyrite (FeS2) in HCl atmosphere have positive standard Gibbs free energy change, namely that these reactions cannot occur spontaneously.

AlCl3 + 3/4O 2 (g) = 1/2Al2 O3 + 3/2Cl2 (g)

(29)

AlCl3 (g) + 3/4O 2 (g) = 1/2Al2 O3 + 3/2Cl2 (g)

(30)

Al2 Cl6 (g) + 3/2O 2 (g) = Al2 O3 + 3Cl2 (g)

(31)

CuFeS2 + 2Cl2 (g) = CuCl2 + FeCl2 + 2S

(32)

FeS2 + Cl2 (g) = FeCl2 + 2S

(33)

Fe 4.5 Ni 4.5S8 + 9Cl2 (g) = 4.5FeCl2 + 4.5NiCl2 + 8S

(34)

Mg 3Si 4 O10 (OH) 2 + 3Cl2 (g) = 3MgCl2 + 4SiO 2 + H 2 O(g) + 1.5O 2 (g)

(35)

Mg 3Si 2 O5 (OH) 4 + 3Cl2 (g) = 3MgCl2 + 2SiO 2 + 2H 2 O(g) + 1.5O 2 (g)

(36)

Fe3 O 4 + 3.75Cl2 (g) = 1.5FeCl2 + 1.5FeCl3 + 2O 2 (g)

(37)

FeCl3 + 3/4O 2 (g) = 1/2Fe 2 O3 + 3/2Cl2 (g)

(38)

FeCl3 (g) + 3/4O 2 (g) = 1/2Fe 2 O3 + 3/2Cl2 (g)

(39)

FeCl3 = FeCl2 + 1/2Cl2 (g)

(40)

FeCl3 (g) = FeCl2 + 1/2Cl2 (g)

(41)

FeCl2 + 1/2Cl2 (g) = FeCl3

(42)

Fe 2 Cl6 + 3/2O 2 (g) = Fe 2 O3  3Cl2 (g)

(43)

It can be seen from Fig. 1c, the generation of Cl2 from AlCl3 occurs effortlessly in the presence of O2 (Eq. (29)-(31)). The chlorination reactions of chalcopyrite (CuFeS2), pyrite (FeS2), and pentlandite (Ni4.5Fe4.5S8) can be spontaneously completed in free Cl2 atmosphere. However, the chlorination reactions of talc [Mg3Si4O10(OH)2], lizardite [Mg3Si2O5(OH)4] 7

and magnetite [Fe3O4] cannot occur spontaneously at 1 atm. Compared to the HCl atmosphere, the chlorination reactions of chalcopyrite (CuFeS2) and pentlandite (Ni4.5Fe4.5S8) have more negative standard Gibbs free energy change (  G  ), demonstrating that the chlorination ability of Cl2 against metal sulfides is stronger than HCl. Additionally, the produced FeCl2 and FeCl3 from the chlorination of iron-bearing mineral phases participate in the reactions which are shown in Eq. (38)-(43). The condensed and gaseous FeCl3 can be easily oxidized into Fe2O3 by O2 accompanying by the generation of Cl2 (Eq. (38),(39), and (43)). Furthermore, the gaseous FeCl3(g) can be decomposed into FeCl2 and Cl2(g) spontaneously under a temperature below 500°C. Fig. S9 gives the equilibrium composition of FeCl3 system from 25 to 600°C; as seen, the major gaseous ferric chloride is Fe2Cl6 but a little amount of FeCl3(g) exists at high temperature; and the amount of Cl2(g) obtained from the decomposition of ferric chloride firstly increase at low temperature but decrease slightly the increasing temperature In the chlorination process, FeCl3, FeCl3(g), and Fe2Cl6(g) participate in the chlorination of nickel and copper through generating Cl2 at a suitable oxidizing atmosphere, it continues until the exhausting of FeCl3 to deliver a final product of Fe2O3. Hence, iron can significantly facilitate the chlorination of nickel and copper by the hand of the oxidizing of FeCl3. According to the above thermodynamic analysis to the potential dominating chlorination agent of AlCl3, HCl, and Cl2, the suitable roasting atmosphere should possess a proportionate of O2 but less free water (overmuch free water will promote the generation of redundant HCl which will impede the generation of Cl2). Based on the above analysis, the roasting temperature, the mass ratio of AlCl3 to the ore, the roasting time, the particle size of the ore and the roasting atmosphere were investigated in the following experiments.

3.2 Roasting process 3.2.1 Effect of roasting temperature In order to determine the volatilization behavior of anhydrous AlCl3, thermal analysis for anhydrous AlCl3 at constant temperatures in 10% O2+90% N2 atmosphere (total flow rate, 100 ml/min), the results are shown in Fig. 2.

8

Fig. 2 DSC-TG curves of anhydrous AlCl3 at different conditions: a-DSC; b-TG; c-DTG. (10% O2+90% N2 atmosphere, total flow rate 100 ml/min; the tests start from room temperature to target temperatures (250°C/ 350°C/ 450°C) with a heating rate of 10 °C/min; then fix the target temperature for 2 h)

As seen in Fig. 2, the violent endothermic peaks on DSC curves (Fig. 2a) at different temperatures correspond to the volatilization of AlCl3, the major mass loss on TG curves (Fig. 2b) concentrates at the heating temperature below than 200°C, which the rapid volatilization of AlCl3 happens below 200°C. However, the volatilization of AlCl3 cannot proceed completely due to the oxidization of AlCl3 to Al2O3 in an oxidizing atmosphere. The different weight of residue for the different heating temperatures is attributed to the oxidization amount of AlCl3, the condition of 350°C for 2 h obtains the biggest amount of residue (Al2O3), while the condition of 250°C for 2 h obtains the smallest amount of residue, accordingly, temperature plays a vital role in the oxidation extent of AlCl3 to Al2O3. As known from the above thermodynamic analysis, the oxidization of AlCl3 to generate Cl2 is beneficial for the chlorination of metal sulfides, the roasting temperature is an important parameter for the chlorination of metals. The preliminarily groping experiments indicated that the roasting products at the low roasting temperature (<200°C) were difficult to filtration because of the generation of tiny particles, even though taking about 24 h to complete the filtration by vacuum filter, the filtrate was still turbid and the tested concentration of nickel and copper are low, so the roasting temperatures were set ranging from 250-500°C under the conditions that the mass ratio of AlCl3 to ore was 2:1, the roasting time was 2 h, the particle size of the ore was 96~80 μm, the proportion of O2 in the introducing gas was 10% (90% N2), the results are displayed in Fig. 3.

9

Fig. 3 Effect of roasting temperature on the extraction rates of nickel, copper, iron and magnesium

Fig. 4 XRD patterns under different roasting temperature (under the conditions that the mass ratio of AlCl3 to ore was 1:1, the roasting time was 2 h, the particle size of the ore was 96~80 μm, the proportion of O2 in the introducing gas was 10% (90%N2)

As shown in Fig. 3, the extraction rate of copper and magnesium increases with the elevated roasting temperature, reaching their maximum of 80.2% and 40.5% at 500°C, respectively. While the extraction rate of nickel increases rapidly from 200°C and yields a maximum of 74.3% at 400°C, then decreases with the enhanced roasting temperature, earning an extraction rate of 57.5% at 500°C. The extraction rate of iron decreases quickly with the elevated roasting temperature, decreasing from 60.8% at 200°C to 21.1% at 500°C. Fig. 4 shows the XRD patterns of the samples under different roasting temperatures. The newly generated phases in the sample under 200°C are iron (II) chloride hydrate [FeCl2(H2O)2] and iolite (Mg2Al4Si5O18), indicating that part of the ironbearing mineral and magnesium silicate have been converted into iron chlorides and iolite at this temperature, respectively. The diffraction peaks of ferric chloride (and its hydrate) and ferric oxide appear in the sample at the roasting temperature of 250°C, manifesting the intensification of the chlorination reactions and part of ferrous chloride having been transferred 10

into ferric oxide, supporting the decreasing of the extraction rate of iron in the experimental results. The main phases in the roasting product at 350°C are iron sulfides and ferric oxide, the disappearance of the diffraction peaks of ferric chloride disappear indicates that ferric chloride has been oxidized into ferric oxide with the enhanced roasting temperature in comparison with 300°C. The diffraction peaks of NiS2 presenting in the sample under 400°C denotes that iron has been stripped from pentlandite (Ni4.5Fe4.5S8) to participate in the chlorination reactions. Moreover, nickel in sulfide-type are mainly hosted in pentlandite (Ni4.5Fe4.5S8), namely iron is the first to be chlorinated in pentlandite, our previous study also proved this[39]. Associating with the above thermodynamic analysis, as the scanty of free water in the early chlorination roasting process, anhydrous AlCl3 firstly erodes magnetite and magnesium silicates to produce iron chlorides, free water, and magnesia-alumina silicates, manifesting the low restrictions against temperature on the chlorination of iron, thereby iron has the highest extraction rate at 200°C in the experimental results. Moreover, the generated water reacts with AlCl3 to form HCl that takes part in the chlorination reactions. At a higher roasting temperature, as the increasing reaction rate with the increasing temperature, there will develop more oxidizable iron oxides at high temperatures than at low temperatures, corresponding to the increasing amount of generated Cl2. Hence, the extraction of nickel and copper increases with the increasing roasting temperatures. However, when the roasting temperature is too high, the volatilization rate of AlCl3 will surpass the chlorination rate resulting from the insufficient time left to chlorination reactions. This is why the extraction rate of nickel decrease when the roasting temperature is higher than 400°C. Therefore, a roasting temperature of 400°C was chosen as the following experiments. 3.2.2 Effect of the mass ratio of AlCl3 to ore

Fig. 5 Effect of the mass ratio of AlCl3 to ore on the extraction rates of nickel, copper, iron, and magnesium

11

Fig. 6 XRD patterns of the samples under the different mass ratio of AlCl3 to ore (under the conditions that the roasting temperature was 400°C, the roasting time was 2 h, the particle size of the ore was 96~80 μm, and the proportion of O2 in the introducing gas was 10% (90%N2)

Fig. 5 shows the effect of mass ratio of ACl3 to ore ranging from 0.2:1 to 2:1 under the conditions that the roasting temperature was 400°C, the roasting time was 2 h, the particle size of the ore was 96~80 μm, and the proportion of O2 in the introducing gas was 10% (90%N2). As shown in Fig. 5, the extraction rate of nickel and magnesium has a tendency of firstly increases then obtains the maximum of 85.8% at 1:1 and 36.7% at a mass ratio of 1.25:1, respectively, but then decreases with the increased dosage of AlCl3. The extraction rate of copper increases with the increased dosage of AlCl3 and reaches at the maximum at a mass ratio of 2:1 of 77.3%, while the extraction rate of iron decreases with the increased dosage of AlCl3, decreases by 26.6% from the mass ratio of 0.2:1 to 2:1. Fig. 6 gives the XRD patterns of the samples with different mass ratio of AlCl3 to ore; as seen, the main phases in the roasting product are ferrous chloride hydrate [FeCl2(H2O)2], ferric oxide (Fe2O3) and copper sulfide (CuS) when the mass ratio of AlCl3 to ore is 0.5:1. The diffraction peaks of ferric oxide are intensified, while nickel chloride (NiCl2) and magnesium chloride (MgCl2) appear as the mass ratio of AlCl3 to ore reaches 1:1. The main phases of the samples with a mass ratio of AlCl3 to ore is 2:1 are identified as iron chlorides and oxide. Hereby, we can see that the amount of the generated ferric oxide which mainly comes from the oxidized of ferric chloride increase with the increased dosage of AlCl3, on account of the generation of Cl2 in the oxidizing process of ferric chloride, so the chlorination extent of nickel and copper is deepened resulting in the increase of their extraction rate. However, a too high mass ratio of AlCl3 to ore will hinder the generation reaction of Cl2 and the reactions between Cl2 and metal sulfide due to the partial pressure of O2 being impaired by the volatilized gaseous AlCl3, resulting in the decrease of the extraction rate of nickel. In addition, magnesia-alumina silicates and magnesia-alumina spinel will be possibly formed due to the abundant AlCl3 in the roasting atmosphere (the calculations of the standard Gibbs free energy 12

change are shown in Table 2) causing the decrease of the extraction of magnesium with the increased dosage of AlCl3, which beneficial to the selective extraction of valuable metals. In the following experiments, the mass ratio of AlCl3 to ore of 1:1 was recommended in the following experiments. Table 2 The standard Gibbs energy and the logarithm of the equilibrium constant of the reactions at 400°C

G 400C (kJ·mol-1)

log K 400C

AlCl3 + 1/2MgCl2 (g) + O 2 (g) = 1/2MgAl2O 4 + 2Cl2 (g)

-255.48

19.83

AlCl3 (g) + 1/2MgCl2 (g) + O 2 (g) = 1/2MgAl2O 4 + 2Cl2 (g)

-250.38

19.43

Al2Cl6 (g) + MgCl2 (g) + 2O 2 (g) = MgAl2O 4 + 4Cl2 (g)

-494.51

38.38

AlCl3 + Mg 3Si 4O10 (OH) 2 = 1/2Mg 3Al2Si3O12 + H 2O(g) + 5/2SiO 2 + 3/2MgCl2

-159.89

12.41

AlCl3 + Mg 3Si 4O10 (OH) 2 = 1/2Mg 3Al2Si3O12 + H 2O(g) + 5/2SiO 2 + 3/2MgCl2

-158.78

12.01

AlCl3 + Mg 3Si 4O10 (OH) 2 = 1/2Mg 3Al2Si3O12 + H 2O(g) + 5/2SiO 2 + 3/2MgCl2

-303.32

25.54

AlCl3 + Mg 3Si 2O5 (OH) 4 =1/2 Mg 3Al2Si3O12 + 2H 2O(g) + 1/2SiO 2 + 3/2MgCl2

-182.02

14.13

AlCl3 (g) + Mg 3Si 2O5 (OH) 4 =1/2 Mg 3Al2Si3O12 + 2H 2O(g) + 1/2SiO 2 + 3/2MgCl2

-176.92

13.73

Al2Cl6 (g) + 2Mg 3Si 2O5 (OH) 4 = Mg 3Al2Si3O12 + 4H 2O(g) + SiO 2 + 3MgCl2

-347.61

26.98

3.2.3 Effect of the roasting time Under the conditions that the roasting temperature of 400°C, the mass ratio of AlCl3 to ore of 1:1, the particle size of the ore was 96~80 μm, and the proportion of O2 in the introducing gas was 10% (90% N2), the effect of roasting time were studied ranging from 30 min to 180 min, the results are shown in Fig. 7. The XRD patterns of the samples under different roasting times are presented in Fig. 8.

Fig. 7 Effect of roasting time on the extraction rates of nickel, copper, iron, and magnesium

13

Fig. 8 XRD patterns of the samples under different roasting time

As seen in Fig. 7, the extraction rate of nickel, copper, and magnesium increase with the extension of the roasting time from 30 to 150 min and reach their corresponding maximum at 150 min of 80.54%, 90.68%, and 40.83%, respectively, but then change a little with the prolonging of roasting time. However, the extraction rate of iron firstly increases with the extension of roasting time, then reach a peak at the roasting time of 120 min, but decreases with the prolonging of roasting time. As seen in Fig. 8, the main phases in the roasting product of 10 min are ferrous chloride hydrate, ferric oxide, and talc, indicating the oxidizing of iron chloride to ferric oxide starts in the initial roasting stage. With the prolonged roasting time, the diffraction peaks of the talc disappear at 20 min, but the appearance of the phase of willemseite

(Ni,Mg)3Si 2 O10 (OH) 2 at 30 min, this is evidence that a part of nickel is surely hosted in silicates which matches with the previous chemical analysis of the ore. The diffraction peaks of magnesium sulfate (MgSO4) appear in the roasting product of 50 to 180 min, the reason is that the reaction between Cl2 and metals sulfides will form sulfur (S) which may participate in the reaction of the formation of magnesium sulfate, as shown in Eq. (44) that has a negative standard Gibbs free energy and large equilibrium constant, indicating that magnesium sulfate is stable than magnesium chloride in this condition. In addition, the diffraction peaks of nickel chloride and ferric oxide are intensified with the prolonged roasting time from 50 to 180 min. This explains the change in the extraction rates of metals with increased roasting time. Therefore, the roasting time of 150 min was selected in the following experiments.

MgCl2 + S + 2O 2 (g) = MgSO 4 + Cl2 (g)； G 400C = - 468.85 kJ  mol1 , logK 400C  36.39

3.2.4 Effect of the particle size of ore 14

(44)

Fig. 9 Effect of the particle size of the ore on the extraction rates of nickel, copper, iron, and magnesium

Fig. 9 shows the effect of of the particle size of the ore on the extraction rate of metals on the conditions that the roasting temperature of 400°C, the mass ratio of AlCl3 to ore of 1:1, the roasting time of 150 min and the proportion of O2 in the inlet gas was 10% (90% N2). It can be seen that the extraction rate of nickel and copper increases with the shrinking of the particle size of the ore because small particle size offers more chances to contact the reaction agents. The extraction rate of iron and magnesium increase a little from 830-380 μm to 250-180 μm and then keep the same level with the decreased particle size of the ore. Hence, a particle size of smaller than 75 μm was chosen for the following experiments. 3.2.5 Effect of the content of O2 As learned from the thermodynamic analysis, O2 plays a crucial role in the chlorination reaction process. Therefore, the content of O2 (the rest is N2) of the roasting atmosphere were investigated under the conditions of the roasting temperature of 400°C, the mass ratio of AlCl3 to ore of 1:1, the roasting time of 150 min and particle size of the ore of smaller than 75 μm, the result are shown in Fig. 10, and the XRD patterns of the samples under a different roasting atmosphere having a different content of O2 are shown in Fig. 11.

15

Fig. 10 Effect of the content of O2 on the extraction rates of nickel, copper, iron, and magnesium

As shown in Fig. 10, the extraction rate of nickel increases rapidly from 49.3% ( 5% O2) to 90.5% (20% O2) and then decreases with the increased content of O2 in the roasting atmosphere from 20% to 25%, and the extraction rate of iron has a similar trend. The extraction rate of copper increases but magnesium varies little with the increased content of O2 in the roasting atmosphere.

Fig. 11 The XRD patterns of the samples under a different roasting atmosphere having different content of O2

16

Fig. 12 The equilibrium composition of AlCl3 (1mol) +FeCl3 (1mol) under different amount of introducing O2 at 400°C (calculated by factsage)

It can be seen from Fig. 11, the main phases in the sample at the roasting atmosphere without O2 (0% O2+100% N2) are ferrous chloride and hydrate, however, when the content of O2 is 5%, the diffraction peaks of ferrous chloride disappear but ferric oxide and copper chloride appear. The diffraction peaks of magnesia-alumina silicates appear with the increased content of O2 at the roasting atmosphere of 10% and 20%, which could explain the decrease of the extraction rate of magnesium. In order to distinguish the oxidation order of AlCl3 and FeCl3, we calculated the equilibrium composition of the mixture of AlCl3 and FeCl3, the results are shown in Fig. 12. As seen, the oxidation reactions of AlCl3 (including other forms) finish while the oxidation reactions of FeCl3 (including other forms) start at the introducing amount of O2 is 0.75 mol, demonstrating that the oxidation of AlCl3 is more prior to FeCl3. Meanwhile, the amount of Cl2 increases gradually and then no longer changes with the increasing amount of O2, the maximum amount of Cl2 corresponds to the depletion of AlCl3 and FeCl3. Therefore, the chlorination effect of FeCl3 occurs after the exhaustion of AlCl3, which could explain the missing of the diffraction peaks of AlCl3 at XRD patterns. Fig. S10 shows the DSC curves under different roasting atmosphere for a heating rate of 10°C/min; as seen, the DSC curve at 80% N2 + 20% O2 atmosphere has a more obvious exothermic peak than at pure N2 atmosphere ranging from 300 to 500°C, which indicates the chlorination reactions occurs vigorously at the atmosphere that contains 20% O2. The reason is that the inlet O2 participates in the oxidizing of aluminum chloride and ferric chloride accompanying by the generation of Cl2 which promotes the chlorination extent of nickel and copper significantly, which is an evidence of the increase of the extraction rate of nickel and copper with the increased content of O2 in the introducing gas. However, when the content O2 exceeds a certain value, the oxidation will suppress the chlorination of metals. Hence, a suitable content of O2 of 20% in the roasting atmosphere was chosen 17

in the following experiments.

3.3 Orthogonal experiments After the roasting process, the water leaching process is a simple dissolution process of metal salts, parameters in the leaching process have a comparatively small effect on the extraction of metals. The leaching experimental results showed that nearly all of the dissolvable metal salts had been dissolved into the water at the fixed leaching condition (liquid-solid of 4:1, at 70°C for 2 h). In order to seek the optimal experimental condition and the major parameters in the chlorination process, an L16(45) orthogonal experiment was implemented based on the results of the single-factor experiments. The results of the orthogonal experiments are shown in Table 3. Table 3 The results of the orthogonal experiments Factor

Temperature (℃)

Mass ratio

Content of O2

Time (min)

Extraction rate (%)

Particle size (μm)

Ni

Cu

Mg

Fe

1

300

0.5:1

10

90

250~180

38.7

38.18

11.03

34.86

2

300

1:01

15

120

120~109

40.3

42.8

12.3

31.1

3

300

1.5:1

20

150

96~80

91.3

60.5

28.4

24.5

4

300

2:1

25

180

75~

75.4

62.6

13.2

32.7

5

350

0.5:1

15

150

75~

37.9

11.6

31.6

6

350

1:01

10

180

96~80

76.3

43.6

14.0

29.4

7

350

1.5:1

25

90

120~109

45.7

41.3

15.4

41.3

8

350

2:01

20

120

250~180

66.74

75.67

18.17

30.24

9

400

0.5:1

20

180

120~109

63.45

90.16

21.44

42.63

10

400

1:01

25

150

250~180

72.89

59.2

18.32

29.82

11

400

1.5:1

10

120

75~

85.93

55.89

24.42

32.34

12

400

2:01

15

90

96~80

46.52

58.29

16.7

34.44

13

450

0.5:1

25

120

96~80

72.39

77.2

19.31

39.06

14

450

1:01

20

90

75~

68.6

60.32

25.41

38.06

15

450

1.5:1

15

180

250~180

91.63

88.51

28.39

16.38

16

450

2:01

10

150

120~109

81.63

76.61

24.37

21.26

45.2

Rroasting time=26.833>Rmass ratio=23.685>Rroasting temperature=20.083>Ratmosphere=16.613>Rparticle size=13.879 Rroasting temperature=24.537> Rroasting time=17.638> Rmass ratio=16.758> Ratmosphere=15.590> Rparticle size=9.785 Rroasting temperature=9.552> Rmass ratio=8.308>Ratmosphere=6.787> Rroasting time=3.548> Rparticle size=1.229 Rroasting time=15.408>Rmass ratio=13.36>Rroasting temperature=9.178>Ratmosphere=7.228>Rparticle size=7.217 Note: R means the range analysis; Ratomosphere=Rthe content of oxygen

As can be seen from Table 3, the affecting order on the extraction of nickel is roasting time, the mass ratio of AlCl3 to the ore, roasting temperature, the content of O2 and particle size of the ore. The optimal experimental condition of the extraction of nickel is the roasting time of 3 h, the roasting temperature of 450°C, the mass ratio of AlCl3 to ore of 1.5:1, the content of O2 of 20%, and particle size of the ore of 96-80 μm. The affecting order on the extraction of copper is roasting temperature, roasting time, the mass ratio of AlCl3 to the ore, the content of O2, and the particle size of the 18

ore. On the basis of the priority of the extraction of nickel, the optimal condition of nickel was selected to the optimal condition of the chlorination roasting process. Under that condition, the extraction rate of nickel, copper, iron, and magnesium of 91.6%, 88.5%, 28.4%, and 16.4% were obtained, respectively. Fig. 13 shows the SEM image and EDS of the roasting product and Fig. 14 shows the XRD patterns of the leaching residue obtained from the optimal chlorination roasting condition. As seen, the magnesia-alumina silicates have an irregular bulk, and metals chlorides present as irregular agglomeration. The leaching residue contains ferric oxide, talc, lizardite, and magnesia-alumina silicate, which can be used in ceramics.

Fig. 13 SEM image and EDS of point 1, 2 and 3 (p-1, p-2 and p-3)

19

Fig. 14 XRD patterns of the leaching residue

3.4 Chlorination kinetics analysis The thermal experiment of the chlorination process of the ore (mass ratio of 1.5:1, 8 mg) was fulfilled under a mixed gas (80%N2+20%O2, total flow rate of 50ml/min), different heating rate (5, 10, 15 and 20°C·min-1), and the heating temperature ranging from 30-800°C by using a METTLER 1600HT synchronous thermal analyzer, the results are presented in Fig. 15.

20

Fig. 15 The DSC, TG and DTG curves of different heating rate from 30-800°C

As seen in Fig. 15, the DSC curve presents one obvious endothermic peak and two exothermic peaks, and TG curve goes through several slope changes, indicating three steps that are discerned by stair lines, but the DTG curve shows wave lines due to the influence of the volatilized AlCl3 on the heat stability. In the air atmosphere, the anhydrous AlCl3 system has violent endothermic peaks below 200°C (see in Fig. S7), corresponding to the volatilization and melting of AlCl3.While the raw ore has an obvious exothermic peak at 446.4°C (see in Fig. S8), corresponding to the sulfation reactions of metal sulfides in the air atmosphere. Therefore, the three steps are corresponding to the volatilization of AlCl3, the chlorination process, and the direct oxidizing of FeCl2 to Fe2O3. In this paper, we just focus on the second step, the chlorination process. The initial reaction temperature (Ti), the end reaction temperature (Te), and the mass change on TG curves, the peak temperature on DSC curves are presented in Table 4. Table 4 The initial reaction temperatures (Ti), the end reaction temperatures (Te), and peak temperatures for the second stage with different heating rates

β (°C/min)

Ti /°C

Tf /°C

Peak temp. (DSC) /°C

Δm/m0 (%)

5

209.8

374.2

352.9

10.14

10

223.9

407.3

392.8

9.78

15

234.0

424.8

402.5

9.77

20

236.5

444.7

422.4

10.32

3.4.1 Activation energy The Kissinger[44] and Flynn-Wall-Ozawa (FWO)[45, 46] method were employed to determine the activation energy from the kinetic analysis of nonisothermal data. The Kissinger and Flynn-Wall-Ozawa (FWO) method based on Eq. (44), and (45), which is written for a specified value of extent of conversion for each step, ??, at different heating rates, β，respectively.

21

  ln  2 T  p ln   ln

 AR E 1    ln E R Tp 

(44)

AE E  5.331  1.052 Rg ( ) RT( p )

(45)

where Ea, R, T, and Tp are the activation energy, the universal gas constant, absolute temperature, and the peak temperature on DSC curves, respectively; A and g(??) are the pre-exponential factor and the integral form of the reaction model, respectively. In Eq. (44), for a certain α of DSC curves, the plot of ln(  / Tp2 ) versus 1/Tp, which is produced for different heating rates, would be a straight line yielding the activation energy. In the same way, the plot of ln β versus 1/T(p) (peak temperature Tp for DSC curves, T for TG curves) would be a straight line yielding the activation energy. Fig. S13 shows the variation of ln ?? with 1/T and ln (?? /T2) with 1/T for step 2 obtained from DSC data; as seen, two straight lines are obtained. Fig. S14 shows the variation of ln ?? with 1/T for step 2 obtained fro TG data; as seen, parallel straight lines are obtained for different values of ??. This can be used to calculate the apparent activation energy, the results are shown in Table 5. It can be seen from Table 5, 69.5 kJ·mol-1 and 70.4 kJ·mol-1 of apparent activation energy in the chlorination process are obtained by using FWO (R2=0.9894) and Kissinger method (R2=0.9857), respectively. However, the apparent activation energy obtained from TG value decreases with the increase of conversion rate, from 104.1 kJ·mol1

at ??=0.1 to 72.1 kJ·mol-1 at ??=0.9, with an average of 86.5 kJ·mol-1. The value of (Emax-Emin)/Eaverage=0.37>0.1, this is

believed to have been caused by a greater extent of overlaps between the reactions in the chlorination process. It is effortless to understand that the chlorination process to the ore displays, not just an elementary reaction but with complex reactions occur simultaneously or not. Furthermore, the chlorination process proceeds easier at high conversion, caused by the participation of iron chlorides in the later of the chlorination process. The obtained value of apparent activation energy from DSC data and TG has a difference possibly results from the unstable atmosphere causing by volatile matter, especially for AlCl3. M. Chakravortty and S. Srikanth[47] have reported the apparent activation energy of the chlorination of chalcopyrite by using KCl to be 120 kJ·mol-1 at air atmosphere while 77 kJ·mol-1 at pure oxygen atmosphere, which indicates the influence of the roasting atmosphere on the calculated apparent activation energy. Table 5 The values of the apparent activation energy (Ea) calculated by fitting results

Ea DSC

TG

Kissinger

FWO

(kJ·mol-1)

69.5

70.4

(min-1)

25.14

lnA

Standard error

0.66

0.91

R2

0.9857

0.9894

α/%

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Average

104.1

99.3

92.2

88.8

85.1

85.0

78.7

72.9

72.1

86.5

Ea

(kJ·mol-1)

22

Standard error

0.29

0.39

0.38

0.55

0.58

0.42

0.48

0.44

0.33

R2

0.9990

0.9981

0.9980

0.9951

0.9943

0.9970

0.9954

0.9955

0.9974

3.4.2 The mechanism of the non-isothermal chlorination process The variation of apparent activation energy depending on the conversion rate declare that the chlorination process to the ore is determined by several mechanism functions jointly, so the following estimation aimed to select one or two most possible mechanism function. The chlorination process including the reactions occur between several gases (AlCl3(g), Al2Cl6(g), FeCl3(g), Fe2Cl6(g), HCl(g), Cl2(g), O2(g), N2(g)), liquid phases and solid phases, it is so complex that the calculated results could not guarantee the accuracy, but could offer information for the chlorination reactions. The fit between the experimental data and standard master plots was evaluated to determine the mechanism of chlorination of metals. The mechanism was evaluated at four heating rates, 5, 10, 15 and 20°C/min. Šatava method[48] was used to estimate the most probable mechanism function in the chlorination process. The original equation of Šatava method is given by the general formula (46). lg G ( ) 

1 T

(46)

Where G ( ) is the most probable mechanism function from 43 forms of integral formula in literature, T is the corresponding temperature to a determined α. Hence, the most probable mechanism function can be evaluated by the fitting results of lg G ( ) versus 1/T, and the linearly dependent coefficient (r) that is closest to 1 is the most probable mechanism function. Owing to space constraints, we just show the fitting results of the two

G ( ) ,

G ( )=[(1- ) 1/3  1]2 (Z-L-T equation, 3D diffusion, No. 9) and G ( )=[(1- )1 n  1] / (1  n), n  1.8701 (apparent reaction order, No. 43), which has the biggest r value for 43 forms of integral formula, the results are given in Fig. 16 and Table 6.

Fig. 16 lg G ( ) versus 1/T for step 2 at different α 23

Table 6 The fitting results of the selected two G ( ) G(α) [(1  )

β (°C/min)

1/ 3

[1  (1  )1 n ] / (1  n) , n=1.8701

 1]

2

Ea (kJ·mol-1)

r

SD

Ea (kJ·mol-1)

r

SD

5

123.2

0.9930

0.30

73.9

0.9938

0.17

10

115.1

0.9929

0.29

69.2

0.9943

0.15

15

116.0

0.9914

0.32

69.7

0.9939

0.16

20

108.7

0.9930

0.27

65.2

0.9932

0.16

115.8

0.9926

0.30

69.5

0.9938

0.16

Averag e

As seen in Fig. 16, two sets of parallel straight lines are obtained for different values of α, the No. 43 (apparent reaction order equation G ( )=[(1- )1 n  1] / (1  n), n  1.8701 ) has a greater r ( average 0.9938) and smaller SD (0.16) than No. 9 (Z-L-T equation (average r=0.9926, SD=0.30)). Furthermore, the calculated Ea of 69.5 kJ·mol-1 by No. 43, as shown in Table 6, is more close to the calculated Ea by FWO method. Hence, No. 43 equation is one of the most probable mechanism functions of the chlorination process: integral form G()  0.8701[1  (1  )0.8701 ] ; differential form f ()  (1  )1.8701 .

4 Conclusion The work carried out a selective extraction of valuable metals from the low-grade nickel-copper sulfide-oxide ore by using a chlorination roasting and water leaching process. (1) Thermodynamic analysis indicated that AlCl3 could react with the talc, lizardite, and magnetite directly, while pentlandite and chalcopyrite could react with the generated Cl2 to transfer into their corresponding metal chlorides. (2) In the chlorination roasting process, nickel and copper were transferred into their corresponding metal chlorides, while most of the iron and magnesium were transferred into ferric oxide and magnesia-alumina silicates, respectively. The generated Cl2 from the oxidizing of AlCl3 and FeCl3 could enhance the chlorination extent of nickel and copper obviously. (3) The extraction rate of nickel, copper, iron, and magnesium of 91.6%, 88.5%, 28.4%, and 16.4% was obtained under the optimal conditions that the roasting time was 3 h, the roasting temperature was 450°C, the mass ratio was AlCl3 to ore of 1.5:1, the content of O2 was 20%, and the particle size of the ore was 96-80 μm from the orthogonal experiments. (4) The thermoanalysis indicated that the chlorination reactions occurred around 200 to 450°C, the apparent activation energy calculated from DSC data was 70.4 kJ·mol-1, while calculated from TG data was 86.5 kJ·mol-1 (average) by 24

using FWO method. One of the most probable mechanism functions of the chlorination process was

G ( )  0.8701[1  (1   )0.8701 ] (integral form) and f ( )  (1   )1.8701 (differential form).

Acknowledgment This research was jointly supported by the National Basic Research Program of China (Grant 2014CB643405) and National Natural Science Foundation of China (Grant 51874371).

References [1]

USGS,

Nickel

Statistics

and

Information-Mineral

Commodity

Summaries,

in,

USGS,

https://www.usgs.gov/centers/nmic/nickel-statistics-and-information, 2019. [2] S.S. Jounela, Method for beneficiating nickel sulfide concentrates and corresponding mixtures, unsuitable for smelting, in, Google Patents, 1995. [3] E. Yalcin, S. Kelebek, Flotation kinetics of a pyritic gold ore, International Journal of Mineral Processing, 98 (2011) 48-54. [4] M.D. Adams, Towards a virtual metallurgical plant 2: application of mineralogical data, Minerals engineering, 20 (2007) 472-479. [5] M.P. Hay, R. Roy, A case study of optimising UG2 flotation performance. Part 1: Bench, pilot and plant scale factors which influence Cr2O3 entrainment in UG2 flotation, Minerals Engineering, 23 (2010) 855-867. [6] M.P. Hay, A case study of optimising UG2 flotation performance part 2: Modelling improved PGM recovery and Cr2O3 rejection at Northam’s UG2 concentrator, Minerals engineering, 23 (2010) 868-876. [7] N.O. Lotter, Modern Process Mineralogy: An integrated multi-disciplined approach to flowsheeting, Minerals Engineering, 24 (2011) 1229-1237. [8] C. Edwards, W. Kipkie, G. Agar, The effect of slime coatings of the serpentine minerals, chrysotile and lizardite, on pentlandite flotation, International Journal of Mineral Processing, 7 (1980) 33-42. [9] V. Kirjavainen, K. Heiskanen, Some factors that affect beneficiation of sulphide nickel–copper ores, Minerals Engineering, 20 (2007) 629-633. [10] Y. Gao, G. Zhang, M. Wang, D. Liu, The Critical Role of Pulp Density on Flotation Separation of Nickel-Copper Sulfide from Fine Serpentine, Minerals, 8 (2018) 317. [11] K. Zhao, G. Gu, C. Wang, X. Rao, X. Wang, X. Xiong, The effect of a new polysaccharide on the depression of talc and the flotation of a nickel–copper sulfide ore, Minerals Engineering, 77 (2015) 99-106. [12] B. Feng, Y. Lu, Q. Feng, M. Zhang, Y. Gu, Talc–serpentine interactions and implications for talc depression, Minerals Engineering, 32 (2012) 68-73. [13] D.A. Beattie, L. Huynh, G.B. Kaggwa, J. Ralston, The effect of polysaccharides and polyacrylamides on the depression of talc and the flotation of sulphide minerals, Minerals Engineering, 19 (2006) 598-608. [14] A. Ducret, W. Rankin, Liquidus temperatures and viscosities of FeO ‐ Fe2O3 ‐ SiO2 ‐ CaO ‐ MgO slags at compositions relevant to nickel matte smelting, Scandinavian journal of metallurgy, 31 (2002) 59-67. [15] W.G. Davenport, E. Partelpoeg, Flash smelting: analysis, control and optimization, Elsevier, 2015. [16] R.U. Pagador, M. Hino, K. Itagaki, Distribution of minor elements between MgO saturated FeOx–MgO–SiO2 or FeOx–CaO–MgO–SiO2 slag and nickel alloy, Materials transactions, JIM, 40 (1999) 225-232. [17] F. Crundwell, M. Moats, V. Ramachandran, T. Robinson, W.G. Davenport, Extractive metallurgy of nickel, cobalt and platinum group metals, Elsevier, 2011. 25

[18] Y. Xie, Y. Xu, L. Yan, R. Yang, Recovery of nickel, copper and cobalt from low-grade Ni–Cu sulfide tailings, Hydrometallurgy, 80 (2005) 54-58. [19] X. Li, J. Chen, R. Kammel, F. Pawlek, Application of attrition grinding in acid leaching of nickel sulfide concentrate, TRANSACTIONS OF NONFERROUS METALS SOCIETY OF CHINA (1997). [20] R. Bredenhann, V. Vuuren, The leaching behaviour of a nickel concentrate in an oxidative sulphuric acid solution, Minerals Engineering, 12 (1999) 687-692. [21] E. Peters, Direct leaching of sulfides: chemistry and applications, Metallurgical Transactions B, 7 (1976) 505-517. [22] H. Kobayashi, H. Shoji, S. Asano, M. Imamura, Selective Nickel Leaching from Nickel and Cobalt Mixed Sulfide Using Sulfuric Acid, MATERIALS TRANSACTIONS, (2018) M2018080. [23] Y. Li, V.G. Papangelakis, I. Perederiy, High pressure oxidative acid leaching of nickel smelter slag: Characterization of feed and residue, Hydrometallurgy, 97 (2009) 185-193. [24] Z.-G. Deng, C. Wei, X.-B. Li, C.-X. Li, G. Fan, Leaching Mo and Ni by Direct Oxygen Pressure from Black Shale with any other only water, in:

2015 4th International Conference on Sustainable Energy and Environmental Engineering,

Atlantis Press, 2016. [25] B. Pandey, D. Bagchi, V. Kumar, A. Agrawal, Premchand, Pressure sulpuric acid leaching of a sulphide concentrate to recover copper, nickel and cobalt, Mineral Processing Extractive Metallurgy, 111 (2002) 106-109. [26] F. Forward, V. Mackiw, Chemistry of the ammonia pressure process for leaching Ni, Cu, and Co from Sherritt Gordon sulphide concentrates, JOM-US, 7 (1955) 457-463. [27] X. Meng, K.N. Han, The principles and applications of ammonia leaching of metals—a review, Mineral Processing Extractive Metullargy Review, 16 (1996) 23-61. [28] N. Arbiter, B.D. Cooley, Sulfate precipitation during oxidative ammonium leach of Cu, Ni, Zn sulfide ores, in, Google Patents, 1979. [29] W.P. Duyvesteyn, H. Liu, M.J. Davis, Heap leaching of nickel containing ore, in, Google Patents, 2001. [30] W. Qin, S. Zhen, Z. Yan, M. Campbell, J. Wang, K. Liu, Y. Zhang, Heap bioleaching of a low-grade nickel-bearing sulfide ore containing high levels of magnesium as olivine, chlorite and antigorite, Hydrometallurgy, 98 (2009) 58-65. [31] A.-K. Halinen, N. Rahunen, A.H. Kaksonen, J.A. Puhakka, Heap bioleaching of a complex sulfide ore: Part I: Effect of pH on metal extraction and microbial composition in pH controlled columns, Hydrometallurgy, 98 (2009) 92-100. [32] H. Watling, The bioleaching of nickel-copper sulfides, Hydrometallurgy, 91 (2008) 70-88. [33] P.R. Norris, Selection of thermophiles for base metal sulfide concentrate leaching, Part II: Nickel-copper and nickel concentrates, Minerals Engineering, 106 (2017) 13-17. [34] T. Norgate, S. Jahanshahi, Low grade ores–smelt, leach or concentrate?, Minerals Engineering, 23 (2010) 65-73. [35] X.-w. Liu, Y.-l. Feng, H.-r. Li, Z.-c. Yang, Z.-l. Cai, Recovery of valuable metals from a low-grade nickel ore using an ammonium sulfate roasting-leaching process, International Journal of Minerals, Metallurgy, Materials, 19 (2012) 377383. [36] D. Yu, T.A. Utigard, M. Barati, Fluidized bed selective oxidation-sulfation roasting of nickel sulfide concentrate: Part I. Oxidation roasting, Metallurgical Materials Transactions B, 45 (2014) 653-661. [37] T. Mukherjee, P. Menon, P. Shukla, C. Gupta, Chloridizing roasting process for a complex sulfide concentrate, JOMUS, 37 (1985) 29-33. [38] C. Xu, H.-w. Cheng, G.-s. Li, C.-y. Lu, X.-g. Lu, X.-l. Zou, Q. Xu, Extraction of metals from complex sulfide nickel concentrates by low-temperature chlorination roasting and water leaching, International Journal of Minerals, Metallurgy, Materials, 24 (2017) 377-385. [39] F. Cui, W. Mu, S. Wang, H. Xin, H. Shen, Q. Xu, Y. Zhai, S. Luo, Synchronous extractions of nickel, copper, and cobalt by selective chlorinating roasting and water leaching to low-grade nickel-copper matte, Separation Purification Technology, 195 (2018) 149-162. 26

[40] F. Cui, W. Mu, S. Wang, Q. Xu, Y. Zhai, S. Luo, Controllable Phase Transformation in Extracting Valuable Metals from Chinese Low-Grade Nickel Sulphide Ore, JOM-US, 69 (2017) 1925-1931. [41] F. Cui, W. Mu, S. Wang, H. Xin, Q. Xu, Y. Zhai, S. Luo, Sodium sulfate activation mechanism on co-sulfating roasting to nickel-copper sulfide concentrate in metal extractions, microtopography and kinetics, Minerals Engineering, 123 (2018) 104-116. [42] T. Kuzuya, S. Hirai, V.V. Sokolov, J. Seppur, Recovery of valuable metals from a spent nickel–metal hydride battery: Selective chlorination roasting of an anodic active material with CCl 4 gas, Separation Purification Technology, 118 (2013) 823-827. [43] T. Warner, N. Rice, N. Taylor, Thermodynamic stability of pentlandite and violarite and new EH-pH diagrams for the iron-nickel sulphur aqueous system, Hydrometallurgy, 41 (1996) 107-118. [44] H.E. Kissinger, Reaction kinetics in differential thermal analysis, Analytical chemistry, 29 (1957) 1702-1706. [45] T. Ozawa, A new method of analyzing thermogravimetric data, Bulletin of the chemical society of Japan, 38 (1965) 1881-1886. [46] J.H. Flynn, L.A. Wall, A quick, direct method for the determination of activation energy from thermogravimetric data, Journal of Polymer Science Part B: Polymer Letters, 4 (1966) 323-328. [47] M. Chakravortty, S. Srikanth, Kinetics of salt roasting of chalcopyrite using KCl, Thermochimica Acta, 362 (2000) 25-35. [48] V. Šatava, Mechanism and kinetics from non-isothermal TG traces, Thermochimica Acta, 2 (1971) 423-428.

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Fuhui Cui: Conceptualization, Methodology, Software, Formal analysis, Investigation, Writing Original Draft, Writing - Review & Editing Wenning Mu: Resources, Data Curation Yuchun Zhai: Funding acquisition, Conceptualization Xueyi Guo: Funding acquisition, Supervision, Project administration

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

No

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