Factors Affecting the Upgrading of a Nickeliferous Limonitic Laterite Ore by Reduction Roasting, Thermal Growth and Magnetic Separation

minerals

Article Factors Affecting the Upgrading of a Nickeliferous Limonitic Laterite Ore by Reduction Roasting, Thermal Growth and Magnetic Separation

Filipe Rodrigues, Christopher A. Pickles *, John Peacey, Richard Elliott and John Forster

The Robert M. Buchan Department of Mining, Queens’s University, Kingston, ON K7L3N6, Canada; [email protected] (F.R.); [email protected] (J.P.); [email protected] (R.E.); [email protected] (J.F.) * Correspondence: [email protected]; Tel.: +1-613-533-2759

Received: 25 July 2017; Accepted: 19 September 2017; Published: 20 September 2017

Abstract: There is considerable interest in the development of new processes to extract the nickel from the oxidic nickeliferous laterite deposits, as the global nickel sulphide resources are rapidly becoming more difficult to access. In comparison to sulphide ores, where the nickel-containing mineral can be readily concentrated by flotation, nickel laterites are not amenable to significant upgrading, due to their complex mineralogy. In this paper, firstly, a brief overview of the conventional techniques used to process the nickeliferous limonitic laterites is given, as well as a review of current research in the area. Secondly, a thermodynamic model is developed to simulate the roasting process and to aid in the selection of process parameters to maximize the nickel recovery and grade and also to minimize the magnetite content of the concentrate. Thirdly, a two-stage process involving reduction roasting and thermal growth in either a tube furnace or a rotary kiln furnace, followed by magnetic separation, was investigated. Thermogravimetric, differential thermal and mineral liberation analyses techniques were utilized to further understand the process. Finally, the nickel grades and recovery results were compared to those available in the literature.

Keywords: laterites; limonite; nickel; reduction roasting; magnetic separation

1. Introduction Nickel metal is primarily employed in high nickel alloys and superalloys. Nickel utilization is increasing at a rate of about 4% per year [1]. Nickel is used in stainless steels, non-ferrous alloys and plating applications. In the earth’s crust, nickel can exist in either sulphidic or oxidic form. As a consequence of the declining worldwide nickel sulphide supply, there is increasing interest in the economic development of the nickeliferous laterite deposits, which represent about 59.5% of the known nickel resources [2]. The nickel laterite ores are mainly found in Australia, the Philippines, and Indonesia. These deposits are located close to the surface of the earth and therefore are mined using surface mining methods. Since the nickel oxide is in solid solution in the host rock, it is not feasible to produce a high grade concentrate using ﬂotation [3]. Additionally, it has been reported that the environmental damage associated with extracting the nickel from these oxide deposits is greater than that for sulphide deposits [4]. There are two main types of nickel laterite deposit: limonite and saprolite. These are separated by a layer of smectite clay. The nickel concentrations in these layers are roughly 1% to 1.6% for limonite, 2% to 5% for saprolite and 1% to 1.5% for smectite [5]. The upper limonitic layer of the deposit consists of mainly goethite ((Fe,Ni)O·OH) with some hematite (Fe2O3) and some magnesium silicates. These ores contain substantial amounts of water, and the nickel has traditionally been extracted via hydrometallurgical processes. Another approach for the processing of these ores for the

Minerals 2017, 7, 176; doi:10.3390/min7090176 www.mdpi.com/journal/minerals Minerals 2017, 7, 176 2 of 21 successful recovery of the nickel and potentially the cobalt would be to selectively reduce all the nickel and cobalt oxides and some of the iron oxide via low temperature solid state reduction to produce an intermediate product, known as ferronickel. This reduced product could be further upgraded downstream, by removing the unreduced oxides, so as to produce a concentrate, and magnetic separation could be considered as a suitable method for concentration. One problem in such a process, particularly with the limonitic ores, is the formation of magnetite, which will contaminate the ferronickel concentrate, thereby diluting the nickel grade. In the present paper, the existing industrial practices are discussed along with previous research performed on the reduction roasting and magnetic separation of the nickeliferous limonitic laterites. A thermodynamic model is utilized to predict the grade and recoveries of the ferronickel and also the grade of the concentrate. Experimental results obtained in both a stationary tube furnace and a rotary kiln are discussed and compared to the other results available in the literature. Thermogravimetric, differential thermal and mineral liberation analyses techniques were utilized to further understand the reduction process.

2. Current Industrial Practices Presently, there are three commercial processing routes for nickel laterite ores as shown in Figure1 and the method selected depends on the ore composition. Limonitic ores are typically processed by hydrometallurgical leaching methods in order to avoid the high costs associated with dehydrating, heating and smelting of a large volume of relatively low grade ore. In some cases, limonitic ores may be smelted in a blast furnace to produce nickel pig iron, although this is an energy intensive process, with a high volume waste stream, and generally yields low quality products [6]. The most common method for treating limonitic ores is High Pressure Acid Leaching (HPAL), although other leaching methods have been employed [7]. Nickel bearing ore is leached with sulphuric acid in an autoclave, followed by the recovery of nickel metal from the solution by electrowinning or hydrogen reduction. A high purity nickel metal can be produced by this method, and nickel recoveries are typically in the range of 90% to 92% [8]. While several of these processes have been implemented, operating costs have been high and production levels unsatisfactory [8,9]. In addition to the high acid consumption, the resultant waste stream is a signiﬁcant environmental concern [4]. The Caron process uses a combined pyro/hydrometallurgical approach to produce a high-grade ferronickel from limonite/saprolite blended feeds. Because of the low recoveries and high operating costs, the Caron process is generally considered to be uneconomic [1,10]. Each of these three processing options faces different challenges, although in general they are costly, energy intensive and produce a high volume waste stream. A low cost method of producing a nickel concentrate could signiﬁcantly reduce the operating costs of the current nickeliferous laterite processing methods. Minerals 2017, 7, 176 3 of 21 Minerals 2017, 7, x FOR PEER REVIEW 3 of 24

LIMONITE

Nickel Pig Iron HPAL Caron Process

High Pressure Reduction Drying Acid Leach Roasting

CCD & Ammoniacal Sintering Neutralization Leaching

Purification & Purification & BF Smelting Recovery Recovery

Ni Pig Iron Mixed Ni & Co Ni Carbonate

Sulphide or

76 Hydroxide

77 Figure 1: Simplified flowsheet of the current commercial production methods for the extraction of Figure 1. Simpliﬁed ﬂowsheet of the current commercial production methods for the extraction of 78 nickel from the nickeliferous limonitic laterite ores, adapted from previous work by Dalvi et al. [1]. nickel from nickeliferous limonitic laterite ores, adapted from previous work by Dalvi et al. [1]. 79 3. Previous Experimental Research 803. Previous3.1. Reduction Experimental Roast and Research Upgrade 81 3.1. Reduction Roast and Upgrade 82 Various degrees of success in selectively reducing and upgrading limonitic ores at the experimental 83 Variousbench scale degrees level of have success been inachieved. selectively De reducingGraaf [11] andshowed upgrading that selective limonitic reduction ores at of the limonite experimental to

84bench the scale magnetite/wustite level have been phase achieved. boundary De Graaf was [ 11possible] showed using that a selective CO/CO2 reduction and H2/H of2O limonite reducing to the 85magnetite/wustite atmosphere. The phase presence boundary of water was possiblevapour in using the reducing a CO/CO atmosphere2 and H2/H was2O reducingfound to atmosphere.have a 86The presencedeleterious of effect water on vapour the extraction in the reducingof nickel. Using atmosphere hydrogen was and found carbon to monoxide have a deleterious as reductants effect in on the extraction of nickel. Using hydrogen and carbon monoxide as reductants in both a tube furnace 87 both a tube furnace and a fluidized bed, Antola et al. [12] were able to produce a high grade (70 to and a fluidized bed, Antola et al. [12] were able to produce a high grade (70% to 80% Ni) product 88 80% Ni) product by solid state selective reduction. Pure metallic nickel could not be produced by by solid state selective reduction. Pure metallic nickel could not be produced by selective reduction, 89 selective reduction as the nickel was always found in a nickel‐iron alloy phase. Low partial pressures as the nickel was always found in a nickel-iron alloy phase. Low partial pressures of reducing gas with 90 relativelyof reducing short processing gas with relatively times provided short processing the optimum times provided nickel grade. the optimum Varying nickel the temperaturegrade. Varying in the 91range the of 700temperature to 1000 ◦ inC hadthe range little effectof 700 onto 1000°C the reduction had little selectivity effect on althoughthe reduction reduction selectivity proceeded although faster 92at higherreduction temperatures. proceeded Reduction faster at higher occurred temperatures. five to seven Reduction times occurred faster in thefive fluidizedto seven times bed comparedfaster in to the tube furnace. The authors proposed that the ferronickel could be magnetically separated. Zevgolis et al. [13] reviewed the physicochemical properties of the reduction roasting of Greek laterite ores. The researchers concluded that the reduction degrees of the iron and nickel oxides in Greek laterites between 700 to 900 ◦C could not exceed 33% and 76%, respectively, even though adequate amounts of carbon remained in the calcine. The reduction was finished after 20 to 40 min due Minerals 2017, 7, 176 4 of 21

to the formation of iron silicates such as fayalite (2FeO·SiO2) and forsterite (Mg2SiO4), which coated the oxide grains, impeding further reduction. Valix and Cheung [14] studied the phase transformations of laterite ores under reducing conditions and temperatures up to 800 ◦C. When limonite ores were reduced, the mineral phases formed were magnetite, taenite and fayalite. Taenite is an iron-nickel alloy mineral containing 20% to 65% nickel. X-Ray Diffraction (XRD) patterns showed that taenite becomes more dominant as the temperature increased from 650 to 750 ◦C, but upon further heating, the taenite disappeared. On cooling the reduced limonite from 800 to 25 ◦C in air, there were no evident changes in the phases, suggesting the stability of the phases formed at higher temperatures. They also conducted leaching tests using an ammoniacal leach and achieved an optimum nickel recovery of 80% at 600 ◦C. Kawahara et al. [15] investigated the reducibility of laterite ores using hydrogen over a temperature range of 400 to 1000 ◦C. The reducibility was based on the degree of reduction of the oxides of iron, nickel and cobalt to their metallic forms. The nickel reduction degree varied from 19% to 70%, and the reduction process was concluded after 40 min. The reducibility of nickel oxide increased with increasing temperature and the iron content of the ore. The magnesia and silica in the 2+ 2+ ore combined together to form olivine ((Mg,Fe,Ni)2SiO4), and Ni was substituted for Mg in the silicate matrix. The activity of nickel oxide in the silicate solid solution is low, and this explained the poor reducibility using gaseous reductants. The effect of precalcination on the reducibility of the limonitic ore in a H2/CO2 atmosphere was studied by O’Connor et al. [16]. The function of precalcination was to dehydroxylate goethite. During calcination, the goethite became more porous and the surface area of the ore increased, exposing the reduced nickel phase for recovery via an ammoniacal leach solution. Under reducing conditions, the goethite was dehydroxylated to hematite, and then reduced to magnetite at 500 ◦C. The formation of taenite and fayalite simultaneously occurred at this temperature. Precalcination produced recrystallized hematite, which was less susceptible to reduction and this resulted in reduced ferronickel formation. Utigard and Bergman [17] found that with pretreatment of the ore in air or nitrogen followed by reduction in an H2/CO2 atmosphere, the limonite ore was easier to reduce to the metallic state. Valix and Cheung [14] found that a prolonged reduction period resulted in over-reduction of limonite, resulting in decreased nickel recovery. Reoxidation of the reduced wustite phase to magnetite impeded the effectiveness of magnetic separation to remove the gangue. Chander and Sharma [18] found that cooling in an inert atmosphere prevented reoxidation to magnetite. The reoxidation was limited to the surface layers of the sample. Quenching the samples in either water or air resulted in extensive reoxidation.

3.2. Effect of Sulphur A number of studies have been performed on the effects of sulphur-containing additives on the reduction and/or recovery of nickel from nickeliferous laterite ores. Sulphur has been added in various forms such as: elemental sulphur, pyrite or pyrrhotite or sodium sulphate. In general, it has been shown that sulphur additions are beneﬁcial in terms of both improved nickel grade and nickel recovery. A number of researchers have concluded that the addition of elemental sulphur promotes the growth of ferronickel particles during the selective reduction of the saprolite ores [19–22]. With regards to the addition of elemental sulphur, it would be expected that as the limonitic ore is heated, the elemental sulphur is converted to sulphur gas and a large proportion would escape. However, a fraction would react with hematite to form pyrite (FeS2) as follows:

Fe2O3 + 2.75S2(g) = 2FeS2 + 1.5SO2(g) (1) or under reducing conditions:

Fe2O3 + 2.5S2(g) + CO(g) = 2FeS2 + SO2(g) + CO2(g) (2) Minerals 2017, 7, 176 5 of 21

At higher temperatures, the pyrite decomposes to pyrrhotite (FeS) as follows:

FeS2 = FeS + S2(g) (3)

In a patent by Diaz et al. [23], where the sulphur additions to a limonitic ore were in the range of 2% to 4%, it was postulated that the ferronickel particles form via a Fe-S-O liquid phase. This liquid phase could facilitate the agglomeration of the ferronickel particles. In many cases, the ferronickel particles are found associated with an FeS-rich phase, possibly indicating the ferronickel particles precipitated on cooling [19,20,22–24]. Also, it has been shown that when sulphur is added, the ferronickel particles are larger [20,22]. Since some iron is sulphidized, the iron content of the ferronickel alloy is reduced and consequently, the nickel grade is increased [25]. From a kinetic perspective, the release of the gaseous sulphur could open up the mineral structure and facilitate dehydroxylation [26]. Also, this would increase the particle surface area available for gaseous reduction. The ﬁssures/voids that remain after sulphur volatilization could provide nucleation sites for ferronickel particle growth. It has also been suggested that sulphur can reduce the surface tension of the ferronickel particles, enhancing the agglomeration process [27]. Valix and Cheung [21] reported an increase in the nickel recovery from 29% to 81% for a precalcined limonitic ore initially containing 5 wt % sulphur. Thus, although sulphur additions to limonitic ores have been shown to lead to larger ferronickel particles, improved recoveries and higher grades, there is little consensus on the mechanism or mechanisms by which these improvements are achieved.

3.3. Thermodynamic Considerations Of interest in the reduction roasting and the concentration of the limonitic laterites are the nickel grades and recoveries in the ferronickel. Furthermore, if the ferronickel is concentrated by magnetic separation, then the behavior of magnetite needs to be considered. From a thermodynamic perspective, it would be expected that the major factors affecting the grade, the recovery and the amount of magnetite, would be as follows: the ore composition and any additives, the reduction temperature, the type of reducing agent and the amount of reducing agent per unit of ore. Previously, a thermodynamic model has been developed for the limonitic ores [28], and it is utilized here to determine the inﬂuence of variables on the grades and recoveries and in particular, the amount of magnetite, for the particular ore composition utilized in this research. A ferronickel phase containing some cobalt was assumed to form, while the other condensed species were considered to be part of the oxide phase. Activity coefﬁcient data available in the literature for various species were utilized as described previously [28–30]. The ore was mainly composed of goethite, but for the purposes of the calculation, the ore was assumed to be dewatered, and therefore the iron content was reported as 91.88% hematite (Fe2O3). The nickel oxide (NiO) content was 1.62% and the minor impurities were silica (SiO2), magnesia (MgO) and alumina (Al2O3). Figure2 shows the calculated nickel recovery as a function of the amount of carbon added and also the reaction temperature. The carbon addition is represented as kg of carbon per 100 kg of ore. At low carbon additions (less than 1.5 kg/100 kg) and across the whole temperature range, metallic nickel was not formed, as the carbon was utilized to reduce the hematite to magnetite. As the carbon addition approached 2 kg/100 kg, metallic nickel began to be recovered. For all carbon additions, metallic nickel did not form until about 400 ◦C. However, for the lower carbon additions, reoxidation could occur at higher temperatures, which lowered the nickel recovery. Above about 600 ◦C and for carbon additions of over about 4 kg/100 kg, nickel recoveries approaching 100% were readily achieved. Figure3 shows the nickel grade in the ferronickel alloy as a function of the amount of carbon added and also the reaction temperature for the same conditions as in Figure2. The nickel grade was inversely proportional to the amount of metallic iron in the ferronickel. Since NiO was less stable than FeO, then metallic nickel should be selectively reduced, to some degree, at reduction potentials which were less reducing than those for the formation of metallic iron. Again, at temperatures below about Minerals 2017, 7, 176 6 of 21 Minerals 2017, 7, 176 6 of 21

Consequently,400 ◦C and carbon over additions both a range below of carbon about 1.5 additions kg/100 and kg,there a range was of no temperatures, metallic nickel. nickel However, grades ascould the approachconditions 95%. became However, more reducing these conditions in terms of were both theshort-lived temperature as metallic and carbon iron additions,was produced then thein increasingnickel grades amounts rose quite with dramatically,both increasing to temperatur values approachinge and carbon 95%. additions. Consequently, By inspection over both of Figures a range 2of and carbon 3, it additionscan be seen and that a rangeboth high of temperatures, grades and high nickel recoveries grades could can be approach achieved 95%. at above However, 600 °C. these For theseconditions reasons, were 600 short-lived °C was used as metallic as the primary iron was roas producedting temperature in increasing in amountsthis work. with Above both 600 increasing °C, the nickeltemperature recoveries and approached carbon additions. 100%, but By inspectionthe nickel grade of Figures dropped2 and with3, it canincreasing be seen temperature that both high to a relativelygrades and constant high recoveries value of 3% can to be 5%. achieved at above 600 ◦C. For these reasons, 600 ◦C was used as the primaryThe behaviour roasting of temperature magnetite is in shown this work. in Figure Above 4, 600again◦C, as the a function nickel recoveries of temperature approached and carbon 100%, additions.but the nickel From grade room dropped temperature with increasingupwards, the temperature amount of to magnetite a relatively increased constant as value the hematite of 3% to was 5%. reducedThe and behaviour reached of amagnetite maximum is at shown about in200 Figure °C for4, all again carbon as a additions. function of Subsequently, temperature andthe amount carbon ofadditions. magnetite From decreased room temperature as wustite began upwards, to form the amountwith increasing of magnetite temperature increased and as thethe hematiteamount was essentiallyreduced and independent reached a maximum of the amount at about of 200carbon◦C for added all carbon until additions.about 600 Subsequently,°C, where the theamount amount of carbonof magnetite had a decreasedstrong influence. as wustite At beganhigher to temper form withatures, increasing the amount temperature of magnetite and thedecreased amount with was increasingessentially independentcarbon addition of the and amount for any of carbongiven addedtemperature, until about the amount 600 ◦C, where of magnetite the amount was of linearly carbon dependenthad a strong on inﬂuence. the carbon At higheraddition. temperatures, A carbon ad thedition amount of about of magnetite 7 kg/100 decreased kg corresponded with increasing to the stoichiometriccarbon addition requirement and for any for given the reduction temperature, of allthe the amountNiO to Ni of metal magnetite and all was of linearlythe Fe2O dependent3 to FeO as givenon the by carbon the following addition. reactions: A carbon addition of about 7 kg/100 kg corresponded to the stoichiometric requirement for the reduction of all the NiO to Ni metal and all of the Fe2O3 to FeO as given by the NiO + C = Ni + CO(g) (4) following reactions: FeNiO2O3 + C = 2FeO Ni + CO+ CO(g)(g) (5)(4) At high temperatures and high carbon additions, the magnetite is reduced to wustite and Fe2O3 + C = 2FeO + CO(g) (5) metallic iron, while at lower temperatures, only wustite forms in decreasing amounts with decreasing temperature.At high These temperatures results indicate and high that carbon the amount additions, of magnetite the magnetite is a complex is reduced function to wustite of both andthe carbonmetallic addition iron, while and at lowerthe temperature. temperatures, A onlycomparison wustite formsof Figures in decreasing 2–4 shows amounts that withif intermediate decreasing temperaturestemperature. are These utilized results to indicateachieve thatboth thehigh amount nickel ofrecoveries magnetite and is agrades, complex then function the amount of both of magnetitethe carbon is addition also high. and Additionally, the temperature. even if A the comparison amount of ofmagnetite Figures2 formed–4 shows at high that iftemperatures intermediate is small,temperatures on cooling, are utilizedthe wustite to achieve and metallic both highiron nickelrevert recoveriesto magnetite and under grades, equilibrium then the conditions. amount of Consequently,magnetite is also the high. amount Additionally, of magnetite even in if thethe amountreduced of product magnetite would formed be athigh high and temperatures essentially independentis small, on cooling, of the reduction the wustite conditions. and metallic Therefore, iron revert to minimize to magnetite the underamount equilibrium of magnetite conditions. at room temperatureConsequently, it thewould amount be necessary of magnetite to quench in the th reducede reduced product material would and/or be highcool andin a essentiallycontrolled atmosphere.independent Also, of the reoxidation reduction could conditions. occur at Therefore, room temperature to minimize and the therefore amount in of the magnetite present research, at room sampletemperature analysis it wouldwas performed be necessary as quickly to quench as possible. the reduced material and/or cool in a controlled atmosphere. Also, reoxidation could occur at room temperature and therefore in the present research, sample analysis was performed as quickly as possible.

100

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(

R 60

V O

C 40

i 1200 N ) 20 1000 o C ( 800 E R U 0 600 T 9 A 8 400 R 7 E 6 P 5 200 M 4 E CA T RB 3 0 ON ( 2 kg/10 1 0kg) Figure 2. EffectsEffects of temperature and carbon additions on the nickel recovery for the limonitic ore.

Minerals 2017, 7, 176 7 of 21 Minerals 2017, 7, 176 7 of 21 Minerals 2017, 7, 176 7 of 21

100 100

) 80

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(

( L

E E

A 60

D N

G 40

N i

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F E

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0 o O C F 20 200 ) o (

O 20 200 C 400 E ( 400 R 600 U E 0 T R 600 A U 0 10 800 R T E A 10 8 800 R E 8 6 1000 MP 1000 E CA 6 4 T MP RBO 1200 E CAR N (k 4 2 T BON g/100 2 1200 (kg/1 kg) 00kg ) FigureFigure 3. EffectsEffects of of temperature temperature and carbon additions on the nickel grade of the limonitic ore. Figure 3. Effects of temperature and carbon additions on the nickel grade of the limonitic ore.

0.35 0.35

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3 0.15 F e 0.15 0 F 0 ) 200 o C 0.10 ( ) 200 o C 0.10 400 E ( 400 R 600 U E 0.05 T R 600 A U 0.05 2 800 R T E A 2 4 800 P R E 4 6 1000 MP 6 1000 E CA 8 T M RBO 8 1200 E CAR N (k 10 T BON g/100 10 1200 (kg/1 kg) 00kg ) Figure 4. Effects of temperature and carbon additions on the amount of magnetite for the limonitic ore. FigureFigure 4. 4.EffectsEffects ofof temperature temperature and and carbon carbon additions additions on on the the amount amount of of magnetite magnetite for the limonitic ore. Figure 5 shows the nickel grade of the simulated concentrate as a function of the amount of Figure 5 shows the nickel grade of the simulated concentrate as a function of the amount of carbonFigure added5 shows and the the reduction nickel grade temperature. of the simulated Here concentrate, the concentrate as a function was assumed of the amount to consist of carbon of the carbon added and the reduction temperature. Here, the concentrate was assumed to consist of the ferronickeladded and thealloy reduction plus magnetite. temperature. Again Here, at low the temp concentrateeratures wasand assumedlow carbon to consistadditions, of the there ferronickel was no ferronickel alloy plus magnetite. Again at low temperatures and low carbon additions, there was no nickelalloy plusproduced. magnetite. Above Again about at low400 temperatures°C, the nickel and grade low carbonincreased additions, rapidly. there At relatively was no nickel low nickel produced. Above about 400 °C, the nickel grade increased rapidly. At relatively low temperaturesproduced. Above and/or about carbon 400 additions,◦C, the nickel the amounts grade increased of wustite rapidly. and metallic At relatively iron were low low, temperatures while the temperatures and/or carbon additions, the amounts of wustite and metallic iron were low, while the amountand/or carbonof magnetite additions, was high, the amounts which lowered of wustite the grad ande. metallic On the other iron were hand, low, at high while carbon the amount additions of amount of magnetite was high, which lowered the grade. On the other hand, at high carbon additions and/ormagnetite temperatures, was high, which the amount lowered of the metallic grade. Oniron the was other high, hand, which at high again carbon lowered additions the and/orgrade. and/or temperatures, the amount of metallic iron was high, which again lowered the grade. Consequently,temperatures, the the amount nickel grade of metallic of the iron simulated was high, conc whichentrate again was lowered relatively the uniform grade. Consequently, over most of the Consequently, the nickel grade of the simulated concentrate was relatively uniform over most of the reductionnickel grade conditions of the simulated and varied concentrate in the range was of relatively about 1.5% uniform to 2%. over most of the reduction conditions reduction conditions and varied in the range of about 1.5% to 2%. and varied in the range of about 1.5% to 2%.

Minerals 2017, 7, 176 8 of 21 Minerals 2017, 7, 176 8 of 21

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FigureFigure 5. 5.Effects Effects of of temperaturetemperature and and carbon carbon additions additions on the on grade the grade of the ofmagnetic the magnetic concentrate concentrate (Fe + Ni + Co + Fe3O4) for the limonitic ore. (Fe + Ni + Co + Fe3O4) for the limonitic ore.

4. Experimental methods 4. Experimental methods

4.1.4.1. Raw Raw Materials Materials The as-received ore was dried at 100 °C for seven days to ensure the removal of free water. Two The as-received ore was dried at 100 ◦C for seven days to ensure the removal of free water. kilograms of the ore were crushed and separated into eight representative samples of approximately Two kilograms of the ore were crushed and separated into eight representative samples of 180 g using a riffle splitter. Subsequently, the ore was pulverized in a ring pulverizer for 15 s and approximatelythen screened 180 tog a using size passing a rifﬂe splitter.100 mesh Subsequently, (150 μm). Oversized the ore wasmaterial pulverized was repulverized in a ring pulverizer for an for 15additional s and then 5 s then screened rescreened to a size until passing one hundred 100 mesh percent (150 ofµ them). sample Oversized was minus material 100 wasmesh. repulverized In order for anto maintain additional the 5ore s then in a dry rescreened condition until prior one to use hundred in the reduction percent experiments, of the sample the was screened minus samples 100 mesh. In orderwere tostored maintain at 65 °C the in ore a drying in a dry oven. condition prior to use in the reduction experiments, the screened samplesThe were chemical stored analysis at 65 ◦C of in the a drying dried limonitic oven. ore is provided in Table 1. The dried ore had a nickel oxideThe content chemical of 1.48%, analysis a very of thehigh dried iron content limonitic (58.71%) ore is and provided low concentrations in Table1. of The magnesia dried ore(MgO), had a nickelsilica oxide (SiO content2) and alumina of 1.48%, (Al a very2O3), highwhich iron are content typical (58.71%)of the nick andeliferous low concentrations limonitic ores. of magnesiaX-Ray Diffraction (XRD) analysis identified goethite (FeO·OH) as the major mineral phase present in the (MgO), silica (SiO2) and alumina (Al2O3), which are typical of the nickeliferous limonitic ores. X-Raylimonitic Diffraction ore. On (XRD) a dehydroxylated analysis identiﬁed basis, the goethite calculated (FeO ·ironOH) oxide as the (Fe major2O3) and mineral nickel phase oxide present (NiO) in contents were 91.88% and 1.62%, respectively. the limonitic ore. On a dehydroxylated basis, the calculated iron oxide (Fe2O3) and nickel oxide (NiO) contents were 91.88% and 1.62%, respectively. Table 1. Analysis of the dried limonitic ore.

Species NiO Table 1. CoOAnalysis Total of the Fe dried limonitic SiO2 ore. MgO Al2O3 (wt %) 1.48 0.045 58.71 2.15 0.22 2.92 Species NiO CoO Total Fe SiO2 MgO Al2O3 For the(wt experiments, %) 1.48 the limonitic 0.045 ore was mech 58.71anically 2.15 mixed with 0.22 predetermined 2.92 amounts of bituminous coal and sulphur. The composition of the bituminous coal is given in Table 2. Reagent grade elemental sulphur was used. In order to promote both gas-solid and solid-solid reactions, the reagentsFor the and experiments, the ore were the compacted limonitic together. ore was mechanicallyThe ore, coal and mixed sulphur with mixture, predetermined with a total amounts mass of bituminousof 3 g, was coal briquetted and sulphur. at a pressure The composition of 55.2 MPa of the into bituminous 1.3 cm diameter coal is cylinders given in Tablewith 2a .height Reagent of grade1.0 elementalcm. Initially, sulphur a single was used.briquette In order was utilized. to promote Howeve bothrgas-solid in later tests, and three solid-solid briquettes reactions, were combined the reagents andto the give ore a weretotal sample compacted mass together. of 9 g and The five ore, briquettes coal and were sulphur combined mixture, to give with a sample a total mass mass of of about 3 g, was briquetted15 g. The at control a pressure briquette of 55.2 contained MPa into 6% 1.3 bituminous cm diameter coal cylindersand 4% sulphur. with a height of 1.0 cm. Initially, a single briquette was utilized. However in later tests, three briquettes were combined to give a total sample mass of 9 g and ﬁve briquettes were combined to give a sample mass of about 15 g. The control briquette contained 6% bituminous coal and 4% sulphur. Minerals 2017, 7, 176 9 of 21 Minerals 2017, 7, 176 9 of 21 Minerals 2017, 7, 176 9 of 21

TableTable 2.2. ChemicalChemical analysisanalysis of the bituminous coal. Table 2. Chemical analysis of the bituminous coal.

ComponentComponent Total Total Carbon Carbon Fixed Fixed CarbonCarbon Volatiles Volatiles Moisture Moisture Ash Ash Sulphur Sulphur (%) 73.0 51.6 36.6 1.2 7.9 2.3 (%)(%) 73.073.0 51.6 36.6 36.6 1.2 1.2 7.9 7.9 2.3 2.3

4.2. Stationary Tube Furnace Experiments 4.2.4.2. Stationary Stationary TubeTube Furnace Furnace ExperimentsExperiments A schematic diagram of the tube furnace utilized for the thermal upgrading of the ore is shown AA schematic schematic diagramdiagram ofof thethe tubetube furnacefurnace utilized for the thermal upgrading upgrading of of the the ore ore is is shown shown in Figure 6. The process comprised of two isothermal stages: roasting at 600 °C followed by thermal in Figure 6. The process comprised of two isothermal stages: roasting at 600 °C◦ followed by thermal inparticle Figure 6growth. The process at either comprised 1000 or 1100 of two °C. isothermalA quartz tube stages: in a roastinghorizontal at resistance 600 C followed furnace bywas thermal used particle growth at either 1000 or 1100◦ °C. A quartz tube in a horizontal resistance furnace was used particleas the reaction growth atchamber. either 1000 The orsample 1100 wasC. A placed quartz in tube a quartz in a horizontal boat, which resistance was lined furnace with magnesium was used as theas reactionthe reaction chamber. chamber. The The sample sample was was placed placed in a quartzin a quartz boat, boat, which which was linedwas lined with with magnesium magnesium oxide oxide powder to minimize the reaction of the sample with the boat. The furnace was then sealed at powderboth ends to minimize and purged the reactionwith high of purity the sample nitrogen with gas the boat.at a flow The rate furnace of 300 was mL/min then sealed throughout at both endsthe andboth purged ends and with purged high purity with high nitrogen purity gas nitrogen at a ﬂow gas rate at ofa flow 300 mL/min rate of 300 throughout mL/min throughout the experiment. the experiment. After initial purging, the hot zone temperature was increased by 20 °C/min to 600 °C and After initial purging, the hot zone temperature was increased by 20 ◦C/min to 600 ◦C and the boat the boat containing the sample was introduced into the hot zone and reacted for 60 min. containing the sample was introduced into the hot zone and reacted for 60 min. Subsequently, the Subsequently, the hot zone temperature was ramped at 20 °C/min to reach the particle growth hot zone temperature was ramped at 20 ◦C/min to reach the particle growth temperature of 1000 or temperature of 1000 or 1100 °C. The samples were held at the growth zone temperature for 60 min 1100 ◦C. The samples were held at the growth zone temperature for 60 min then withdrawn to the cool then withdrawn to the cool zone for 40 min. The temperature of the cooled sample ranged from 60 to zone for 40 min. The temperature of the cooled sample ranged from 60 to 80 ◦C. Magnetic separation 80 °C. Magnetic separation and subsequent analysis were performed immediately after removing the and subsequent analysis were performed immediately after removing the processed samples from the processed samples from the furnace in order to minimize reoxidation. furnace in order to minimize reoxidation.

Figure 6. Schematic diagram of the tube furnace. FigureFigure 6.6. SchematicSchematic diagram of the tube furnace. 4.3. Rotary Kiln Experiments 4.3.4.3. Rotary Rotary KilnKiln ExperimentsExperiments In order to obtain larger and more representative samples for the subsequent magnetic InIn order order to to obtain obtain larger larger and moreand more representative represen samplestative samples for the subsequentfor the subsequent magnetic separationmagnetic separation tests, experiments were performed in a rotary kiln, which is shown in Figure 7. tests,separation experiments tests, experiments were performed were inperformed a rotary kiln,in a rotary which kiln, is shown which in is Figure shown7. in Figure 7.

Figure 7. Schematic diagram of the rotary kiln. Figure 7. Schematic diagram of the rotary kiln. Figure 7. Schematic diagram of the rotary kiln.

Minerals 2017, 7, 176 10 of 21

The kiln rotated at a constant speed of 2 rpm. Two separate resistance furnaces were placed around the kiln to establish both the reduction zone at 600 ◦C and the growth zone at 1000 ◦C. Nitrogen was introduced into the kiln at a rate of 1.5 L/min in order to maintain a neutral atmosphere. A charge consisting of one hundred of the 3 g briquettes (300 g in total) was placed inside a quartz capsule and inserted into the kiln. The capsule had openings at both ends to allow both the purge gas to pass through and the reaction gases to escape. Initially, the capsule was held in the cooling zone at the front of the furnace while the temperature of the reaction zone was ramped up to the set point temperature. The capsule was then moved into the reaction zone for 60 min and then into the growth zone for another 60 min. After completion of the two-stage process, the capsule was withdrawn into the cool zone until it could be safely handled.

4.4. Magnetic Separation Tests Magnetic separation tests were performed to test the viability of upgrading the processed samples to a ferronickel concentrate. Prior to magnetic separation, the processed samples were pulverized using a ring pulverizer to enhance liberation of the reduced material. The samples were pulverized for 20 s and then passed through a 200 mesh (74 µm) screen. Oversized material was repulverized for an additional 5 s then rescreened. Water was added to the minus 200 mesh material prior to magnetic separation to produce a slurry. Initial magnetic separation test work was performed using a Davis Tube Tester (DTT). In the DDT tests, the sample mass was 10 g and the magnetic field strength was fixed at 4800 G at various water flowrates and rotation speeds. In order to able to vary the magnetic field intensity, additional magnetic separation studies were conducted using Wet High Intensity Magnetic Separation (WHIMS). Outokumpu Technology’s (Espoo, Finland) Carpco WHIMS Model 3x4L (Serial number 227-05) was utilized and here the magnetic field intensity could be varied from 0 to 17.5 kG. The magnetic media used was 6.4 mm iron spheres, and this matrix media provide a large surface area, which is more suited for collecting small magnetic particles and also minimizes entrapment of non-magnetic material.

4.5. Analytical Techniques The chemical compositions of the samples were determined using wet analytical techniques. Total digestion of the samples was performed using aqua regia and the solutions were analyzed for nickel and iron using a Thermo Fisher Scientiﬁc (Waltham, MA, USA) iCE 3000 Series Atomic Absorption Spectroscopy (AAS) unit. Powder X-Ray Diffraction (XRD) was used to characterize the mineralogy of the raw materials and selected processed samples. An Xpert Pro Phillips powder diffractometer (Almelo, The Netherlands) with Cu kα radiation linked with the commercial software X-pert High Score was utilized. Scanning Electron Microscopy (SEM) was performed using a Mineral Liberation Analysis (MLA) 650 FED Environmental SEM (Hillsboro, OR, USA). Back scattered electron imaging was the primary mode used to analyze the samples. Energy dispersive X-Ray spectroscopy (EDS) helped verify the chemical composition of the mineral phases detected. Thermogravimetric and Differential Thermal Analysis (TGA/DTA) studies were conducted using a Netzsch STA 449 F3 Jupiter system (Selb, Germany). The samples were processed in 5 mL alumina crucibles. In order to simulate the actual reduction experiments, samples were ramped at 20 ◦C/min or 50 ◦C/min then held at a reduction temperature of 600 ◦C for 60 min and then ramped at 20 ◦C/min to a thermal growth temperature of 1000 or 1100 ◦C for 60 min. A neutral atmosphere was maintained by using nitrogen as the carrier gas at ﬂowrates of 50 to 150 mL/min.

5. Results and Discussion

5.1. Thermogravimetric and Differential Thermal Analysis Figure8 shows the TGA results for the samples containing 6% coal and 4% sulphur for a reduction temperature of 600 ◦C and thermal growth temperatures of either 1000 or 1100 ◦C. If all of the iron Minerals 2017, 7, 176 11 of 21 of the iron in the ore was present as goethite, then the mass loss due to dehydroxylation would be about 9.5%. Up to 600°C, the mass loss was much higher at about 15%, and this additional 5.5% in the ore were present as goethite, then the mass loss due to dehydroxylation would be about 9.5%. mass loss could◦ mainly be attributed to the evaporation of the majority of the sulphur and some reductionUp to 600. DuringC, the the mass isothermal loss was hold much at higher 600°C at, there about was 15%, only and a this small additional mass loss, 5.5% typically mass loss less could mainly be attributed to the evaporation of the majority of the sulphur and some reduction. than 1%. Considerable mass loss and◦ thus reduction occurred during the second heating stage up to During the thermal the isothermal growth holdtemperature at 600 sC,, particularly there was only for a th smalle sample mass heatedloss, typically to 1100 less°C than. This 1%. was attributedConsiderable to the mass presence loss and of thus residual reduction reductant. occurred For during the the sample second at heating 1000°C stage, additional up to the thermalreduction growth temperatures, particularly for the sample heated to 1100 ◦C. This was attributed to the presence occurred during the initial stages of the isothermal hold. For the sample at 1100°Cthere was little of residual reductant. For the sample at 1000 ◦C, additional reduction occurred during the initial stages mass loss during the hold, however, there was an overall additional 5% mass loss and this was of the isothermal hold. For the sample at 1100 ◦C there was little mass loss during the hold; however, indicative of considerable over-reduction and excessive formation of metallic iron. As a major there was an overall additional 5% mass loss, and this was indicative of considerable over-reduction research objective was to avoid the formation of significant metallic iron, most of the roasting and excessive formation of metallic iron. As a major research objective was to avoid the formation tests were performed with the lower thermal growth temperature of 1000°C. of signiﬁcant metallic iron, most of the roasting tests were performed with the lower thermal growth temperature of 1000 ◦C.

Thermal Growth at 1000oC 5 Thermal Growth at 1100oC

1000oC 600oC 10 or 1100oC

20 MASS LOSS (%) LOSS MASS 25

35 0 20 40 60 80 100 120 140 160 180 200 TIME (mins) ◦ FigureFigure 8: TGA 8. TGA results results of of mass loss loss versus versus time time for reduction for reduction at 600 C at and 600 thermal°C and growth thermal at 1000 growth or at ◦ 1000 or1100 1100C for°C afor sample a sample containing containing 6% coal and 6% 4% coal sulphur. and 4% sulphur.

FigureFigure 9 shows9 shows both boththe TGA the TGA and andDTA DTA curves, curves, where where the the sample sample was was again again heated heated to 600 °C◦C for 60for mins 60 min and and then then heated heated to to a a thermal thermal growth growth temperature temperature of of 1000 1000◦°CC,, but but in in this this case, case excess excess sulphursulphur and and coal coal were added added so so as as to amplify to amplify their their effects. effects. A mixture A mix of limonite,ture of coal limonite, and sulphur coal and sulphurwas used were with employed a mass ratio with of a 2:1:1.mass Again,ratio of the 2:1:1 majority. Again, of the the mass majority loss occurredof the mass during loss the occurred two duringheating the stages. two heating During stages the first. During heating the stage, first the heating mass lossstage, was the about mass 27%, loss and was this about was due27% to and thisdewatering, was due the to removaldewatering, of some the sulphur remova andl of some some reduction. sulphur andAgain, some at the reduction. roasting temperature Again, at the roastingof 600 ◦ temperatureC, the mass loss of was 600 only°C, theabout mass 1%, with loss the was majority only about of this 1%, occurring with within the majority the first of ten this occurringminutes. within During the heating first fromten minutes. 600 to 1000 During◦C, a furtherheating mass from loss 600 of aboutto 1000 2%°C was, a observed.further mass Another loss of aboutmass 2% loss of was about obs 1%erved. occurred Another at the thermalmass loss growth of abouttemperature 1% occurred of 1000 ◦C at and the subsequently thermal growth the temperaturesample mass of remained 1000°C constant.and subsequently The DTA curvethe sample showed mass two endothermicremained constant. peaks: (1) The one DTA at about curve showed296 ◦C, two corresponding endothermic to thepeaks: very (1) large one mass at lossabout due 296 to°C the, dehydroxylationcorresponding to of goethite,the very (2)large and mass a losssecond due atto 490 the◦ C dehydroxylation corresponding to of the goethite, decomposition (2) and of pyrite a second to pyrrhotite. at 490°C During corresponding cooling of the to the sample, an exothermic peak was observed at 908 ◦C. In the Fe-FeS system, the melting temperature of ◦ the eutectic composition is 988 C at 1 atm, and thus14 this peak could be due to the solidification of the proposed Fe-O-S melt. This would support the proposal by Diaz et al. [23] that a Fe-S-O liquid forms, which facilitates the growth of the ferronickel particles. It should be noted that DTA evidence of melting or partial melting of the sample was not observed during the heating stage. decomposition of pyrite to pyrrhotite. During cooling of the sample, an exothermic peak was observed at 908°C. In the Fe-FeS system, the melting temperature of the eutectic composition is 988°C at 1 atm and thus this peak could be due to the solidification of the proposed Fe-O-S melt. This would support the proposal by [25] that a Fe-S-O liquid forms, which facilitates the growth of the ferronickel particles. It should be noted that no prior DTA evidence of melting or partial melting of the sample was observed during the heating stage. Minerals 2017, 7, 176 12 of 21

0 2.0 o o 490oC 600 C 1000 C

5 1.5 296oC 1.0 10 DTA 0.5 15

0.0 V/mg) 20 o 908 C -0.5 ( DTA

MASS LOSS (%)LOSS MASS 25 TGA -1.0

30 -1.5

35 -2.0 0 20 40 60 80 100 120 140 160 180 200 TIME (mins)

Figure 9. TGA/DTA results for reduction at 600 ◦C and thermal growth at 1000 ◦C for a mixture of Figure 9limonite,: TGA/DTA coal and results sulphur for with reduction a mass ratio at of 600°C 2:1:1. and thermal growth at 1000°C for a mixture of limonite, coal and sulphur with a mass ratio of 2:1:1. 5.2. Magnetic Separation Using Davis Tube Tester (DTT) 4.2. Magnetic Separation using Davis Tube Tester Several preliminary tests were performed using the DTT to magnetically separate the concentrate. SeveralThe effect preliminary of sulphur tests additions, were performed particle growth using zonethe Davis temperature Tube Tester and the to difference magnetically between separate the theas-received concentrate. ore and The calcined effect of ore sulphur (at 600 ◦ C) additions, were investigated. particle growth However, zone there temperature was no significant and the differencedifference between between the the as results.-received There ore wereand calcined very little ore tailings (at 600 with°C) 90% were of inve thestigated. feed reporting However, as theremagnetics was no to significant the concentrate. difference The average between analyses the resul of thets concentrate. There were from very these little tests tailings were as with follows: 90% of 1.35%the feed Ni andreporting 77.2% Fe.as Themagnetics nickel and to ironthe concentrate. recoveries were The 87.0% average and 89.9%, analyses respectively. of the concentrate The high frompercentage these tests magnetics, were as the follows: low nickel 1.35% grades Ni and and the 77.2% high iron Fe. recoveries The nickel indicated and iron that separationrecoveries was were 87.0%not efficient.and 89.9%, Similar respectively problems were. The encountered high percentage by Iwasaki magnetics, et al. [31 the], who low used nickel X-Ray grades Diffraction and the highanalysis iron of recoveries the roasted indicated material to that confirm separation that the was presence not of efficient a small. amountSimilar of problems magnetite were in encounteredthe wustite by was [33] sufficient who used to preventX-Ray efficientDiffraction magnetic analysis separation. of the roasted Thus, inmaterial the current to confirm research, that theall presence subsequent of magnetica small amount separation of testsmagnetite were performed in the wustite using WHIMS. was sufficient to prevent efficient magnetic5.3. Magnetic separation. Separation Thus Using, in Wetthe current High Intensity research, Magnetic all subsequent Separation (WHIMS) magnetic separation tests were performed using WHIMS. Figure 10 shows the percent magnetics as a function the magnetic field strength for the WHIMS 4.3.tests. Magnetic The percent Separation magnetics using increased Wet from High about Intensity 20% at Mag 2 kGnetic to about Separation 60% at 14 kG. These values were considerably lower than the 90% magnetics obtained with the DTT, and this indicates improved Figureseparation. 10 shows Figure the 11 percenta,b show magnetics the effect as of magnetica function field the strength magnetic on field the nickel strength grade for and the recovery WHIMS tests.and The the ironpercent grade magnetics and recovery, increased respectively. from At about low magnetic 20% at field2 kG strengths to about of about60% at 2 kG, 14 thekG. nickel These valuesgrade were was about considerably 4%, and this lower indicated than considerable the 90% magneticsconcentration obtained of the nickel. with As the the magneticDTT, and field this strength increased, the nickel grade decreased rapidly and at much higher magnetic field strengths of about 14 kG, the grade was similar to that of the as-received ore. At 2 kG, the nickel recovery was about 65% and decreased somewhat to about 55%15 at 14 kG. For the case of iron, the grade decreased only slightly with increasing magnetic field strength and had an average value of 79.9%. On the other hand, the iron recovery increased rapidly from about 10% at 2 kG to about 70% at 14 kG. These results indicated that that the highest nickel grades and recoveries were achieved at low field strengths. As the field strength increased, more iron was recovered, and therefore, the nickel grade decreased. Consequently, in subsequent tests a field strength of 3 kG was utilized. Minerals 2017, 7, 176 13 of 21 Minerals 2017, 7, 176 13 of 21 Minerals 2017, 7, 176 13 of 21

100 100

80 80

60 60

MAGNETICS (%) 40 MAGNETICS (%) 20 20

0 0 2 4 6 8 10 12 14 16 0 0 2MAGNETIC 4 6 FIELD 8STRENGTH 10 (kG) 12 14 16 MAGNETIC FIELD STRENGTH (kG) Figure 10. Percent magnetics as a function of magnetic field strength for WHIMS. FigureFigure 10. 10.Percent Percent magnetics magnetics as as a a function function of of magnetic magnetic ﬁeld field strength strength for for WHIMS. WHIMS.

5 100

4 80

3 60

2 40

2 40 NICKEL GRADE (%) NICKEL RECOVERY(%)

NICKEL GRADE (%) 1 20 NICKEL RECOVERY(%) 1 20

0 0 0246810121416 0 0 0246810121416MAGNETIC FIELD STRENGTH (kG) MAGNETIC FIELD(a )STRENGTH (kG) (a)

100 80

80 60 80 60 60

60 40

40 40

IRON GRADE (%) GRADE IRON 40

20 IRON RECOVERY (%) IRON GRADE (%) GRADE IRON 20 20 IRON RECOVERY (%) 20

0 0 0246810121416 0 0 0246810121416MAGNETIC FIELD STRENGTH (kG) MAGNETIC FIELD(b) STRENGTH (kG) (b) Figure 11.11. ((a)) EffectEffect ofof magneticmagnetic ﬁeldfield strengthstrength onon nickelnickel gradegrade andand recovery;recovery; ((b)) EffectEffect ofof magneticmagnetic ﬁeldfieldFigure strengthstrength 11. (a on)on Effect ironiron gradeofgrade magnetic andand recovery.recovery. field strength on nickel grade and recovery; (b) Effect of magnetic field strength on iron grade and recovery.

FigureMinerals 122017 shows, 7, 176 the effect of sulphur additions on the percent magnetic material. Initially,14 of 21 the amount of magnetics increased only slowly with sulphur but then more quickly as the sulphur Minerals 2017, 7, 176 14 of 21 addition increased. Figures 13(a) and (b) show the effect of sulphur on the nickel grade and Figure 12 shows the effect of sulphur additions on the percent magnetic material. Initially, the recoveryFigure and 12 theshows iron the gradeeffect of and sulphur recovery, additions respectively. on the percent Sulphur magnetic additions material. promotedInitially, the both increasedamount ofnickel magnetics grades increased and recoveries. only modestly In a with manner sulphur similar but then to morethe percent intensively magnetics, as the sulphur both the additionamount of increased. magnetics Figure increased 13a,b only show modestly the effect with of sulphur sulphur but on then the more nickel intensively grade and as recovery the sulphur and nickeladdition grade increased. and the Figures nickel 13a,b recoveries show the first effect increased of sulphur slowly on the with nickel nickel grade addition and recovery and andthen the more quickly.the iron The grade iron and recovery recovery, also respectively. initially Sulphur increased additions slowly promoted with sulphur both increasedadditions nickel and then grades more andiron recoveries.grade and recovery, In a manner respectively. similar to Sulphur the percent additions magnetics, promoted both both the nickelincreased grade nickel and grades the nickel and rapidly. Consequently, the increased iron recovery limited the increase in grade of the recoveriesrecoveries. ﬁrst In increaseda manner modestly similar to with the sulphur percent addition magnetics, and thenboth more the intensively.nickel grade The and iron the recovery nickel concentrate, despite the increasing nickel recovery. The iron grade showed only a slight increase alsorecoveries initially first increased increased modestly modestly with with sulphur sulphur additions addition and then and more then intensively. more intensively. Consequently, The iron the with sulphur addition. These results show that increasing sulphur additions result in increased increasedrecovery also iron recoveryinitially increased limited the modestly increase inwith the sulphur grade of additions the concentrate, and then despite more the intensively. increasing nickel and iron recoveries but the increase is higher for nickel than iron and thus the nickel grade nickelConsequently, recovery. the The increased iron grade iron showed recovery only limited a slight the increase increase in with the grade sulphur of the addition. concentrate, These despite results increases. The increase in iron grade with increasing sulphur would suggest improved separation showthe increasing that increasing nickel sulphurrecovery. additions The iron result grade in showed increased only nickel a slight and increase iron recoveries with sulphur but the addition. increase ofisThese the higher gangue results for nickelconstituents.show thanthat increasing iron, and thussulphur the nickeladditions grade result increases. in increased The increase nickel and in iron iron grade recoveries with increasingbut the increase sulphur is wouldhigher suggestfor nickel improved than iron, separation and thus of the the nickel gangue grade constituents. increases. The increase in iron grade with increasing sulphur would suggest improved separation of the gangue constituents. 60

MAGNETICS (%) MAGNETICS 20

0 0 1 2 3 4 5 SULPHUR (S ) ADDITION (wt%) 2 FigureFigure 12: Effect12. Effect of of sulphur sulphur additions on on the the percent percent magnetic magneti material.c material.

3.0 80

2.5

2.0

1.5 40

1.0 NICKEL GRADE (%)

20 RECOVERYNICKEL (%)

0.5

0.0 0 012345 18 SULPHUR (S) ADDITION (wt%) (a)

Figure 13. Cont.

Minerals 2017, 7, 176 15 of 21 Minerals 2017, 7, 176 15 of 21

100 80

80 60

40 IRON GRADE (%) GRADE IRON

20 IRON RECOVERY (%) 20

0 0 012345

SULPHUR (S) ADDITION (wt%) (b)

Figure 13. ((aa)) EffectEffect ofof sulphursulphur additionsadditions onon nickel nickel grade grade and and recovery recovery at at a magnetica magnetic field field strength strength of of3 kG;3 kG; (b) Effect(b) Effect of sulphur of sulphur additions additions on the on iron the grade iron grade and recovery and recovery at a magnetic at a magnetic field strength field strength of 3 kG. of 3 kG. Figure 14a,b show the MLA results for the cumulative percent passing as a function of particle Figures 14a,b show the MLA results for the cumulative percent passing as a function of particle size for various nickel grade ranges, for the briquettes containing 6% coal and 6% coal plus 4% sulphur, size for various nickel grade ranges, for the briquettes containing 6% coal and 6% coal plus 4% respectively and Figure 15 shows the cumulative 80% passing size (d80) versus the nickel grade ranges sulphur, respectively and Figure 15 shows the cumulative 80% passing size (d80) versus the nickel for the two cases. For both the sulphurless and the sulphur-containing samples, it can be seen that grade ranges for the two cases. For both the sulphurless and the sulphur-containing samples, it can ferronickel particles are produced with widely varying nickel grades and as the particle size decreases be seen that ferronickel particles are produced with widely varying nickel grades and as the particle then the nickel grade increases. This would indicate that as the particles grow then the nickel grade size decreases then the nickel grade increases. This would indicate that as the particles grow then the decreases, suggesting that a major factor affecting particle growth is both the formation of additional nickel grade decreases, suggesting that a major factor affecting particle growth is both the formation metallic iron and incorporation into the growing ferronickel particles. For the sulphurless sample, the of additional metallic iron and incorporation into the growing ferronickel particles. For the largest particles are observed for the grade range of 0% to 2% Ni, with a maximum size of 53 µm and sulphurless sample, the largest particles are observed for the grade range of 0 to 2% Ni, with a a d80 of about 9 µm. For the higher nickel grade ranges, the curves are significantly shifted to much maximum size of 53 μm and a d80 of about 9 μm. For the higher nickel grade ranges, the curves are lower particle sizes, with both the curves and the d80 values shifting slightly to lower sizes as the significantly shifted to much lower particle sizes, with both the curves and the d80 values shifting grade increases. The range of d80 values for these grades was very narrow and varied from about 3.5 slightly to lower sizes as the grade increases. The range of d80 values for these grades was very narrow to 2.5 µm. Thus, magnetic separation of this material would preferentially remove the larger particles, and varied from about 3.5 to 2.5 μm. Thus, magnetic separation of this material would preferentially which would have a nickel grade of less than 2%. For the sample containing sulphur, although the remove the larger particles, which would have a nickel grade of less than 2%. For the sample maximum size for the 0% to 2% Ni range curve has shifted from 53 µm to a lower particle size of 38 µm, containing sulphur, although the maximum size for the 0% to 2% Ni range curve has shifted from 53 the d80 value has increased from 9 to 18 µm. Also, the d80 values for the other nickel grade curves have μm to a lower particle size of 38 μm, the d80 value has increased from 9 to 18 μm. Also, the d80 values shifted from the range of 3.5 to 2.5 µm for the sulphurless samples to larger values of 17 to 8.5 µm. for the other nickel grade curves have shifted from the range of 3.5 to 2.5 μm for the sulphurless Thus, in contrast to the sulphurless sample, the curve for the 0% to 2% nickel grade range is more samples to larger values of 17 to 8.5 μm. Thus, in contrast to the sulphurless sample, the curve for the closely associated with the other curves. These larger particle sizes for the sulphur-containing sample 0% to 2% nickel grade range is more closely associated with the other curves. These larger particle would be expected to lead to improved recovery and higher grades during magnetic separation. sizes for the sulphur-containing sample would be expected to lead to improved recovery and higher Figure 16 shows an X-ray diffraction pattern of a sample reduced at 600 ◦C and held for grades during magnetic separation. thermal growth at 1100 ◦C. The species detected were wustite (FeO), kamacite (FeNi with Ni ≤ 10%), Figure 16 shows an X-ray diffraction pattern of a sample reduced at 600 °C and held for thermal fayalite (Fe SiO ), magnesioferrite (Mg(Fe3+) O ) and pyrrhotite (FeS). The main constituent was growth at 11002 °C.4 The species detected were wustite2 4 (FeO), kamacite (FeNi with Ni ≤ 10%), fayalite wustite, demonstrating that the conditions were sufficiently reducing so as to minimize the formation (Fe2SiO4), magnesioferrite (Mg(Fe3+)2O4) and pyrrhotite (FeS). The main constituent was wustite, of magnetite. The detection of kamacite was indicative of the presence of the ferronickel alloy. Fayalite demonstrating that the conditions were sufficiently reducing so as to minimize the formation of formed as a result of the reaction of some of the iron oxide with silica and also, some of the iron oxide magnetite. The detection of kamacite was indicative of the presence of the ferronickel alloy. Fayalite reacts with magnesium oxide to form magnesioferrite, which is magnetic. Pyrrhotite was detected formed as a result of the reaction of some of the iron oxide with silica and also, some of the iron oxide which demonstrates that the majority of the sulphur is present as FeS. reacts with magnesium oxide to form magnesioferrite, which is magnetic. Pyrrhotite was detected which demonstrates that the majority of the sulphur is present as FeS.

Minerals 2017, 7, 176 16 of 21

100

40 Minerals 2017, 7, 176 16 of 21 30 0 to 2% Ni in FeNi 20 2 to 5% Ni in FeNi 5 to 7.5% Ni in FeNi 7.5 to 10% Ni in FeNi

CUMULATIVE PERCENT PASSING (%) PERCENT CUMULATIVE 10 10 to 15% Ni in FeNi 0 2 1 0 53 45 38 32 27 22 19 16 9.6 8.1 6.8 5.7 4.8 4.1 3.4 2.9 2.4 1.2 13.5 11.4 1.75 1.45 0.87 0.73 0.62 0.52 μ SIZE ( m) (a)

100 0 - 2 % Ni in FeNi 90 2 - 5 % Ni in FeNi 5 - 7.5 % Ni in FeNi 80 7.5 - 10 % Ni in FeNi (a) 10 - 15 % Ni in FeNi 70 100 60 0 - 2 % Ni in FeNi 90 2 - 5 % Ni in FeNi 5 - 7.5 % Ni in FeNi 50 80 7.5 - 10 % Ni in FeNi 10 - 15 % Ni in FeNi 40 70

30 60

50 20 40

CUMULATIVE PERCENT PASSING (%) PERCENT CUMULATIVE 10 30 0 20 2 1 0 38 32 27 22 19 16 9.6 8.1 6.8 5.7 4.8 4.1 3.4 2.9 2.4 1.2 13.5 11.4 1.75 1.45 0.87 0.73 0.62 0.52

CUMULATIVE PERCENT PASSINGCUMULATIVE PERCENT (%) 10 SIZE (μm) 0 2 1 0 38 32 27 22 19 16 9.6 8.1 6.8 5.7 4.8 4.1 3.4 2.9 2.4 1.2 13.5 11.4 (b) 1.75 1.45 0.87 0.73 0.62 0.52 SIZE (μm) (b) Figure 14. (a) Results of the MLA studies showing the cumulative percent passing as a function of particle sizeFigure for various 14. (a) Results nickel of gradethe MLA ranges studies forshowing the briquettethe cumulative containing percent passing 6% coal;as a function (b) Results of of the particle size for various nickel grade ranges for the briquette containing 6% coal; (b) Results of the MLA studies showing the cumulative percent passing21 as a function of particle size for various nickel MLA studies showing the cumulative percent passing as a function of particle size for various nickel grade rangesgrade for ranges the briquette for the briquette containing containing 6% 6% coal coal plus plus 4% sulphur. sulphur.

18 6% coal 6% coal, 4% sulphur 16

12 m) μ 10 ( 80 d 8

0 0-2 2-5 5-7.5 7.5-10 10-15 NICKEL GRADE RANGE (%)

Figure 15. Effect of nickel grade range on the cumulative 80% passing size (d80) as a function of the nickel Figure 15. Effect of nickel grade range on the cumulative 80% passing size (d ) as a function of grade range for the sample containing 6% coal and the sample containing 6% coal plus 4% sulphur.80 the nickel grade range for the sample containing 6% coal and the sample containing 6% coal plus 4% sulphur. Minerals 2017, 7, 176 17 of 21 Minerals 2017, 7, 176 17 of 21 Minerals 2017, 7, 176 17 of 21 3500 W: Wustite 3500 W K: Kamacite 3000 W:F: FayaliteWustite W K:M: Kamacite Magnesioferrite 3000 F:P: Fayalite Pyrrhotite 2500 M: Magnesioferrite K P: Pyrrhotite 2500 2000 W K 2000 W W 1500 COUNTS W 1500 COUNTS 1000 W W F F P M 1000 F M W 500 W F F P M F M 500 0 010 20406080100 30 50 70 90 0 θ ( ) 010 20406080100 302 degrees 50 70 90 2θ (degrees) FigureFigure 16. X-Ray 16. X-Ray Diffraction Diffraction pattern pattern of a reduced of a reduced briquette briquette at 600 ◦atC with600 °C a thermal with a growththermal temperature growth temperature of 1100 °C. of 1100Figure◦C. 16. X-Ray Diffraction pattern of a reduced briquette at 600 °C with a thermal growth temperature of 1100 °C. 5.4. Nickel Grade and Recovery 5.4. Nickel Grade and Recovery 5.4. NickelTypically, Grade inand both Recovery the tube furnace and the rotary kiln tests, the nickel recoveries were only about 65%,Typically, and therefore, in both additional the tube magnetic furnace andseparation the rotary tests were kiln performed tests, the on nickel the tailings recoveries to determine were only Typically, in both the tube furnace and the rotary kiln tests, the nickel recoveries were only about aboutif improved 65%, and nickel therefore, recovery additional could be magnetic achieved. separationThe metal grades tests wereand recoveries performed for onthethe rotary tailings kiln to 65%, and therefore, additional magnetic separation tests were performed on the tailings to determine determinetests are if shown improved in Figure nickel 17a,b. recovery A rougher could and be two achieved. scavenger The stages metal were grades performed. and recoveries In the rougher for the if improved nickel recovery could be achieved. The metal grades and recoveries for the rotary kiln rotarystage, kiln the tests nickel are and shown iron grades in Figure were 17 4.31%a,b. Aand rougher 85.1%, respectively. and two scavenger In the first stages scavenger were stage, performed. the tests are shown in Figure 17a,b. A rougher and two scavenger stages were performed. In the rougher nickel recovery to the concentrate increased by 14.8% and in the second by 5.4% to give an overall In thestage, rougher the nickel stage, and theiron nickel grades andwere iron 4.31% grades and 85.1%, were respectively. 4.31% and 85.1%,In the first respectively. scavenger stage, In the the first nickel recovery of 86.5%. The nickel grade of the final concentrate was 3.2% and contained 80.9% iron. scavengernickel recovery stage, the to nickelthe concentrate recovery increased to the concentrate by 14.8% and increased in the bysecond 14.8% by and5.4% in to the give second an overall by 5.4% The overall iron recovery to the final concentrate was 27.9%, and the iron grade was 80.9%. Somewhat to givenickel an recovery overall of nickel 86.5%. recovery The nickel of grade 86.5%. of Thethe final nickel concentrate grade of was the 3.2% final and concentrate contained was80.9% 3.2% iron. and similar results were obtained for the tube furnace tests as shown in Figure 18a,b. The nickel grades containedThe overall 80.9% iron iron. recovery The to overall the final iron concentrate recovery was to 27.9%, the final and concentratethe iron grade was was 27.9%,80.9%. Somewhat and the iron of the rougher concentrate and the final concentrate were 4.16% and 3.87%, respectively. Meanwhile, gradesimilar was results 80.9%. were Somewhat obtained similar for the tube results furnace were tests obtained as shown for thein Figure tube furnace18a,b. The tests nickel as grades shown in the iron grades of the rougher concentrate and the final concentrate were both 90.3%. The nickel Figureof the 18 roughera,b. The concentrate nickel grades and ofthe the final rougher concentrate concentrate were 4.16% and and the 3.87%, final concentrate respectively. were Meanwhile, 4.16% and recoveries of the rougher concentrate and the final concentrate were 64.2% and 83.6%, respectively. 3.87%,the respectively.iron grades of Meanwhile, the rougher theconcentrate iron grades and of the the final rougher concentrate concentrate were both and the90.3%. final The concentrate nickel The corresponding iron recoveries were 26.0% and 39.8%. Thus, in order to obtain nickel recoveries recoveries of the rougher concentrate and the final concentrate were 64.2% and 83.6%, respectively. wereof both about 90.3%. 85% The to nickelthe final recoveries concentrate of the and rougher nickel concentrategrades in the and range the final of concentrate3.2% to 3.8%, were three 64.2% The corresponding iron recoveries were 26.0% and 39.8%. Thus, in order to obtain nickel recoveries and 83.6%,concentration respectively. stages were The correspondingrequired. Under ironthese recoveries conditions, were 30% 26.0%to 40% and of the 39.8%. iron was Thus, recovered in order to of about 85% to the final concentrate and nickel grades in the range of 3.2% to 3.8%, three obtainwith nickel iron recoveriesgrades in the of final about concentrate 85% to the of final 80% concentrateto 90%. and nickel grades in the range of 3.2% to 3.8%,concentration three concentration stages were stages required. were Under required. these Under conditions, these 30% conditions, to 40% of 30% the toiron 40% was of recovered the iron was with iron grades in the final concentrate of 80% to 90%. recovered with iron grades in5 the final concentrate of 80% to 90%. 100 Ni (%) Fe (%) 5 100 4 80 Ni (%) Fe (%) 4 80 3 60

3 60 2 40 IRON GRADE (%) NICKEL GRADE (%) 2 40 1 20 IRON GRADE (%) NICKEL GRADE (%)

1 20 0 0 Rougher Scavenger 1 Scavenger 2 Overall Tails in conc. 0 0 Rougher Scavenger 1 Scavenger(a) 2 Overall Tails in conc.

(a)

Figure 17. Cont.

Minerals 2017, 7, 176 18 of 21 Minerals 2017, 7, 176 18 of 21

100 Ni (%) 90 Fe (%) 80

20 NICKEL ORNICKEL IRON RECOVERY (%) 10

0 Rougher Scavenger 1 Scavenger 2 Overall Tails to conc.

(b)

FigureFigure 17. 17.(a )(a Nickel) Nickel and and iron iron grades grades in in the the concentrates concentrates and and tails tails for for the the rotary rotarykiln kiln tests;tests; ((bb)) NickelNickel andand iron iron recoveries recoveries in in the the concentrates concentrates and and tails tails for for the the rotary rotary kiln kiln tests. tests.

5 100

Ni (%) Fe (%) 4 80

3 60

2 40 IRON GRADE (%) NICKEL GRADE (%)

1 20

0 0 Rougher Scavenger 1+2 Overall Tails in conc.

(a) (a)

90 90 80 Ni (%) 80 FeNi (%)(%) Fe (%) 70 70

6060

5050

4040

3030

20 20 NICKEL OR IRONRECOVERY (%)

NICKEL OR IRON RECOVERY (%) 10 10 0 0 Rougher Scavenger 1+2 Overall Tails Rougher Scavenger 1+2to Overallconc. Tails to conc.

(b(b) ) CONCLUSIONS FigureFigure 18. 18.(a ()a Nickel) Nickel and and iron iron grades grades in in the the concentrates concentrates and and tails tails for for the the tube tubefurnace furnace tests;tests; ((bb)) NickelNickel andand iron iron recoveries recoveries in in the the concentrate concentrate and and tails tails for for25 the the tube tube furnace furnace tests. tests.

Minerals 2017, 7, 176 19 of 21

6. Conclusions (1) A thermodynamic model was developed to describe the carbothermic reduction roasting process of a limonitic laterite ore using HSC Chemistry 7.1 and was utilized to predict the grade of the ferronickel alloy and the nickel recovery to the ferronickel alloy. The grade and recovery were studied as a function of the carbon addition and reduction temperature. Since it was proposed to utilize a reduction roast followed by magnetic separation to concentrate the nickel, then the effects of operating parameters on the formation of magnetite were investigated. Nickel recoveries approaching 100% and nickel grades from 3% to 5% could be achieved at temperatures above 600 ◦C. A significant amount of magnetite formed at high temperatures and on cooling, reversion occurred. Thus, either quenching or rapid cooling under a controlled atmosphere was recommended. (2) TGA/DTA studies were performed on the ore plus carbon plus sulphur mixture for a temperature profile, which consisted of heating to two isothermal steps, one for reduction at 600 ◦C for 60 min and another for particle growth at 1000 or 1100 ◦C for 60 min, followed by cooling. The TGA curves showed that the majority of the mass loss occurred during heating to 600 ◦C, and this was attributed to dehydroxylation, evaporation of sulphur and some reduction. Also, some mass loss and therefore reduction occurred during heating to 1000 ◦C. There was only a small amount of mass loss during the two isothermal stages. The DTA curve showed three peaks: (1) an endothermic peak at about 296 ◦C, corresponding to a very large mass loss due to the dehydroxylation of goethite, (2) an endothermic peak at 490 ◦C corresponding to the decomposition of pyrite to pyrrhotite, and (3) an exothermic peak at 908 ◦C, which could be due to the solidification of the proposed Fe-O-S melt. (3) Magnetic separation using the Davis Tube Tester was not successful with most of the nickel, and also most of the iron reporting to the magnetics and thus the nickel grade was low. Improved results were obtained using Wet High Intensity Magnetic Separation. The nickel grades and recoveries decreased with increasing field strengths, but at low kG in the range of 2 to 4, nickel grades in the range of 3% to 4% and nickel recoveries in the range of 60% to 70% could be achieved. (4) The effects of sulphur additions on the grade and recovery of both nickel and iron were investigated. The addition of sulphur significantly improved the nickel recovery, and also there was a slight improvement in grade. Similarly, the iron recovery increased but was not as pronounced as that for nickel and the iron grade increased only slightly. This demonstrated improved separation of the gangue constituents. Mineral Liberation Analysis of the reaction product demonstrated that with the addition of sulphur, the ferronickel particles were larger and had a higher grade, which would explain the improvements in recovery and grade in the magnetic separation process. (5) Although high nickel grades could be achieved in the concentrate with one pass through the WHIMS, the nickel recovery was low and in the range of 60% to 70%. Therefore, second and third scavenger stages were performed on the tailings. For the tube furnace tests, the final nickel grade was 3.87% and the nickel recovery was 83.6%. Similar results were obtained in the rotary kiln tests with a nickel grade of 3.2% and recoveries of 86.5%. (6) It is recommended that future research be directed towards optimizing the additives usage as well as the separation processes.

Acknowledgments: The authors wish to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) and XSTRATA for their support of this research. In addition, Kingston Process Metallurgy (KPM) is acknowledged for the use of their rotary kiln. The authors thank William Anthony for his assistance in preparing the manuscript. Author Contributions: Filipe Rodrigues, Richard Elliott and John Forster performed the experimental work. John Peacey was the principal supervisor of the project. Christopher A. Pickles performed the analysis of the results and the interpretation of the data. Conﬂicts of Interest: The authors declare no conﬂict of interest. Minerals 2017, 7, 176 20 of 21

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