Upgrading a manufactured fine aggregate for use in concrete using dry rare-earth magnetic separation

Minerals Engineering 143 (2019) 105942

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Upgrading a manufactured fine aggregate for use in concrete using dry rareearth magnetic separation Flávio P. Andréa, Hayla Micelia, Luanna C. Mouraa, Reiner Neumannb, Luís Marcelo Tavaresa,

T

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Department of Metallurgical and Materials Engineering, Universidade Federal do Rio de Janeiro – COPPE/UFRJ, Cx. Postal 68505, CEP 21941-972 Rio de Janeiro, RJ, Brazil b Centre for Mineral Technology (CETEM), Rio de Janeiro, RJ, Brazil a

A R T I C LE I N FO

A B S T R A C T

Keywords: Manufactured fine aggregate Manufactured sand Biotite Magnetic separation Dry processing

Manufactured fine aggregates are progressively more used worldwide as an alternative to natural sand in concrete. In a recent study by the authors the removal of contaminants from manufactured fine gneiss aggregates by dry rare-earth magnetic separation was investigated. The present work initially demonstrates that single-stage dry high-intensity magnetic separation of the unclassified fine fraction (−2.36 mm) of the aggregate produced by a vertical shaft impact crusher allows simultaneous partial removal of the flaky biotite particles as well as reduction in the proportion of fines, thus improving its potential for application as a replacement of natural sand in concrete. The work then investigates the effect of magnetic field intensity, specific throughput, roll velocity, splitter position and moisture content on the performance of a bench-scale separator following a full factorial design of experiments. The non-magnetic product from an additional test was then used in combination with the coarser (+2.36 mm) unconcentrated fraction produced by a vertical shaft impact crusher in tests in concrete. Concretes with different strengths were prepared and results show that the upgraded non-magnetic manufactured aggregate was able to match the performance of concrete using natural sand, demanding one third less rheology additive, with comparable strengths at 28 days of curing.

1. Introduction The shortage of natural sand coupled to environmental restrictions has motivated the search for replacement materials in many parts of the world, in particular to meet the demands from the construction and building industry (Gonçalves et al., 2007; Anand and Reddy, 2014; Cordeiro et al., 2016). Manufactured fine aggregate is often distinguished from natural sand due to its particle shape, fines content and mineralogical composition. Whereas the characteristics of the latter are defined by natural agents, such as weathering and transport, besides type of deposit, characteristics of manufactured fine aggregate are essentially co-determined by rock properties and processing route. Crushing is an important stage in processing manufactured fine aggregates, and several studies have demonstrated the benefit of impact (vertical shaft impact or VSI) over compressive crushing as a method to improve particle shape (Gonçalves et al., 2007; Åkesson and Tjell, 2010; Nanthagopalan and Santhanam, 2011; Cepuritis et al., 2016). Particle shape has been known to influence workability, durability and strength of concrete. Whereas natural aggregates are composed predominantly of round-shaped particles, manufactured fine aggregates

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usually contain particles that are more irregularly-shaped and with well-defined edges (Gonçalves et al., 2007; Cepuritis et al., 2017). Yet, Åkesson and Tjell (2010) demonstrated that fines from granites that contained large amounts of feldspars and small amounts of mica , produced using a VSI crusher, presented particle shapes that were similar to those of a natural sand. Crushed fine aggregates, in particular when resulting from VSI crushing, typically contain larger proportions of fines than natural sand. However, given the usually non-reactive nature of these fines, their presence may be tolerated in concrete (Ahn, 2000; Gonçalves et al., 2007). Indeed, standards for fine aggregates have been revised worldwide, now making allowances for larger proportions of fines in the case of manufactured fine aggregates. For instance, the American standard (ASTM C33/C33M, 2018) now tolerates up to 7% of material finer than 75 µm for use in concrete, whereas the Brazilian standard (ABNT NBR 7211, 2007) tolerates up to 12% of material finer than this size, provided that fines are not composed of micaceous, ferruginous, and/or expansive clays, otherwise the limit is lowered to 5%. In spite of that, often concrete producers favor the manufactured fine aggregate following the restrictions of the natural material, that is, with a smaller

Corresponding author. E-mail address: [email protected] (L.M. Tavares).

https://doi.org/10.1016/j.mineng.2019.105942 Received 8 April 2019; Received in revised form 15 August 2019; Accepted 17 August 2019 Available online 24 August 2019 0892-6875/ © 2019 Elsevier Ltd. All rights reserved.

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proportion of fines, so as to avoid segregation in silos, and to guarantee good concrete workability. The result is that several technological solutions, including dry air classification (Johansson and Evertsson, 2012; Cepuritis et al., 2015), have been proposed and successfully applied to partially remove fines contained in manufactured fine aggregates. Mineral composition has also been known to influence the effectiveness of manufactured fine aggregates in mortars and concrete. Minerals resulting from weathering of feldspars (kaolinite and illite), as well as phyllosilicates, such as biotite and chlorite, have been associated to deleterious effects in mortars and concrete (Anastasio et al., 2016). For instance, Åkesson and Tjell (2010) have demonstrated that little or no improvement in particle shape was achieved when processing rocks with large amounts of mica and amphiboles on a vertical shaft impact (VSI) crusher due to the elongated shape of the minerals, breaking preferably as thin sheets. Wakizaka et al. (2001) reported poor performance of mortars with manufactured fine aggregate due to the presence of biotite. The weak bond between the cement paste and the smooth surface of biotite particles and their irregularity in shape was considered responsible for, ultimately, the poorer workability, the lower compressive, tension and flexural strengths, as well as the higher water demand of concrete using the manufactured fine aggregate. In summary, the successful application of manufactured fine aggregates in mortars and concrete has been found to require good control of particle shape, which can be achieved by application of VSI crushers (Gonçalves et al., 2007; Nanthagopalan and Santhanam, 2011; Cepuritis et al., 2016), provided that the content of foliated minerals is small (Åkesson and Tjell, 2010). Also, control of fines content has also proven beneficial for application of this material, which has been often achieved by dry classification (Johansson and Evertsson, 2012; Cepuritis et al., 2015). Control of manufactured fine aggregate composition, through the removal of deleterious minerals has only been studied recently and carried out using dry rare-earth magnetic separation (Miceli et al., 2017; Moura et al., 2019). Important advances have been made in the last decade in the field of dry rare-earth magnetic separation (REMS). One of the main advantages of REMS is its low energy consumption and high throughput, while the high wear rate of the belt is one of its main challenges. REMS has been known to separate efficiently particles in the size range from 25 mm to 75 µm and has been very successful in separation of moderately paramagnetic minerals, given the recent developments in powerful rare-earth ceramic magnets (Tripathy et al., 2017). Several variables have been known to influence dry REMS, including roll diameter, magnetic field intensity and gradient – this later related to the magnetsteel ratio, thickness of the belt, number of stages of separation, feed rate and roll speed (Tripathy et al., 2015, 2017; Alp, 2009; Ibrahim et al., 2002; Koca et al., 2000; Atesok et al., 1999; Yildinm et al., 1996), splitter position (Ibrahim et al., 2017; Tripathy et al., 2017; Ozdemir et al., 2011; Koca et al., 2000; Yildinm et al., 1996) and feed size and distribution (Dwari et al., 2013, 2014; Grieco et al., 2014; Ibrahim et al., 2017; Leaper et al., 2002; Ozdemir et al., 2011). The present work analyzes in detail the application of dry REMS to upgrade a manufactured fine aggregate from a gneiss rock from Brazil, building up on the previous work by the authors (Miceli et al., 2017). In the work the bulk separation strategy, that is, separation of the unclassified −2.36 mm material, is analyzed. A sensitivity analysis of separation in dry REMS to a number of operating variables has then been conducted. The performance of the upgraded product is then compared to a natural sand used in concretes of different strengths.

Fig. 1. Estimated proportion of liberated particles (both felsic and mafic) as a function of particle size for the as-received sample (modified from Miceli et al., 2017).

Complex (Silva et al., 2015). It corresponded to the product of a vertical shaft impact crusher after classification in a vibrating screen with a 4.00 mm opening. The size distribution of the sample was analyzed by wet-dry sieving. Earlier work by the authors (Miceli et al., 2017) demonstrated that the mafic minerals, mainly biotite, are not properly liberated at coarser sizes (Fig. 1). As such, the material was classified in the lab using the 2.36 mm sieve, with the material coarser, corresponding to 8% of the sample, stored and the fine material prepared for separation tests. This screen size, which is above the liberation size (Fig. 1), was selected for convenience considering a potential future industrial classification operation. A sample of natural sand was also collected from a concrete producer and served as the control of the tests in concrete. It is a dredged sand consisting predominantly of quartz from a local artificial water body. 2.2. Magnetic separation tests Magnetic separation tests were conducted using a laboratory scale dry rare-earth magnetic separator (Fig. 2). The device consists of a 100 mm diameter rare-earth roll coupled to an idler roll and supported

2. Experimental 2.1. Material A sample of fine manufactured aggregate was collected from a quarry located in the metropolitan area of Rio de Janeiro, Brazil. Petrological analyzes identified it as a gneiss from the Rio Negro

Fig. 2. Bench-scale dry rare-earth magnetic separator used in the tests. Feed hopper (1); vibratory feeder (2); feeder controller (3); magnetic roll (4); rolls frequency controller (5); product splitter (6); product collection boxes (7). 2

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statistical significance. Identification of the optimal conditions has been carried out using the method proposed by Derringer and Suich (1980) (Myers et al., 2009), in which the objective function transforms the existing values of the response variables to a scale-free value called desirability. Desirability is the ratio between the optimal value of each response variable when optimized individually and as a part of a multiresponse optimization. This is available through the Response Optimization tool of Minitab® 18 software. Batches of approximately 10 kg of the dry −2.36 mm material split using a longitudinal pile of the entire sample were used in each test. In the case of tests with the moist material, the required volume of water was added to the sample, which was mixed for a few minutes in a plastic bag, until moisture appeared to be uniformly distributed. Tests were then conducted by introducing the sample in the hopper on top of the vibration feeder (Fig. 2), setting the test conditions and then conducting the experiment, making sure that the hopper level was maintained constant during the test to prevent excessive fluctuations in feed rate.

by a 0.8 mm thickness polyurethane belt. The separator also includes two flow splitters, one that separates the non-magnetic from the middling material (Fig. 2) and the other that separates the middling from the magnetic product. The magnetic roll is composed of two different groups of rare-earth permanent magnets separated by a large steel roll, thus allowing to investigate separation at two different magnetic field intensities by moving lateraly the positions of the feed silo and the vibratory feeder (Fig. 2). Their magnetic field intensity was measured using a gaussmeter (model TLMP-HALL-20 k-T0 by GlobalMag Inc.). The reading was made over the conveyor belt, corresponding to the magnetic field intensity to which particles are subjected to during tests. Their average magnetic field intensities were 0.40 T and 0.58 T. The section with lower field intensity consists of 20 rare-earth magnetic disks while the group with higher field intensity is composed of 18 rare-earth magnetic disks, resulting in different magnet-steel ratios and, therefore, different field intensities and gradients (Tripathy et al, 2017). The tests consisted of feeding the entire −2.36 mm (unclassified) material to the separator. Although the magnetic separator generates a middling product, it was incorporated in either the non-magnetic or the magnetic product in the tests, so that only two products were generated in each experiment. Magnetic separation experiments started with an exploratory (preliminary) test in which the separation performance was analyzed in great detail. This test was conducted with the dry feed, with the roll speed set to 139 rpm, the feed rate to 10 kg/h (76 kg/h·m) and with the magnetic field intensity of 0.40 T. Both magnetic and nonmagnetic products were screened using the 1.18, 0.425, 0.212 and 0.053 mm sieves and chemical and mineralogical analyzes performed on each fraction. In this test, the non-magnetic product was combined with the middling material. Tests were then conducted following a factorial (Montgomery, 2013) design, in which the sensitivity of the separator performance to five operating variables was analyzed, namely magnetic field intensity, feed rate, moisture content (dry basis), roll speed and splitter position. Table 1 summarizes the ranges of values of parameters used in the 25 design (Montgomery, 2013), which resulted in 32 factorial runs and four replicates. The actual number of experiments conducted was 16, in addition to the two replicates, since two splitter positions were analyzed in each test: one that consisted of the combination of the nonmagnetic material and the middling product and the other considering only the split of the non-magnetic product, which was carried out with the splitter shown in Fig. 2. Earlier experiments have demonstrated that moisture contents above 0.8% lead to nearly no separation, being this the reason for the selection of the upper level of moisture at 0.5% in the factorial design (Table 1). A final test was conducted to prepare the sample for concrete, which was carried out with dry feed, with the roll speed at 181 rpm, feed rate at 90 kg/h·m, magnetic field intensity at 0.58 T and with a change in the inter splitter position in order to match the necessary amount of fines of the optimal grading defined by ABNT NBR 7211 (2007). In the test the non-magnetic product was combined with the middling material. Analyzes of variance of the main effects as well as interactions of main variables were carried out analyzing the confidence intervals of the effects (Montgomery, 2013), with a significance level (α) of 0.05 using the Minitab® 18 software. As such, any factor with a p-value lower than 0.05 was considered to influence the response variable with

2.3. Physical, chemical and mineralogical analyzes Samples of both products of the separator were immediately weighed after each experiment and those corresponding to tests with moist feed were dried in a lab oven at 100 °C ± 5 °C for 24 h, being weighed afterwards. Quartered samples of the products were then subjected to size, chemical and mineralogical analyzes. Size analysis were conducted in a Ro-Tap® sieve shaker using sieve openings ranging from 2.36 until 0.300 mm ( 2 Tyler series) and the material passing the 0.300 mm screen was then analyzed by dry laser diffraction using Mytos® (Sympatec GmbH). Particle shape analyzes of selected samples were performed by dynamic image analysis using Camsizer XT (Retsch GmbH). The chemical composition of the feed and products was determined by semiquantitative X-ray fluorescence spectroscopy using a SHIMADZU Ray Ny EDX-720, with 3 kW rhodium sealed tube and Si(Li) detector, cooled by liquid nitrogen, in standardless mode. Prior to chemical analyses, the samples were ground in a planetary mill to sizes below 0.075 mm, dried at 105 °C for 12 h, mixed in proportion of 1 g of sample to 0.5 g of boric acid (H3BO3) and then compacted in pressed pellets with a boric acid matrix. A fraction of the ground material was calcined to 1050 °C, in duplicate, and the loss on ignition estimated from the percentage mass loss. Prior to mineralogical analyzes samples were initially ground wet below 0.075 mm for 10 min in a McCrone Micronizer mill using agate grinding media, followed by overnight drying in vacuum oven at 60 °C. The dry powder samples were loaded into the appropriate sample holders, and X-ray diffraction analyzes were performed using a D4 Endeavour diffractometer (Bruker-AXS), with cobalt radiation and a silicon drift LYNXEYE detector. Total measurement time per sample was about one hour. Quantitative phase analyzes were carried out by X-ray diffraction using the Rietveld refinement method (Kern and Coelho, 1998) implemented in the software TOPAS. Structure files were sourced from the Bruker Crystal Structure Database. Mass flows, as well as size and chemical analyses results from the magnetic separation experiments were reconciled using the JKSimMet® software (Grimes and Keenan, 2015).

Table 1 Parameters analyzed in factorial experiments. Levels

Low (−) High (+)

Factors Magnetic field intensity (T)

Feed rate (kg/h·m)

Moisture content (%)

Roll speed (rpm)

Splitter position* (–)

0.40 ( ± 0.03) 0.58 ( ± 0.05)

76 ( ± 15) 155 ( ± 22)

0.0 0.5

139 222

NMAG + MD/MAG NMAG/MD + MAG

* NMAG: Non-magnetic product; MD: middling material; MAG: magnetic waste 3

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Table 2 Mixture proportions of the concrete formulations (in kg/m3). Component

Portland cement Fine aggregate Coarse aggregate Water

Table 3 Mineralogical composition of the −2.36 mm fraction of the feed sample.

Nominal compressive strength at 28 days of curing

Mineral

Chemical formula

Content (%)

25 MPa

45 MPa

278 834 1006 195

435 685 1014 196

Feldspars Quartz Biotite

KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8 SiO2 K2(Mg,Fe2+)6-4(Fe3+,Al, Ti)0-2Si6-5Al23O20(OH,F)4 Ca2(Mg2.5-4.5,Fe2+ 0.5-2.5)4Si8O22(OH)2* (Fe,Mg)5Al(Si3Al)O10(OH)8** CaCO3 Al2Si2O5(OH)4

53.19 34.02 8.70

Hornblende Chlorite group Calcite Kaolinite

2.4. Tests in concrete

2.51 0.60 0.57 0.42

* Actinolite. ** Clinochlore-chamosite series.

Tests in concrete were conducted using a selected sample of the nonmagnetic −2.36 mm product, mixed with the corresponding part of the material previously classified in the size range 4.00–2.36 mm. Two formulations, shown in Table 2, corresponding to nominal compressive strengths of 25 and 45 MPa, were prepared for testing using the manufactured fine aggregate sample and the natural sand sample. These corresponded to the actual formulations used by a local manufacturer in producing concretes using only natural fine aggregate. Water from the local supplier, and coarse aggregates contained in size range 6.3–19 mm from the same quarry as the manufactured fine aggregate, were used in preparing the concrete. Brazilian type III Portland cement (containing 35–70% of blast furnace slag in its composition) (ABNT NBR 16697, 2018) was adopted in the study. The same superplasticizer (Mira® Set 28 from GCP Applied Technologies Inc.), with 29.4% oven-dried residue and 1.24 g/cm3 density, was added in different proportions according to the fine aggregate of the mixture used to ensure similar rheological response. Rheology in wet state was assessed on the basis of the overall aspect of the mixture, and slump (ABNT NBR NM 67, 1998) was kept constant at 160 ± 10 mm for both strengths. Cylindrical specimens (75 mm diameter and 150 mm height) of each mixture were prepared, left to set for 24 h and then demolded and cured underwater until their compressive strength test carried at the ages of 7, 14 and 28 days, following the Brazilian standard ABNT NBR 5739 (2018).

intermediate and fine size ranges. As mentioned in Section 2.1, the material that was subjected to magnetic separation tests was finer than 2.36 mm, representing 92% of the as-received sample. Results of mineralogical analyzes of this material are summarized in Table 3, which demonstrates that it is predominantly (over 87%) composed of feldspars and quartz. Minerals that compose the material studied are basically aluminum silicates, with the exception of quartz, composed only by SiO2, and calcite. Regarding the main paramagnetic minerals, the sample contains 8.7% of biotite and 2.5% of hornblende (Table 3). Chlorite was identified in lower concentrations and is a mineral that has been associated to a weak paramagnetic response (Borradaile, 1988). Kaolinite was also identified in the sample with low concentrations. Both of these minerals usually indicate a certain degree of alteration in gneiss rocks (Ahrens, 1995). The sample is from the same quarry that was object of an earlier study by the authors (sample A from the work by Miceli et al., 2017) and a comparison between results of the present study and the previous work shows good correspondence. Nevertheless, the higher biotite content in the sample from the present work may be explained by the difference in top size when compared to Miceli et al. (2017), given the smaller proportion of biotite in the coarser fraction (+2.36 mm) that has been removed prior to these analyzes. X-ray fluorescence spectroscopy provided the chemical composition of the sample, presented in Table 4. As expected from the high proportion of feldspar and quartz in the sample (Table 3), large proportions of SiO2 and Al2O3 were identified. K2O is a major component of alkaline feldspars and biotites, while the 4.1% of Fe2O3 is shared between biotites and hornblendes. Calcium constitutes a solid solution with sodium, forming different minerals in the plagioclase series of feldspars (Klein and Dutrow, 2008). Measured loss on ignition corresponded to 0.6% of the sample. More detailed analyzes of the samples may be found in another recent work by the authors (Moura et al., 2019).

3. Results and discussion 3.1. Feed sample characterization Results from analysis of the as-received material, with particle size below 4.00 mm, are presented in Fig. 3, which show the combined size and mineralogical analyzes, besides the Fe2O3 content. It is evident that more than half of the material is contained in classes from 1.18 to 0.212 mm, and also that biotite is present in all size ranges. Variation of Fe2O3 content also shows the concentration of iron-rich minerals in the

3.2. Preliminary magnetic separation test On the previous work by the authors (Miceli et al., 2017), dry magnetic separation of crushed sand was carried out with the material previously classified in size ranges. However, dry classification of Table 4 Chemical composition of the −2.36 mm fraction of the feed sample.

Fig. 3. Distribution of minerals, as well as Fe2O3 content (solid line), as a function of size in the feed (as-received −4.00 mm sample).

Chemical Composition (%)

Content (%)

SiO2 Al2O3 K2O Fe2O3 CaO TiO2 Others, including LOI*

63.85 19.35 4.44 4.10 3.20 0.64 4.42

* LOI: loss on ignition. 4

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Fig. 5. Comparison of efficiency in magnetic separation of the unclassified or bulk material (present work, −2.36 mm) and classified (size-by-size) (Miceli et al., 2017).

considered desirable in the manufactured fine aggregate (feldspars, quartz and calcite) and the rejection of those that are undesirable (biotite, hornblende, kaolinite and chlorite). An efficiency of 100% would result from recovery of all desirable components in the nonmagnetic concentrate and rejection of all undesirable components in the magnetic concentrate (Miceli et al., 2017). A comparison is presented in Fig. 5, which shows that the efficiency of separation at coarser sizes obtained in the present work using the bulk strategy was actually higher than that reached using the size-by-size approach. Nevertheless, separation of material coarser than 1.18 mm was inefficient in both cases, which can be explained by the very limited liberation of the components in these sizes (Fig. 1). As already suggested by Fig. 4, selectivity in separation reduced significantly for the 0.212–0.053 mm size range, and became absent for the material finer than 0.053 mm when the bulk separation strategy was adopted. As such, convenience in separation of the material without previous classification (bulk separation) led to a sacrifice in efficiency of separation in finer sizes. Yet another analysis of the separation results is possible by comparing the size distributions of the products and the feed from the test. Fig. 6 shows that separation was significantly influenced by particle size, with a very limited proportion of fines reporting to the non-magnetic product. As such, while the low efficiency in separation and recovery of the fine material could be, at first, considered a limitation of the bulk separation approach, its ability to simultaneously concentrate desirable minerals and removing fines may be regarded as advantages of this strategy. Indeed, the proportion of fines, namely the material finer than 0.075 mm, was reduced from 10.7% in the original −2.36 mm material to only 1.6% in the non-magnetic product (Fig. 6).

Fig. 4. Comparison of yield and recovery of different minerals for the different size ranges in magnetic separation following the bulk separation strategy in this work (top) and the size-by-size separation strategy (Miceli et al., 2017) (bottom). Product is here the combination of the non-magnetic and the middling material.

particles in several size ranges in an industrial plant can be challenging, limiting the feasibility of that approach. As such, magnetic separation of the −2.36 mm material, hereby called bulk separation, has been conducted and results compared to those from size-by-size separation (Miceli et al., 2017) with a similar sample from the same deposit. Fig. 4 shows that the yield varied significantly as a function of particle size, with the coarser sizes presenting substantially higher yields than the finer ones as a result of bulk separation. Removal of the mafic minerals, biotite and hornblende, as well as of minerals from the chlorite group, and concentration of quartz and feldspars in the non-magnetic product was evident, in particular in the size range from 1.18 to 0.212 mm. Quartz was the most effectively recovered non-magnetic mineral, while biotite the most effectively rejected magnetic mineral. Fig. 4 also shows that some results from the present work differ significantly from those obtained in size-by-size separation of a similar sample (Miceli et al., 2017), in particular in the 0.212–0.053 mm size range, where separation still occurred in the case of the latter and very little upgrading took place as a result of bulk separation. In addition, results from the present work contrast with those from size-by-size separation (Miceli et al., 2017) in regard to kaolinite, which suffered no selective separation in the bulk separation strategy adopted in the present work while some concentration at fine sizes was observed when the size-by-size separation strategy was used (Fig. 4). An additional comparison between the two strategies of conducting the test is possible by analyzing the efficiency of separation as a function of size. Separation efficiency (Wills and Napier-Munn, 2006) is hereby defined as the product of the recovery of minerals that are

Fig. 6. Comparison of size analyzes of the feed and products of the preliminary magnetic separation test, conducted at 139 rpm, with a feed rate of 10 kg/h (76 kg/h·m) and with a magnetic field intensity of 0.40 T. 5

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Table 5 Factors, coefficients and standard errors for the performance indices analyzed for the non-magnetic product from the factorial experiments (values significant at α = 0.05 in bold). Factor

Moist Freq Feed Int Split Moist*freq Moist*feed Moist*int Moist*split Freq*feed Freq*int Freq*split Feed*int Feed*split Int*split Moist*freq*feed Moist*freq*int Moist*freq*split Moist*feed*int Moist*feed*split Moist*int*split Freq*feed*int Freq*feed*split Freq*int*split Feed*int*split Moist*freq*feed*int Moist*freq*feed*split Moist*freq*int*split Moist*feed*int*split Freq*feed*int*split Moist*freq*feed*int*split Standard error

% −75 µm

Yield (%)

% Fe2O3

Coef.

p-Value

Coef.

p-Value

Coef.

p-Value

−1.232 2.521 −1.668 −1.004 −4.934 1.265 −2.228 −0.545 −1.549 0.167 0.715 −1.201 −0.840 −0.136 0.638 0.353 0.615 −0.171 −0.558 −0.346 0.604 0.51 0.006 0.293 0.181 0.537 −0.006 0.224 0.209 0.076 −0.126 ± 0.137

0.001 0.000 0.000 0.002 0.000 0.001 0.000 0.017 0.000 0.291 0.006 0.001 0.004 0.378 0.010 0.062 0.011 0.280 0.015 0.065 0.012 0.020 0.966 0.100 0.257 0.017 0.966 0.178 0.203 0.609 0.412

1.515 −0.257 −0.201 −0.380 −0.940 −0.033 −0.185 −0.328 −0.641 0.376 0.083 −0.026 0.189 0.200 0.288 0.114 −0.050 0.022 0.151 0.209 0.289 −0.064 −0.146 −0.158 −0.117 −0.043 −0.146 −0.161 −0.111 0.188 0.148 ± 0.031

0.000 0.001 0.003 0.000 0.000 0.350 0.004 0.000 0.000 0.000 0.058 0.457 0.004 0.003 0.001 0.022 0.188 0.516 0.008 0.003 0.001 0.112 0.010 0.007 0.020 0.238 0.010 0.007 0.024 0.004 0.009

0.139 0.157 0.001 −0.104 −0.268 0.006 −0.090 −0.071 −0.061 0.082 −0.110 −0.126 −0.007 −0.025 0.108 −0.063 −0.029 −0.018 0.037 0.018 0.022 0.018 −0.012 0.078 0.020 0.075 0.042 −0.006 −0.041 0.008 −0.057 ± 0.014

0.001 0.000 0.953 0.002 0.000 0.709 0.003 0.007 0.012 0.004 0.001 0.001 0.644 0.143 0.001 0.011 0.105 0.275 0.057 0.261 0.189 0.255 0.452 0.005 0.216 0.006 0.038 0.707 0.042 0.591 0.015

Legend: Moist = Moisture content; Freq = Rotation frequency of the rolls; Feed = Feed rate; Int = Magnetic field intensity.

magnetic product containing less fines, with no direct effect on Fe2O3 content in the non-magnetic product. High feed rates can negatively affect separation, as more particles will have to share the field intensity, reducing usable area between the magnets. Increase in magnetic field intensity resulted, as expected, in a reduction, although modest, in yield, as more magnetic material is pulled; it reduced the proportion of fines and the Fe2O3 contamination in the non-magnetic product. Evidently, combination of the non-magnetic and the middling material resulted in increase in yield, Fe2O3 as well as proportion of fines of the non-magnetic product. Several interactions between variables were also found to be significant (Table 5). In order to identify the combined effect of the main and interaction effects on the performance of the separator, the Response Optimizer tool in Minitab® 18 software was used, by maximizing the yield and minimizing both the Fe2O3 as well as the percentage passing 75 µm in the non-magnetic product. Table 6 demonstrates that different sets of conditions were found to optimize the operation considering the various criteria, considering the cases in which the feed of the process contains different moisture levels. It shows that when the feed is dry, the optimal condition would correspond to the combination of the middling material to the non-magnetic product, whereas when the feed contains 0.5% moisture, the product is composed solely of the non-magnetic product. A highlight in Table 6 is the fact that it was possible to reach nearly the same product specification in terms of iron and fines content even when the feed contains different moisture contents. Also evident in Table 6 is the fact that, as long as operating conditions are modified to cope with the moist feed, only yield would be sacrificed, with a reduction of about 3% in comparison to the dry feed. The desirability of the simulations was moderate, which indicates that the multi-objective optimization result required some sacrifice in

This preliminary test resulted in a yield (mass recovery) of 65.6%, with the non-magnetic product containing 1.0% Fe2O3 and 2.9% biotite, from a feed containing 4.1% Fe2O3 (Table 4) and 8.7% biotite (Table 3). 3.3. Factorial experiments A summary of the analysis of variance results from the factorial experiments is presented in the Table 5 in respect to the three response variables selected: yield, Fe2O3 content and percentage passing 75 µm in the non-magnetic product. The aim of the experiment was to reduce the proportion of biotite and hornblende through reduction in the Fe2O3 content, and also to reduce the proportion of fines contained in the non-magnetic product, while maximizing the yield. Iron content was selected as the control component since it is contained exclusively in minerals whose presences in the product are undesirable (Table 3). With the exception of the effect of feed rate on the Fe2O3 content of the non-magnetic product, all main effects influenced the three response variables. This is illustrated graphically in the main effectś plots of Fig. 7. Increase in moisture content reduced the yield of the product, increased its Fe2O3 content as well as its percentage of −0.075 mm, by pulling fine material as well as a larger proportion of iron-containing minerals to the non-magnetic product. An increase in rotation frequency resulted in a significant increase in yield and more contamination of the non-magnetic product with iron-bearing minerals and fines. High rotation frequency of the rolls will increase the centrifugal force component, resulting in the material more readily being projected to the non-magnetic product (Tripathy et al., 2015). This effect has been observed to be amplified when the feed is contained of material from a wider range of sizes (Tripathy et al., 2015), as is the case of the present work. Higher throughputs resulted in lower yields and non6

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Moisture

70

Roll speed

Feed rate

Field intensity

Splitter position

-1

-1

Mean of Yield

68

66 64

62 60 -1

1

-1

1

Moisture

1.7

-1

1

Roll speed

Feed rate

1

1

Field intensity

Splitter position

-1

-1

Mean of Fe2O3

1.6 1.5 1.4 1.3 1.2

-1

-1

Moisture

4.0

Mean of %-0.075mm

1

1

-1

Roll speed

1

Feed rate

1

1

Field intensity

Splitter position

-1

-1

3.5

3.0

2.5

2.0

1.5 -1

1

-1

1

-1

1

1

1

Fig. 7. Main effects plots on the response variables.

Table 6 Optimization results from the factorial experiments. Moisture content (%)

Magnetic field intensity (T)

Feed rate (kg/h·m)

Roll speed (rpm)

Splitter position (–)

Yield (%)

Fe2O3 (%)

% −75 µm

Desirability (–)

0.0 0.5

0.40 0.58

155 76

139 222

NMAG/MD NMAG

67.7 64.4

1.08 1.11

1.40 1.30

0.75 0.68

7

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Table 7 Slump values and additive dosages for the formulated concretes. Strength class

Fine aggregate

Designation

Additive dosage (%)

Slump (mm)

25 MPa 25 MPa 45 MPa 45 MPa

Natural Manufactured Natural Manufactured

NS-25 MFA-25 NS-45 MFA-45

0.6 0.4 0.6 0.4

155 170 160 165

crusher at coarser sizes presented nearly the same shape as the natural material. This agrees with results from Åkesson and Tjell (2010), which were obtained for granites that contained, however, small proportions of mica. On the other hand, particle shapes became more irregular for finer particle sizes, namely in the range below about 0.4 mm. The figure also presents results for the final product as well as for the magnetic product, which confirms the highly flaky shapes of the magnetic waste, with comparatively low aspect ratios, in particular at sizes coarser than about 0.1 mm. The benefit in increasing aspect ratio of the manufactured fine aggregate resulting from its removal was modest, but evident. This demonstrates that although magnetic separation removed a comparatively modest proportion of coarse particles, those particles were composed of highly flaky material, which have a detrimental effect in concrete. Concretes of two compressive strengths for each fine aggregate were prepared, hereafter named NS-25 and NS-45 for the concretes using natural sand, and MFA-25 and MFA-45 for concretes using manufactured fine aggregate with compressive strengths of 25 and 45 MPa, respectively. Slump values for the concretes are summarized in Table 7. All concretes presented good workability with no visible aggregate segregation and water exudation. Also, concretes using manufactured fine aggregated required 1/3rd less additive in order to reach the desired range of slump values. Fig. 10 summarizes results from compressive strengths of the concretes. A significant reduction in compressive strength is observed when using MFA in higher strength (45 MPa) concrete instead of NS at earlier ages. Nevertheless, it is possible to notice that, for both strength classes tested, concretes did not present significant differences in compressive strength after 28 days of curing. As such, one can state that the processing route proposed for the manufactured fine aggregate led to a concrete with equivalent mechanical properties after the expected curing time adopted in civil construction in Brazil, which is 28 days.

Fig. 8. Particle size distributions of natural sand, manufactured fine aggregate and as-received fine aggregate sample (feed). The dashed lines represent the tolerable limits and the dotted lines the optimal limits for the Brazilian standard (NBR 7211).

the optimization of the individual response variables. 3.4. Tests in concrete The feasibility of fully replacing the natural fine aggregate by a manufactured fine aggregate generated by a vertical impact shaft crusher and subsequently classified using a dry rare-earth magnetic separator was investigated. In summary, this test with the −2.36 mm material resulted in a yield (mass recovery) of 77.1%, with the nonmagnetic product containing 1.24% Fe2O3 and with 2.40% passing 75 µm. As described in 2.3, conditions used in this test were selected so as to more closely match the NBR7211 standard, which tolerates larger proportions of fines than reached in the optimization results in Table 6. This material has been combined to the unconcentrated + 2.36 mm material, giving a fineness modulus of 2.24. Fig. 8 presents the corresponding particle size distribution of this fine aggregate along with two different grading limits established by the Brazilian standard NBR7211. It shows that most of the size distribution of the manufactured fine aggregate is contained within the optimal limits established by the standard. This contrasts with the natural sand, with fineness modulus of 2.79, whose distribution falls largely outside them. Through this figure the beneficial role of magnetic separation in controlling the product size and the content of fines becomes also evident. Fig. 9 compares a shape descriptor (aspect ratio) for the different materials as a function of size. A comparison between the control material (natural sand) and the feed material (as-received −4.0 mm product of the VSI crusher) shows that the material produced by the VSI

3.5. Potential industrial application Although tests have been conducted at the bench scale, it is worthwhile analyzing the potential industrial application of the proposed process route. First, the process is carried out fully dry, and this is

Fig. 9. Comparison of aspect ratio (minor axis/major axis of particles) of the non-magnetic product (manufactured fine aggregate), the magnetic waste, the as-received feed and the natural sand samples below 2.36 mm.

Fig. 10. Compressive strengths of concretes containing manufactured fine aggregate (MFA) and natural sand (NS) for 7, 14 and 28 days of curing. 8

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Fig. 11. Schematic flowsheet of the proposed industrial circuit for production of manufactured fine aggregate from biotite-rich fine aggregate, containing the VSI crusher, two stages of screening and a single stage of REMS.

VSI crusher. The undersize of the screen is screened again with a 2.36 mm opening. This passing material is then fed in open circuit to the dry REMS, at a moisture content that would likely not exceed 0.5%, given the saturation moisture content of the material in the stockpile and the marginal ability of the VSI crusher in removing some moisture during its operation due to heat dissipation. Operating conditions would have to be varied in the presence of moist feeds, including a reduction in feed rate, an increase in roll velocity and the rejection of the middling material to the magnetic waste (Table 6). Such changes would result in a reduction in yield, but enable maintaining the product specification, in terms of biotite and fines in the non-magnetic product, constant. The final product for application in concrete would then be obtained by mixing the retained 4.00–2.36 mm from the second screening stage to the −2.36 mm non-magnetic product (Fig. 11). The yield of the process, in respect to the −4.0 mm material, is estimated to be in the order of 80% whenever the feed to the plant is dry. The proposed process flowsheet (Fig. 11) would be responsible for the generation of a magnetic waste, with a typical composition given in Table 8, and representing about 20% of the mass of −4.0 mm material. It is evident that this material is rich in fines (Fig. 6) and mafic minerals. Applications are being sought for this product, including as filler in polymers and rubber composites, rock fertilizers (slow release of potassium) and KCl from its acid leaching. This is work ongoing in the authors’ laboratory.

Table 8 Composition of the as-received material (−4.0 mm feed), final manufactured fine aggregate and magnetic waste (% mass). Composition (%)

Feed

Manufactured fine aggregate

Waste

SiO2 Al2O3 K2O Fe2O3 CaO TiO2 Others, including LOI % −75 µm

63.81 19.46 4.54 3.97 3.20 0.62 4.40 10.8

68.11 19.55 4.10 1.32 3.24 0.32 3.37 2.3

48.05 19.12 6.16 13.72 3.03 1.70 8.22 44.8

due to growing restrictions in the use of water in the minerals industry in Brazil in recent years. The proposed flowsheet involved in the production of the manufactured fine aggregate in question is presented in Fig. 11. The initial feed material, consisting of particles in the size range from 45 to 19 mm, resulting from earlier stages of crushing (primary, secondary and tertiary) in the aggregate plant, represents the feed to the proposed manufactured fine aggregate plant. The selection of this size range has been made on the basis of its saturation moisture, that is, the maximum moisture that the aggregate will contain after water being poured onto it, such as during strong rainfall, and subsequently drained. This value was measured as being 0.8% for the size range and material in question. Given this value, it is proposed that after the material is reclaimed from the stockpile all conveyor belts and equipment must be protected from direct exposure to rainfall, since further reduction in size would lead to higher capacity to retain surface moisture, which would be detrimental to both fine screening and dry magnetic separation operations. The material from the stockpile, which would remain exposed to rainfall, would feed the VSI crusher, which would operate in closed circuit with a multiple deck screen, with an option of returning the material coarser than 4.00 mm directly to the

4. Conclusions Dry REMS of the unclassified −2.36 mm material resulted in very good separation performance for the material contained in the 1.18–0.212 mm range, with limited upgrading in coarser sizes and well as low yields at finer sizes. Separation of the main deleterious minerals, namely biotite and hornblende, occurred efficiently, with recoveries of feldspars and quartz above 75% in the non-magnetic product. 9

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A comparison between results from bulk magnetic separation tests in the present work and those from the earlier work by the authors (Miceli et al., 2017), in which dry REMS was carried out with the material classified in four different size ranges, demonstrated that efficiency was higher for the bulk separation at sizes coarser than 0.212 mm, while recovery of the material contained in sizes finer than this was sacrificed. Fortunately, the lower yield at these finer sizes resulted in a beneficial reduction in the proportion of fines in the product and a classifying effect of magnetic separation, which is beneficial for application in concrete. Statistical analyzes of the factorial experiments demonstrated significant effects of all main variables studied on nearly all responses studied. Rotation frequency increased yield, while feed rate and magnetic field intensity had the opposite effect. Moisture content increased the non-magnetic product contamination with fines, whereas feed rate and magnetic field intensity reduced it. Moisture and rotation frequency increased Fe2O3 contamination in the non-magnetic product, while magnetic field intensity reduced. All interactions between variables were found to influence significantly at least one of the responses studied. Optimization results showed that different combinations of the set variables would allow maintaining the characteristics of the product when the moisture content raises up to 0.5%, with the only detrimental impact of reduction in 3% in yield of the product. The unconcentrated 4.00–2.36 mm material combined to the nonmagnetic −2.36 mm product was used in concrete and results compared to those obtained using natural sand. The resulting size distribution was nearly entirely within the optimal limits established by the Brazilian standard, whereas aspect ratio (shape) was similar to that of the natural sand. Full replacement of natural sand by manufactured fine aggregate resulted in concretes with the same compressive strength after 28 days of curing with one third less demand for rheology controlling additive.

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Acknowledgements The authors would like to thank Mineração Santa Luzia Ltda. for supporting the work, as well as the Brazilian Agencies CNPq (grant number 310293/2017-0), CAPES and FAPERJ (grant number E-26/ 203.024/2016) for partially funding the work. The authors also acknowledge the assistance of Ms. Jaqueline V. Oliveira from CETEM in the X-ray fluorescence analyses. References ABNT NBR 16697, 2018. Portland Cement – Requirements. Brazilian Association of Technical Standards, Rio de Janeiro, Brazil. ABNT NBR 5739, 2018. Concrete – Compression Test of Cylindrical Specimens. Brazilian Association of Technical Standards, Rio de Janeiro, Brazil. ABNT NBR 7211, 2007. Aggregates for Concrete. Brazilian Association of Technical Standards, Rio de Janeiro, Brazil. ABNT NBR NM 67, 1998. Concrete – Slump Test for Determination of the Consistency. Brazilian Association of Technical Standards, Rio de Janeiro, Brazil. Ahn, N.-S., 2000. Experimental study on the guidelines for using higher contents of aggregate micro fines in Portland cement concrete. Ph.D thesis. University of Texas at Austin. Ahrens, T.J., 1995. Rock Physics & Phase Relations: A Handbook of Physical Constants, second ed. American Geophysical Union, Washington, DC. Åkesson, U., Tjell, B., 2010. Geological parameters controlling the improvement of manufactured sand using vertical shaft impact crushers instead of cone crushers. In: XXV Int. Min. Proc. Congr., Brisbane, AUSIMM, pp. 1–12. Alp, I., 2009. Application of magnetic separation technology for the processing of a colemanite ore. J. S. Afr. Inst. Min. Metall. 108, 139–145. Anand, R.S., Reddy, D.V., 2014. Evaluation of suitability 1 of garnetiferous biotite gneiss for M-sand production – a case study. Int. J. Earth Sc. Eng. 7, 1655–1660. Anastasio, S., Fortes, A.P., Kuznetsova, E., Danielsen, S.W., 2016. Relevant petrological properties and their repercussions on the final use of aggregates. Energy Procedia 97, 546–553. ASTM C33/C33M, 2018. Standard Specification for Concrete Aggregates. ASTM

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