Chemistry and Kinetics (TG/DTA-MS) of Metals Carbothermical Reduction

Chemistry and kinetics (TG/DTA-MS) of metals carbothermical reductionin the FeS-Ni3S2-CaO system

E. Selivanov, R. Gulyaeva

Institute of Metallurgy, Ural Branch of RussianAcademy of Sciences,

101, Amundsen St., Yekaterinburg, 620016 Russia, 620016

Absract

The metals reduction processes by carbon and its monoxide in the FeS-Ni3S2-CaO system under continuous heating conditions up to 1250 оC have been studied by means of thermogravimetric analysis together with differential thermal that as well as mass spectrometry of the gases. During heating in an inert atmosphere in FeS-CaO and FeS-Ni3S2-CaO systems along with the sulfide phases the formation of CaFeSO and Ca3Fe4S3O6oxysulfides has been revealed. The carbothermical reduction chemistry of FeS, Ni3S2 and F4.5Ni4.5S8 sulfides together with calcium oxide has been established. It has been shown the reduction products are calcium sulfide and metals, the calcium oxide insertion preventing sulfurous gassing. For temperature range 750–1170 оC the experimental data on metals carbothermal reduction in FeS-Ni3S2-CaO system are denoted by one-stepAvrami-Erofeev model with a limited stage of the formation and growth of nucleuses the and kinetics parameters equaled Е=520 kJ/mole, logA=18.2 s-1, n=0.87. It has been shown the reduction of nickel sulfides by a carbon and its oxide is activated by means of the insertion to the reaction mixture composition of iron sulfide along with calcium oxide thatcontributes the formation of intermediate iron-calcium oxysulfide compounds having a higher reactivity.

Introduction

The phase transformations in the calcium-containing oxide-sulfide systems are of interest to understand the processes proceeded during pyrometallurgical processing of the sulfide ores and non-ferrous metals concentrates. At the same timethe phase transformations proceeded during heating of iron and non-ferrous metals sulfides together with a calcium oxide have been insufficientlystudied. Availabledataintheliteraturedealt with mainly the double systems [1, 2]. Thus V.I.Yarygin et al. [1] has presented data about the influence оf calcium oxide on iron, copper, zinc and lead sulfides on fusibility. The phase equilibriums in Fe-Ca-S-O system and carbothermical reduction of pyrrotite in the presence of lime have been considered in the articles [2, 3]. During the reduction of sulfides of metals the insertion of calcium oxide is known to contribute to bindingand preventing of sulfur evaluation. The possibility of the metals reduction by graphite or coke from nickel sulfides (Ni3S2 and NiS) in the mixture with CaO under isothermal conditions (800 – 1124 оC) have been shown by authors of works [4, 5]. The influence of iron sulfide additions to Ni3S2 – CaO mixture on a chemistry of the metals reduction during a non-isothermal heating is of interest to estimate.

According to data [6, 7] the reduction of iron from MeS-CaO mixtures in compared with nickel proceeds at reduced temperatures, with larger rate and larger incompleteness. One of the reasons of above is a formation of the iron – calcium oxysulfide phases during heating of FeS with CaO whereas nickel sulfide does not form the similar compounds [8]. It can assume additions of FeS to Ni3S2 – CaO mixture will be allowed the reduction processes to be intensified both owing to the interaction of the sulfides followed by iron-nickel formation and decreasing in melting temperature of the forming eutectic in the sulfide and oxysulfide systems.

The work subject was to study the phase transformations in FeS-CaO and FeS-Ni3S2-CaO systems during heating in an inert (argon) and reduction (monoxide carbon) atmosphere and to determine chemistry and the kinetics parameters of the metals reduction processes by carbon.

Experimental

To estimation the transformations proceeding during heating of reaction mixture NETZSCH STA 449 C Jupiter thermal-analyzer has been used. The experiments have been carries out using a sensor for a differential scanning calorimetry (DSC) in platinum crucibles under conditions of continuous heating up to 920 оC and followed by cooling up to 200 оC at 10 оC min-1 rate in a argon flow (70 cm3 min-1). The experiments on carbothermal reduction of the metals from the sulfides have been performed during continuous heating of the samples (about 50 mg) to 1200 - 1250 оC at 10 - 30 оC min-1 rates in an argon flow (60 cm3 min-1) either the gaseous mixture of Ar and CO (60 – 80 vol%) or argon (at reduction by graphite). Herewith the sample holder for differential thermal analysis (DTA) has been used. QMS 403C Aêolos quadrupole mass spectrometer coupled with a thermal analyzer has been used analyzing the composition of the gaseous phase for the experiments on carbothermal reduction. The calculation of the kinetics parameters of the carbothermal reduction and the estimation of the thermodynamical possibility of the reactions has been conducted by means of NETZSCH Thermokinetics 3.0 and Outotec HSC 6.1 programs. X-ray analysis of the samples has been carried out in DRON-2 diffractometer (Cu K-radiation,monochromatoron the outletbeam). All the reflections were identified using the ICDD data base of crystallographic 2012.

The initial nickel sulfide is obtained by alloying of pure metallic powders of nickel and element sulfur in the graphite crucibles. The samples of iron sulfide and pentlandite is obtained by sintering of carbonyl iron, nickel and sulfur in evacuated quartz ampoules for 400 hours at 700 0C. The sulfides had structure of FeS troilite, Ni3S2 heazlewoodite and Fe4.5Ni4.5S8 pentlandite. Pentlandite sample was contained to 32.6 wt.% Fe, 34.2 wt.% Ni, 33.2 wt.% S. Calcium oxide has been obtained by calcination of carbonate calcium (chemical pure) at the temperature 950 оC for 30 min. Size of the particles milled of nickel and pentlandite was less than 120 μm, iron sulfide and calcium oxide one being 40 μm. Graphite is taken in an amount in 120 pct from that required stoichiometrically in order to reduce the metals fully.

Results and discussions

The endothermic effects (Fig. 1) being due to the phases transformations in troilite with onset/maximum at 144/151 and 273/317 оC have been detected by a thermal analysis of FeS and CaO equimolar mixture in an argon flow [9]. The exothermic effects accompanied by a formation of the iron-calcium oxysulfides with onset/maximum at 610/866 оC is connected with the interaction between FeS and CaO. The endothermic effects at 861/863 оC and 943/945/968 оC are due to melting of formed phases. During cooling of the sample the crystallization effects of the phases were observed. Herewith the intense effect has been revealed at 930/924 оC and weak those do at 840 and 810 оC. X-ray analysis results have suggested a formation of CaFeSO and Ca3Fe4S3O6 oxysulfides the data on them are given in the works [1-3, 10-12]. In the products of heating besides oxysulfides CaS, CaO, FeS and Ca2Fe2O5 phases are detected. A formation of the oxysulfide is a possible as follows:

6FeS + 6CaO = Ca3Fe4S3O6 + 3CaS + 2Fe, (1)

FeS + CaO = CaFeSO. (2)

Fig. 1. Thermal analysis curves of FeS-CaO (a) and 2FeS-Ni3S2-2CaO (b) mixtures during heating and cooling at the 10 оC min-1 rate in an argon flow

In 2FeS-Ni3S2-2CaO system during heating up to 920 оC in a argon flow on the DSC curve the endothermic effects featured to phases transformations in FeS as well as due toβ-transition in Ni3S2 - (Fe,Ni)9S8 solid solution [13] with onset/maximum at 501/525 оC and phase passing in pentlanditeat 608/615 оC have been revealed (Fig. 1). The effect having been revealed in the temperature range 817 – 843 оC are related to melting of the sulfides and oxysulfides phases. During cooling the crystallization of the melt takes place at 831/803оC. The heat effect at 469/458оC corresponds to β–transition of the sulfide solid solution. In the products cooled of heating Ni3S2, CaS, Fe4.75Ni4.25S8, Ca3Fe4S3O6, Ca2Fe2O5 phases and иFeStraces is detected by X-ray analysis.

Reduction by carbon monoxide

Heating of the FeS and CaO components taken in equimolarratioat 10оC min-1 rates in flow of the gaseous mixture (60 vol% of CO – 40 vol% of Ar) is accompanied by an increase in the sample weight by 1.56% in the temperature range 200-626 оC (Fig. 2). Above is due to a decomposition of СО with a formation of СО2 and elemental carbon precipitating on the sample and sample holder. Further heating (up to 1000 оC) of the reaction mixture is characterized by a loss in its weight by 5.6% from initial one that corresponds to iron recovery equaled 50.4%. On the DSC curve the endothermic effects to be due to phases passing in FeS, Ca(OH)2 decomposition (363/390 оC), a reduction of iron (combinedeffect with onset/maximum at 696/738 оC) and melting of oxysulfide phases (957/964 оC) have been revealed. The final products ofheating of the mixture to 1000 оC in the reduction medium are CaS, CaFeSO and Fe phases.

A chemistry of iron reduction by carbon monoxide from FeS – CaO mixture corresponds to total – scheme:

FeS + CaO + CO = Fe + CaS + CO2, (3)

The intermediate stages along with reactions (1) and (2) are

Ca3Fe4S3O6 + 6CO = 3CaS + 4Fe + 6CO2, (4)

CaFeSO + CO = CaS + Fe + CO2. (5)

The results of the works [6, 7] have been shown a reduction of iron from the oxysulfides (Ca3Fe4S3O6 and CaFeSO) by carbon monoxide proceeds with essential rates in the temperature range 500 – 800 оC. The formation of intermediate oxysulfides during heating of FeS and CaO mixture is likely to define the iron reduction process features.

The reduction of metals by carbon monoxide from 2FeS-Ni3S2-2CaO mixture during heating at 20 оC min-1 rates is started at 440-540 оC and accompanied by an intensive weight loss at temperatures higher than 700 оC (Fig. 2). Heating of the sample to 1170 оC results in weight loss by 6.7 % and a formation of pentlandite (Fe4.75Ni4.25S8) in the products besides CaS and Ni-Fe alloy that points out on deficiency of СаО for binding of sulfur fully. The exothermic effect at 779/808 оC has been caused by a formation of the oxysulfide phases. The process of the metals reduction in the CO flow has been denoted by the reaction

Ni3S2 + 2FeS + 2CaO + CO = 2CaS + (3-0.25x)Ni + 0.25(x-1)Fe +

+ 0.25Fe(9-x)NixS8 + CO2. (6)

During heating of 2FeS-Ni3S2-4CaO mixture at 10 оC min-1 rate in the CO-Ar flow (Fig. 2) reduction process is started at 560 оC and accompanied by a weight loss reaching 9.58 % at 1120 оC that corresponds to recovery of the metals to 95.9 %. On DSC curve the exothermic effects belonged to phase transformations in FeS, Ni3S2, Fe4.75Ni4.25S8 and melting of the intermediate phases (931 оC) have been revealed. On X-ray analysis data (Fig. 3) the interaction products are CaS and Ni-Fe alloy. Increasing of heating rate to 30 оC min-1 results in an additional formation of a few of Fe4.75Ni4.25S8, Ca3Fe4O6S3 and CaO that points out on an incompleteness of the reduction process.

Reduction by carbon

During heating equimolar FeS and CaO mixture with graphite (Fig. 4) at 10 оC min-1 rate the reduction is started about 900 оC and accompanied by an intense weight loss reaching 16 % in region of temperatures to 1050 оC. Rather heating results in a slowdown of weight loss. On DTA heat flow curve a combined endothermic effect speaking about melting of oxysulfide phase and proceeding of the reduction processwith start at 956 and maximum at 968 and 995 оC has been revealed. A little effect at 860 оC points out on melting of Ca3Fe4S3O6oxysulfide, its quantity is reduced. As following from massspectrometricanalysisofevolvedgasesat temperatures higher than 800оC a formation of carbon dioxide is taken place and higher 950 оC carbon monoxide is evolved. It points out a run of the iron reduction reactions.

Fig. 2. TG, DTG and DTA curves during heating of FeS-CaO (a), 2FeS-Ni3S2-2CaO (b) and 2FeS-Ni3S2-4CaO (c) mixtures at 10 оC min-1 (a, c) rates and 20 оC min-1 (b) in an argon flow of CO(60 %)-Ar

Fig. 3. The XRPD patterns of the products reduction of 2FeS-Ni3S2-4CaO (a, b) and Fe4.5Ni4.5S8-CaO mixtures (c) by monoxide carbon (а) and graphite (b, c) during heating and at 10 оC min-1 rate up to 1250 оC in CO-Ar flow (a) and an argon that (b, c): 1 – CaS, 2 – Fe-Ni, 3 - Fe4.75Ni4.25S8, 4 – CaO, 5 - graphite

Fig. 4. Weight, heat flow changes and mass spectrometric data of gases during heating (10 оC min-1) of FeS-CaO mixture with graphite in an argon flow.

During heating of FeS-CaO mixture with graphite a little gassing of SO2 takes place a formation of the gases such as COS and CS2 has been not detected. A little increase of ion currents with mass numbers (m/z=18 and 44 a.m.u.) belonged to H2O and CO2 with temperature maximums at 390 and 590 оC are due to a decomposition of trace amounts of calcium hydroxide andcarbonate having formed during a preparation of the samples. On X-ray analysis results the main products of heating of FeS-CaO mixture with graphite to 1250 оC is presented by CaS and Fe a little residue amounts of the initial components (CaO, FeS and C).

Carbothermal reduction of iron from its sulfide being a little likely thermodynamically and insertion of calcium oxide activating the process, the total reaction has a form

FeS + CaO + C = Fe + CaS + CO. (7)

The intermediate reactions between FeS and CaO and carbon are presented as (1), (2) and follows

FeS + CaO = CaS + FeO, (8)

FeS + 2FeO = 3Fe + SO2, (9)

Ca3Fe4S3O6 + 3C (CO) = Fe + 3CaFeSO + 3CO (CO2), (10)

CaFeSO + C (CO) = CaS + Fe + CO (CO2), (11)

С + CO2 = 2CO. (12)

According to [3] the calculate pressure of sulfur dioxide on reaction (9) is changed in range 5.5 mPa - 1.9·10-2 mPa. The formation of oxysulfide phases also reduces an equilibrium pressure of SO2. Thus the metallic iron can be obtained during carbothermal reduction of iron sulfide only in CaO presence. Calcium oxide is as medium for an exchange of sulfur anions instead oxygen. The reduction process proceeds by means of a formation of the intermediate low-melting oxysulfide phases withoutsulfur-containing gassing.

Carbothermal reduction of nickel sulfide (Ni3S2) in CaO presence under continuous heating conditions to 1250 оC has been also studied. The results of the measurements have been shown (Fig. 5) in Ni3S2-2CaO-2С mixture a essential weight change connected with the reduction reactions takes place at temperatures higher than 790 оC closed to those of a start of sulfides melting. During heating to 1250 оC the value of the sample weight loss has been 8.2 % from initial wherein the run of TG line points out to on incomplete process. DTA line endothermic effects connect to the phase passing (553/563 оC) and melting (786/792 оC) of Ni3S2 [9, 15] as well as a region of active reduction sulfide (1000/1019 оC). Along with a sample weight change in the gaseous products CO, CO2 and SO2higher contents were observed. It is worth nothing in the gas sulfur dioxide little amounts have been fixed only owing to a high sensitivityof the mass spectrometer. In work [4] where the gaschromatography method has been used the SO2 gassing is not detected. In the products of the reduction CaS and Ni phases also the initial components (Ni3S2, CaO and C) have been reveal.

As following from obtained data the process of the nickel sulfide carbothermical reduction in the presence of calcium oxide can be denote by thermodynamic possiblereactions as follows:

Ni3S2 + 2CaO + 2C = 3Ni + 2CaS + 2CO, (13)

Ni3S2 + 2CaO + C = 3Ni + 2CaS + CO2. (14)

Along with presented reactions those accompanied by evaluation of SO2 are possible

2Ni3S2 + 4CaO + O2 = 6NiO + 4CaS, (15)

Ni3S2 + 4NiO = 7Ni + SO2, (16)

1.5Ni3S2 + 2CaO = 4.5Ni + 2CaS + SO2. (17)

Fig. 5. Weight, heat flow changes and mass spectrometric data of gases during heating (10 оC min-1) of Ni3S2-2CaO mixture with graphite in an argon flow

Carbothermal reduction of the metals from 2FeS-Ni3S2-4CaO mixture during heating at 10 оC min-1 rate is developed at the temperatures higher than 800 оC (Fig. 6) expressed by samples weight loss, beginningof the endothermic effect and evolution of carbon oxides (CO and CO2). The stepwise weight loss in range 300 – 800 оC is connected with the decomposition of Ca(OH)2 and CaCO3. The formation of COS and CS2 gases during heating has been not revealed a little evolution of SO2 takes place when formed the liquid phase (1048 оC). The solid products of the interaction contain CaS, Ni,Fe phases, a little of Fe9Ni9S16 as well as residual amounts of C and CaO (Fig. 3).

Fig. 6. TG and DTA curves and mass spectrometric data of gases during heating (10 оC min-1) of 2FeS-Ni3S2-4CaO mixture with graphite

The data on a change of weight during heating of 2FeS-Ni3S2-4CaO mixture with graphite at 5, 10 and 20 оC min-1 rates at 750–1170 оC have been taken in basis of the kinetics parameters of a determine and a estimation of the reaction model. The results of the calculations are shown (Fig. 7) Avrami-Erofeev one-stepmodel withthe limiting stageof formation and growth of nucleuses denotes the experimental data with correlation coefficient equaled to 0.9987. Kinetics equation has the form [16, 17]:

dα/dτ = A exp[-E/(RT)] n(1 – α)[-ln(1 - α)](n-1)/n, (19)

where: A is pre-exponentialcoefficient; Е is activationenergy; T is temperature, K; R is gaseous constant; α is recovery equaled to ration of present value a weight change by initial its one; n is reaction order; τ is duration. The kinetics parameters of metals carbothermal reduction have been determined as follows Е=520±11 kJ/mole, logA=18.2±0.4 s-1, n=0.87±0.01.

Fig. 7. Weight change during heating of 2FeS-Ni3S2-4CaO mixture with graphite at 5, 10 and 20 оC min-1 rates (points correspond the experiment, lines do the calculation)

The processes in Fe4.5Ni4.5S8–8CaO–8С system have been estimated during its heating at 10 оС min-1 rate in an argon flow. Pentlandite consisted of Fe4.41Ni4.56S8 non-stochiometric phase and a little amount of Fe0.79Ni0.13S. According to the pentlandite thermal analysis [18] obtained during heating of the sample to 900 оC in an argon flow at 20 оC min-1 on heat flow line (DSC) the endothermic effects with onset/maximum at 278/293 (weak peak), 479/485 and at 616/624 and 857/864 оC are highlighted. The effect at 278 and 479 оC are due to ordering of the super-structure and β-transition of Fe0.79Ni0.13S solid monosulfide solution. At the temperature closed to 615 оC pentlandite phase undergoes passing to a cubic system high-temperature modification. High-temperature pentlandite is a stable up to 857 оC but at higher temperature the liquid phase is arisen and a monosulfide solid solution is kept in the sample.

Carbothermal reduction of Fe4.5Ni4.5S8 pentlandite in the mixture with calcium oxide (Fig. 8) results in a sample weight change correlating with data for 2FeS-Ni3S2-4CaO system. The sample weight intense loss as well as followed by a decelerationof the process is observed in temperature range 900-1200 оC. During heating to 1250 оC the weight loss obtained due to a reduction of pentlandite reaches 14.6 %. On DTA line during heating of Fe4.5Ni4.5S8 – CaO mixture with graphite the effects characterized to the phases transformations in pentlandite (615 and 855 оC) as well as a little exothermal that connected with a oxysulfide formation with maximum at 864 оC and endothermic that (932/938 оC) for its melting have been revealed. The combined endothermic effects with start at 1021 and maximum at 1030 and 1063 оC are consequence of melting of the sulfide phases and a development of the reduction process accompanied by an evolution of CO and CO2. CaS, C, (Ni,Fe) solid solution, Fe9Ni9S16 traces and CaO (Fig. 3) are products of the carbothermal reduction of the mixture of pentlandite and calcium oxide during heating to 1250 оC. Above data correspond to those [19]. As following from obtained data the carbothermal reduction process of pentlandite in the presence of calcium oxide is presented by total reactions

Fe4.5Ni4.5S8 + 8CaO + 8C=4.5Fe + 4.5Ni + 8CaS + 8CO, (20)

Fe4.5Ni4.5S8 + 8CaO + 4C = 4.5Fe + 4.5Ni + 8CaS + 4CO2. (21)

Fig. 8. TG and DTA curves and mass spectrometric data of gases during heating (10 оC min-1) of Fe4.5Ni4.5S8 and CaO mixture with graphite

Conclusions

1. The processes proceeding during carbothermal reduction of metals in FeS-CaO, FeS-Ni3S2-CaO and Fe4.5Ni4.5S8-CaO systems under non-isothermal heating conditions have been investigated by thermogravimetricanalysistogetherwithdifferentialthermalone as well as massspectrometryofthegases.

2. The interactions in FeS-CaO and FeS-Ni3S2-CaO systems during heating in argon atmosphere proceed with the formation of iron-calcium oxysulfides (Ca3Fe4S3O6 and CaFeSO) along with sulfides and oxides phases.

3. Heating of FeS-CaO and FeS-Ni3S2-CaO mixtures at 10 - 30 оC min-1 rates in the gaseous medium containing to 60 % of carbon monoxide results in a formation of the metallic phase (iron or Fe-Ni alloy) and calcium sulfide at the temperatures higher than 560 – 630 оC.

4. Carbothermal reduction of iron in FeS-CaO system proceeds actively in 950-1050 оC range with original formation of liquid phase withoutsulfurous gases herewith the intermediate products are iron-calcium oxysulfides.

5. The reduction of nickel sulfides by a carbon and its oxide is activated by means of the insertion to the reaction mixture composition of iron sulfide along with calcium oxide thatcontributes the formation of intermediate iron-calcium oxysulfide compounds having a higher reactivity.