CHEMISTRY AND KINETICS (TG/DTA-MS) OF METALS
CARBOTHERMICAL REDUCTION IN THE FeS-Ni3S2-CaO
SYSTEM
E. Selivanov, R. Gulyaeva
Institute of Metallurgy, Ural Branch of Russian Academy of Sciences,
101, Amundsen St., Yekaterinburg, 620016 Russia, 620016
pcmlab@mail.ru
Absract
The metals reduction processes by carbon and its monoxide in the
FeS-Ni3S2-CaO system under continuous heating conditions up to 1250
о
C 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
Ca3Fe4S3O6 oxysulfides 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-step Avrami-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 that contributes the formation of intermediate ironcalcium 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 time the phase transformations proceeded
during heating of iron and non-ferrous metals sulfides together with a
calcium oxide have been insufficiently studied. Available data in the
literature dealt 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,
204
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 binding and 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 FeSCaO 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
205
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, monochromator on the outlet beam). 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
о
C 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
о
C. X-ray analysis results have suggested a formation of CaFeSO and
Ca3Fe4S3O6 oxysulfides the data on them are given in the works [1-3, 1012]. In the products of heating besides oxysulfides CaS, CaO, FeS and
206
Ca2Fe2O5 phases are detected. A formation of the oxysulfide is a
possible as follows:
6FeS + 6CaO = Ca3Fe4S3O6 + 3CaS + 2Fe,
FeS + CaO = CaFeSO.
(1)
(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
207
transformations in FeS as well as due to β-transition in Ni3S2 - (Fe,Ni)9S8
solid solution [13] with onset/maximum at 501/525 оC and phase
passing in pentlandite at 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 и FeS traces is detected by X-ray analysis.
Reduction by carbon monoxide
Heating of the FeS and CaO components taken in equimolar ratio
о
at 10 C 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 (combined effect with onset/maximum at 696/738 оC) and melting
of oxysulfide phases (957/964 оC) have been revealed. The final
products of heating 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,
CaFeSO + CO = CaS + Fe + CO2.
(4)
(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
208
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-Ni3S22CaO mixture during heating at 20 оC min-1 rates is started at 440-540
о
C 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.
о
(6)
-1
During heating of 2FeS-Ni3S2-4CaO mixture at 10 C min 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
process with start at 956 and maximum at 968 and 995 оC has been
revealed. A little effect at 860 оC points out on melting of Ca3Fe4S3O6
oxysulfide, its quantity is reduced. As following from mass
spectrometric analysis of evolved gases at temperatures higher than
800оC a formation of carbon dioxide is taken place and higher 950 оC
209
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-Ni3S22CaO (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
210
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
211
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 and carbonate 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
212
FeS + CaO = CaS + FeO,
FeS + 2FeO = 3Fe + SO2,
Ca3Fe4S3O6 + 3C (CO) = Fe + 3CaFeSO + 3CO (CO2),
CaFeSO + C (CO) = CaS + Fe + CO (CO2),
С + CO2 = 2CO.
(8)
(9)
(10)
(11)
(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 without sulfur-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 Ni3S22CaO-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 SO2 higher contents were observed. It
is worth nothing in the gas sulfur dioxide little amounts have been fixed
only owing to a high sensitivity of the mass spectrometer. In work [4]
where the gas chromatography 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 possible reactions as follows:
Ni3S2 + 2CaO + 2C = 3Ni + 2CaS + 2CO,
Ni3S2 + 2CaO + C = 3Ni + 2CaS + CO2.
213
(13)
(14)
Along with presented reactions those accompanied by evaluation
of SO2 are possible
2Ni3S2 + 4CaO + O2 = 6NiO + 4CaS,
Ni3S2 + 4NiO = 7Ni + SO2,
1.5Ni3S2 + 2CaO = 4.5Ni + 2CaS + SO2.
(15)
(16)
(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, beginning of 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
о
C). 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).
214
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-Ni3S24CaO mixture with graphite at 5, 10 and 20 оC min-1 rates at 750–1170
о
C 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-step model with the limiting
stage of 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-exponential coefficient; Е is activation energy; 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.
215
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 deceleration of 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
216
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
о
C 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,
Fe4.5Ni4.5S8 + 8CaO + 4C = 4.5Fe + 4.5Ni + 8CaS + 4CO2.
(20)
(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
217
non-isothermal heating conditions have been investigated by
thermogravimetric analysis together with differential thermal one as well
as mass spectrometry of the gases.
2. The interactions in FeS-CaO and FeS-Ni3S2-CaO systems
during heating in argon atmosphere proceed with the formation of ironcalcium oxysulfides (Ca3Fe4S3O6 and CaFeSO) along with sulfides and
oxides phases.
3. Heating of FeS-CaO and FeS-Ni3S2-CaO mixtures at 10 - 30 оC
-1
min 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
without sulfurous gases herewith the intermediate products are ironcalcium 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 that contributes the formation of
intermediate iron-calcium oxysulfide compounds having a higher
reactivity.
1.
2.
3.
4.
5.
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