One-Step Extraction of Antimony in Low Temperature from Stibnite Concentrate Using Iron Oxide as Sulfur-Fixing Agent

Yun Li 1, Yongming Chen 1,*, Haotian Xue 2,*, Chaobo Tang 1, Shenghai Yang 1 and Motang Tang 1 1 School of Metallurgy and Environment, Central South University, Changsha 410083, China; li-yun@csu.edu.cn (Y.L.); chaobotang@163.com (C.T.); yangshcsu@163.com (S.Y.); tangmotangcsu@163.com (M.T.) 2 Qinghai Provincial Research and Design Academy of Environmental Sciences, Xining 810000, China * Correspondence: csuchenyongming@163.com (Y.C.); haotianx@sina.cn (H.X.); Tel./Fax: +86-731-8883-0470 (Y.C.); +86-971-8172-559 (H.X.)


Introduction
China has abundant reserves of antimony resource and it's also the largest producer of antimony in the world.Generally, the technologies for producing antimony mainly comprise pyrometallurgy and hydrometallurgy [1,2].Pyrometallurgy routes for extraction of antimony typically contain roasting-volatilizing-reducing process, bath smelting-continuous fuming process [3] and direct smelting process [4,5].While hydrometallurgy technologies for antimony separation can be classified into alkaline extraction procedure and acidic extraction process according to property of solvents.Alkaline extraction procedure primarily adopts Na 2 S leaching followed by membrane electrowinning [6].Acidic extraction process mainly includes FeCl 3 leaching-electrowinning and chlorination-distillation procedure [1].At present, due to the lengthy flow, poor efficiency, high running cost and large amount of wastewater treatment during hydrometallurgical separation process [7] of antimony, most of antimony smelteries in China use traditional volatilization smelting process in blast furnace to volatilize the resultant antimony trioxide, and then reduce the trioxide with carbon to metallic antimony in reverberatory furnace.However, this process has its obvious drawbacks, such as serious environmental pollution and large energy consumption, which restrict its popularization and application, especially under the background of increasing stringent environmental standard.Therefore, renovation and innovation in traditional antimony metallurgy technology are imperative [8][9][10].
Our research group has done a great deal of investigations [11][12][13][14] and tried to modify and innovate the traditional antimony metallurgy.Some promising achievements have been acquired.Tang et al. [11] were based on the alkaline smelting presented by scholars [15] of the former Soviet Union and then used and developed this technique to smelt low-melting point nonferrous metals in NaOH-Na 2 CO 3 -Na 2 SO 4 -Na 2 S system, such as antimony, lead, bismuth and tin etc.They found that as long as the smelting temperature was adequately higher than corresponding metals' melting point, the extractive reaction would occur efficiently [11].Yang et al. [12] separated 97.07%antimony (96.45% purity) from stibnite concentrate in NaOH-Na 2 CO 3 system at 880 ˝C (1153 K).Ye et al. [14] extracted 92.88% antimony (purity 93.17%) in Na 2 CO 3 -NaCl system at 850 ˝C (1123 K).However, the sulfur-fixing agent they used was ZnO.Sulfur contained in stibnite concentrate was fixed in form of ZnS.In this study, we developed a kind of alternative sulfur-fixing agent iron oxides.
Iron oxide-rich slags are produced in great deal in China [16,17], in particular pyrites cinder [18] generated in acid-making procedure.These residues generally store up in slag dumps and leave untreated.Considerable accumulation of these tailings not only occupies a large amount of land but causes contamination of the environment and constitutes an ecological threat to the surrounding life due to wind erosion and scattering in the regions.However, these kinds of residues are valuable secondary resource of iron.In addition, some precious metals, such as Au and Ag, frequently exist in pyrites cinder in China.The treatment of these massive quantities of residues is not only extraordinarily important but extremely necessary from both environmental and economic perspectives.
In this paper, the renovate process [12] (as shown in Figure 1) for antimony extraction from stibnite concentrate is proposed to overcome the problems in traditional two-step antimony metallurgy, meanwhile, to co-treat iron oxide-rich slags.This process is characterized by low temperature, elimination of SO 2 emission and short flow.It consists of the following steps: feeding stibnite-containing feed, mixed sodium salt, sulfur-fixing agent and powdery coal or coke into a furnace with a temperature of less than 900 ˝C (1173 K).As a result, crude antimony metal, ferrous sulfide, gangue, and regenerated molten salt, mainly containing sodium carbonate, are obtained.The regenerated molten salt is fed into the smelting furnace to reuse as the reaction flux again after filtering operation while it is melting state.The ferrous sulfide can be sold as ferrous sulfide concentrate or roasted to regenerate into iron oxide and used as sulfur-fixing agent again.The studies in this article were focus on the process illustrated in the frame in Figure 1.

Materials
Stibnite concentrate used in this study obtained from Chenzhou Mining Group Co., LTD., Huaihua, Hunan, China.Powdered Fe2O3, Na2CO3 and NaCl with >99.99% purity were purchased from Aladdin Industrial Corporation.Reductant, metallurgical coke, was provided by Xiangtan Iron and Steel Co., LTD. of Hunan Valin.Chemical compositions of stibnite concentrate were analysed by Inductively Coupled Plasma-atomic Emission Spectrometry (Perkin Elmer, Optima 3000 ICP-AES, Norwalk, CT, USA).The prior decomposition of stibnite concentrate was carried out in aqua regia (nitrohydrochloric acid, a 3:1 mixture of concentrated HCl and HNO3 aqueous solution) while shielded the influence of silica by hydrofluoric acid HF and perchloric acid HClO4.The results were presented in Table 1.

Materials
Stibnite concentrate used in this study obtained from Chenzhou Mining Group Co., LTD., Huaihua, Hunan, China.Powdered Fe 2 O 3 , Na 2 CO 3 and NaCl with >99.99% purity were purchased from Aladdin Industrial Corporation.Reductant, metallurgical coke, was provided by Xiangtan Iron and Steel Co., LTD. of Hunan Valin.Chemical compositions of stibnite concentrate were analysed by Inductively Coupled Plasma-atomic Emission Spectrometry (Perkin Elmer, Optima 3000 ICP-AES, Norwalk, CT, USA).The prior decomposition of stibnite concentrate was carried out in aqua regia (nitrohydrochloric acid, a 3:1 mixture of concentrated HCl and HNO 3 aqueous solution) while shielded the influence of silica by hydrofluoric acid HF and perchloric acid HClO 4 .The results were presented in Table 1.Phase compositions of stibnite concentrate were analysed by X-ray Diffraction (XRD, Rigaku 3014, Rigaku Corporation, Tokyo, Japan, Cu-Kα-radiation, λ = 1.54 Å) (as shown in Figure 2).It can be known that stibnite concentrate primarily comprises Sb 48.08%, S 25.13%, Fe 5.14% and 101.05 g/t Au, which attach a significant economic value to recover.Main phase compositions in stibnite concentrate are Sb2S3, FeS and SiO2.

Methods
As an experimental procedure, 100 g stibnite concentrate were mixed evenly with the given amount of coke, sodium carbonate and sodium chloride, sulfur-fixing agent Fe2O3 in every tests, and then the mixture were put into a weighed 100 mL corundum crucible.The crucible was placed in the constant temperature zone of furnace when the furnace temperature was raised to the desired value and held for preset time.After the smelting duration required, the crucible was taken out from furnace and cooled quickly.The product was put into a pre-prepared water (50-85 °C (323-358 K), L/S = (3-5):1) for leaching 2-3 h to separate crude antimony and molten salt slag.Then the lixivium, leaching residue and crude antimony were measured and weighed carefully and sampled.Each of samples was crushed and well-prepared for analysis.The crude antimony solid samples were dissolved in dilute nitric acid.The leaching residue samples were dissolved in aqua regia while shielded the influence of silica.After dissolving the samples, the solid residue in the leaching solution was filtered using filter paper, and the obtained solution was subjected to ICP-AES (Perkin Elmer, Norwalk, CT, USA) analysis.Metals' recoveries were calculated based on the mass balance principle.In addition, the phase compositions of the molten salt slag before and after leaching were characterized by XRD respectively.

Thermodynamic Considerations
Antimony in stibnite concentrate generally exists in form of Sb2S3.The melting point of metallic antimony is about 630.5 °C (904 K), and the melting point of the binary eutectic molten salt Na2CO3-NaCl [13] is around 632-645 °C (905-918 K).Under the smelting temperature, Sb2S3 can react with Fe2O3 and Na2CO3 respectively, and produces metallic antimony, FeS and Na2S.Na2S then will continually react with Fe2O3 to regenerate Na2CO3.Sulfur is fixed in FeS ultimately.NaCl is not involved in any chemical reaction and just plays a role of inert reaction medium.The purpose of

Methods
As an experimental procedure, 100 g stibnite concentrate were mixed evenly with the given amount of coke, sodium carbonate and sodium chloride, sulfur-fixing agent Fe 2 O 3 in every tests, and then the mixture were put into a weighed 100 mL corundum crucible.The crucible was placed in the constant temperature zone of furnace when the furnace temperature was raised to the desired value and held for preset time.After the smelting duration required, the crucible was taken out from furnace and cooled quickly.The product was put into a pre-prepared water (50-85 ˝C (323-358 K), L/S = (3-5):1) for leaching 2-3 h to separate crude antimony and molten salt slag.Then the lixivium, leaching residue and crude antimony were measured and weighed carefully and sampled.Each of samples was crushed and well-prepared for analysis.The crude antimony solid samples were dissolved in dilute nitric acid.The leaching residue samples were dissolved in aqua regia while shielded the influence of silica.After dissolving the samples, the solid residue in the leaching solution was filtered using filter paper, and the obtained solution was subjected to ICP-AES (Perkin Elmer, Norwalk, CT, USA) analysis.Metals' recoveries were calculated based on the mass balance principle.In addition, the phase compositions of the molten salt slag before and after leaching were characterized by XRD respectively.

Thermodynamic Considerations
Antimony in stibnite concentrate generally exists in form of Sb 2 S 3 .The melting point of metallic antimony is about 630.5 ˝C (904 K), and the melting point of the binary eutectic molten salt Na 2 CO 3 -NaCl [13] is around 632-645 ˝C (905-918 K).Under the smelting temperature, Sb 2 S 3 can react with Fe 2 O 3 and Na 2 CO 3 respectively, and produces metallic antimony, FeS and Na 2 S. Na 2 S then will continually react with Fe 2 O 3 to regenerate Na 2 CO 3 .Sulfur is fixed in FeS ultimately.NaCl is not involved in any chemical reaction and just plays a role of inert reaction medium.The purpose of adding the chlorite into sodium carbonate is to form a lower-temperature mixed molten salt.The reaction mechanism can be briefly represented as follows (see Table 2):

Reaction G T ´T (kJ/mol) [19] Equation
The Gibbs free energy G θ T of reactions ( 1)-( 5) were calculated under one atmospheric pressure.Figure 3 showed G θ T and T diagram of it.It illustrats antimony reduction in the presence of ferrous oxide or sodium carbonate becomes thermodynamically favorable at range of smelting temperature 700-900 ˝C (973-1173 K).Increasing temperature will promote the reactions ( 1) and ( 3) and ( 4) while decrease the positive trend of the reactions (2) and ( 5).Sb 2 S 3 is more likely to react with sulfur-fixing agent FeO instead of Na 2 CO 3 to produce metallic antimony.Na 2 CO 3 can regenerate and recycle through reaction (5).As a result, Na 2 CO 3 will not be consumed during whole smelting system and act as a role of intermediate reactant.Ultimately, the mixed molten slat Na 2 CO 3 -NaCl can be recycled together after filtering away the slag.However, the presence of Na 2 CO 3 and NaCl in smelting system significantly decreases temperature of reductive sulfur-fixing reactions.
The Gibbs free energy ∆G of reactions ( 1)-( 5) were calculated under one atmospheric pressure.Figure 3 showed ∆G and T diagram of it.It illustrats antimony reduction in the presence of ferrous oxide or sodium carbonate becomes thermodynamically favorable at range of smelting temperature 700-900 °C (973-1173 K).Increasing temperature will promote the reactions (1) and ( 3) and ( 4) while decrease the positive trend of the reactions (2) and ( 5).Sb2S3 is more likely to react with sulfur-fixing agent FeO instead of Na2CO3 to produce metallic antimony.Na2CO3 can regenerate and recycle through reaction (5).As a result, Na2CO3 will not be consumed during whole smelting system and act as a role of intermediate reactant.Ultimately, the mixed molten slat Na2CO3-NaCl can be recycled together after filtering away the slag.However, the presence of Na2CO3 and NaCl in smelting system significantly decreases temperature of reductive sulfur-fixing reactions.

Molten Salt Dosage
The results of the influence of molten salt dosage on antimony recovery and resultant crude antimony grade are presented in Figure 4.All experiments were carried out under a temperature of 900 °C (1173 K) for 180 min, the charging composition of 100 g stibnite, 30 wt. % coke of stibnite, 1.0 time stoichiometric requirement of sulfur-fixing agent Fe2O3 (α Fe 2 O 3 = 1.0), calculated according to sulfur content in stibnite.The molten salt composition, which is the ratio of weight of sodium chloride to that of mixture of sodium carbonate and sodium chloride (WNaCl:Wsalt), is 10%.The stoichiometric requirement of molten salt (αsalt) was calculated by Equation (3).
Experimental results indicated that antimony recovery improved firstly and then decreased steadily as αsalt increased from 0.8 to 1.5.Insufficient dosage of molten salt (αsalt = 0.8) results in a

Molten Salt Dosage
The results of the influence of molten salt dosage on antimony recovery and resultant crude antimony grade are presented in Figure 4.All experiments were carried out under a temperature of 900 ˝C (1173 K) for 180 min, the charging composition of 100 g stibnite, 30 wt. % coke of stibnite, 1.0 time stoichiometric requirement of sulfur-fixing agent Fe 2 O 3 (α Fe 2 O 3 = 1.0), calculated according to sulfur content in stibnite.The molten salt composition, which is the ratio of weight of sodium chloride to that of mixture of sodium carbonate and sodium chloride (W NaCl :W salt ), is 10%.The stoichiometric requirement of molten salt (α salt ) was calculated by Equation (3).
Metals 2016, 6, 153 6 of 12 of salt will dilute the concentrate of the reactant and increase the total dissolved loss of antimony in molten salt, which results in reducing of crude antimony productivity ultimately [14].Therefore, the appropriate molten salt dosage is αsalt = 1.

Molten Salt Composition
All experiments were carried out under the following conditions: 100 g stibnite concentrate, 30 wt. % coke, αsalt = 1.0, α Fe 2 O 3 = 1.0, a temperature of 900 °C (1173 K), a smelting duration of 180 min.The molten salt composition WNaCl:Wsalt increases from 10% to 60%. Figure 5 showed the results of the influence of molten salt composition on direct recovery of antimony and on resultant crude antimony grade.The results implied that direct recovery of antimony raised from 90.78% to 96.87% when WNaCl:Wsalt increasd from 10% to 40%.In addition, crude antimony grade ascended from 78.59% to 82.45% gradually.Further increase of WNaCl:Wsalt showed no positive effect on crude antimony grade improvement, even deceased the recovery of antimony.It's because addition of NaCl is beneficial to decrease melting point of molten salt and increase its fluidity, which causes reductive sulfur-fixing reaction to occur more easily and improves settling efficiency of antimony particles.However, contents of NaCl in molten salt went beyond 50%, which, on the one hand, intensified the Experimental results indicated that antimony recovery improved firstly and then decreased steadily as α salt increased from 0.8 to 1.5.Insufficient dosage of molten salt (α salt = 0.8) results in a poor fluidity of reaction melt in low temperature.As a result, the settling and accumulating of antimony particles is also inefficiency.Antimony recovery reached maximum value at α salt = 1.Continued increase in the dosage of molten salt was unnecessary.On the contrary, a higher addition of salt will dilute the concentrate of the reactant and increase the total dissolved loss of antimony in molten salt, which results in reducing of crude antimony productivity ultimately [14].Therefore, the appropriate molten salt dosage is α salt = 1.

Molten Salt Composition
All experiments were carried out under the following conditions: 100 g stibnite concentrate, 30 wt. % coke, α salt = 1.0, α Fe 2 O 3 = 1.0, a temperature of 900 ˝C (1173 K), a smelting duration of 180 min.The molten salt composition W NaCl :W salt increases from 10% to 60%. Figure 5 showed the results of the influence of molten salt composition on direct recovery of antimony and on resultant crude antimony grade.
The results implied that direct recovery of antimony raised from 90.78% to 96.87% when W NaCl :W salt increasd from 10% to 40%.In addition, crude antimony grade ascended from 78.59% to 82.45% gradually.Further increase of W NaCl :W salt showed no positive effect on crude antimony grade improvement, even deceased the recovery of antimony.It's because addition of NaCl is beneficial to decrease melting point of molten salt and increase its fluidity, which causes reductive sulfur-fixing reaction to occur more easily and improves settling efficiency of antimony particles.However, contents of NaCl in molten salt went beyond 50%, which, on the one hand, intensified the volatilization of molten salt, on the other hand, was more than the eutectic composition of in Na 2 CO 3 -NaCl binary system [13], as a result, melting point of binary system ascended, and the fluidity of molten salt decreased evidently.Thus further caused the settling and separation efficiency of antimony particles to be deteriorated.Meanwhile, excessive addition of NaCl will result in shorten of service life of furnace lining and body.Therefore, in this study, W NaCl :W salt = 40% was selected as the optimized molten salt composition.

Molten Salt Composition
All experiments were carried out under the following conditions: 100 g stibnite concentrate, 30 wt. % coke, αsalt = 1.0, α Fe 2 O 3 = 1.0, a temperature of 900 °C (1173 K), a smelting duration of 180 min.The molten salt composition WNaCl:Wsalt increases from 10% to 60%. Figure 5 showed the results of the influence of molten salt composition on direct recovery of antimony and on resultant crude antimony grade.The results implied that direct recovery of antimony raised from 90.78% to 96.87% when WNaCl:Wsalt increasd from 10% to 40%.In addition, crude antimony grade ascended from 78.59% to 82.45% gradually.Further increase of WNaCl:Wsalt showed no positive effect on crude antimony grade improvement, even deceased the recovery of antimony.It's because addition of NaCl is beneficial to decrease melting point of molten salt and increase its fluidity, which causes reductive sulfur-fixing reaction to occur more easily and improves settling efficiency of antimony particles.However, contents of NaCl in molten salt went beyond 50%, which, on the one hand, intensified the

Ferric Oxide Dosage
The curves of effect of addition of sulfur-fixing agent Fe 2 O 3 on direct recovery of antimony and crude antimony grade were showed in Figure 6.All experiments were carried out under the following conditions: 100 g stibnite concentrate, α salt = 1.0,W NaCl :W salt = 40%, 30 wt. % coke, a temperature of 900 ˝C (1173 K), a smelting duration of 180 min.α Fe 2 O 3 increases from 0.8 to 1.3.The results indicated that the direct recovery of antimony basically remained constant at above 95% and crude antimony grade descended from 81.75% to 56.76% as increasing of Fe 2 O 3 addition.That's because some Fe 2 O 3 were reduced to metallic Fe and transferred to crude antimony.Results on the influence of dosage of ferric oxide implied that α Fe 2 O 3 = 1.0 is adequate for efficient extraction of antimony.volatilization of molten salt, on the other hand, was more than the eutectic composition of in Na2CO3-NaCl binary system [13], as a result, melting point of binary system ascended, and the fluidity of molten salt decreased evidently.Thus further caused the settling and separation efficiency of antimony particles to be deteriorated.Meanwhile, excessive addition of NaCl will result in shorten of service life of furnace lining and body.Therefore, in this study, WNaCl:Wsalt = 40% was selected as the optimized molten salt composition.

Ferric Oxide Dosage
The curves of effect of addition of sulfur-fixing agent Fe2O3 on direct recovery of antimony and crude antimony grade were showed in Figure 6.All experiments were carried out under the following conditions: 100 g stibnite concentrate, αsalt = 1.0,WNaCl:Wsalt = 40%, 30 wt. % coke, a temperature of 900 °C (1173 K), a smelting duration of 180 min.α Fe 2 O 3 increases from 0.8 to 1.3.The results indicated that the direct recovery of antimony basically remained constant at above 95% and crude antimony grade descended from 81.75% to 56.76% as increasing of Fe2O3 addition.That's because some Fe2O3 were reduced to metallic Fe and transferred to crude antimony.Results on the influence of dosage of ferric oxide implied that α Fe 2 O 3 = 1.0 is adequate for efficient extraction of antimony.

Smelting Temperature
Figure 7 illustrated the influence of temperature on crude antimony grade and direct antimony recovery rate.All experiments were operated under following conditions: 100 g stibnite concentrate, αsalt = 1.0,WNaCl:Wsalt = 40%, α Fe 2 O 3 = 1.0, 30 wt. % coke, a smelting duration of 180 min.It was observed that antimony recovery rate increased steadily from 85.62% to 95.80%, while crude antimony grade decreased from 87.01% to 71.86% with temperature ascending from 800 °C (1073 K) to 950 °C (1223 K).Excessively low temperature cannot ensure reductive sulfur-fixing reaction is thoroughly positive and resultant metallic antimony particles were difficult to settle as well.However, excessively high temperature will lead to volatilization loss of reactants and rise of energy

Smelting Temperature
Figure 7 illustrated the influence of temperature on crude antimony grade and direct antimony recovery rate.All experiments were operated under following conditions: 100 g stibnite concentrate, α salt = 1.0,W NaCl :W salt = 40%, α Fe 2 O 3 = 1.0, 30 wt. % coke, a smelting duration of 180 min.It was observed that antimony recovery rate increased steadily from 85.62% to 95.80%, while crude antimony grade decreased from 87.01% to 71.86% with temperature ascending from 800 ˝C (1073 K) to 950 ˝C (1223 K).Excessively low temperature cannot ensure reductive sulfur-fixing reaction is thoroughly positive and resultant metallic antimony particles were difficult to settle as well.However, excessively high temperature will lead to volatilization loss of reactants and rise of energy consumption.Therefore, 850 ˝C (1123 K) is selected as the optimum smelting temperature.

Smelting Duration
The influence of the smelting duration on antimony recovery rate and crude antimony was illustrated in Figure 8.All experiments were carried out under a charging of 100 g stibnite concentrate and 30 wt. % coke and αsalt = 1.0, of which WNaCl:Wsalt = 40%, α Fe 2 O 3 = 1.0, smelting at 850 °C (1123 K).Smelting duration increased from 60 min to 240 min.The results implied that the direct recovery of antimony decreased steadily from 92.48% to 80.15% and crude antimony grade dropped from 93.1% to 74.99% respectively as smelting duration increased from 60 min to 240 min.It indicated that the reductive sulfur-fixing reaction had sufficiently carried out after 60 min.Continued extension in smelting duration was unnecessary, On the contrary, a prolonged smelting time will caused volatilization loss of antimony and molten salt to increase.In addition, iron oxide will be increasingly reduced into metallic Fe and dilute the crude antimony grade.Accordingly, 60 min is selected as the optimum smelting duration.

Smelting Duration
The influence of the smelting duration on antimony recovery rate and crude antimony was illustrated in Figure 8.All experiments were carried out under a charging of 100 g stibnite concentrate and 30 wt. % coke and α salt = 1.0, of which W NaCl :W salt = 40%, α Fe 2 O 3 = 1.0, smelting at 850 ˝C (1123 K).Smelting duration increased from 60 min to 240 min.The results implied that the direct recovery of antimony decreased steadily from 92.48% to 80.15% and crude antimony grade dropped from 93.1% to 74.99% respectively as smelting duration increased from 60 min to 240 min.It indicated that the reductive sulfur-fixing reaction had sufficiently carried out after 60 min.Continued extension in smelting duration was unnecessary, On the contrary, a prolonged smelting time will caused volatilization loss of antimony and molten salt to increase.In addition, iron oxide will be increasingly reduced into metallic Fe and dilute the crude antimony grade.Accordingly, 60 min is selected as the optimum smelting duration.
recovery of antimony decreased steadily from 92.48% to 80.15% and crude antimony grade dropped from 93.1% to 74.99% respectively as smelting duration increased from 60 min to 240 min.It indicated that the reductive sulfur-fixing reaction had sufficiently carried out after 60 min.Continued extension in smelting duration was unnecessary, On the contrary, a prolonged smelting time will caused volatilization loss of antimony and molten salt to increase.In addition, iron oxide will be increasingly reduced into metallic Fe and dilute the crude antimony grade.Accordingly, 60 min is selected as the optimum smelting duration.

Reductive Agent Dosage
The results of effect of reductive agent dosage on crude antimony grade and direct antimony recovery rate were illustrated in Figure 9.All experiments were carried out under the following conditions: 100g stibnite, αsalt = 1.0,WNaCl:Wsalt = 40%, α Fe 2 O 3 = 1.0, smelting at 850 °C (1123 K) for 60

Reductive Agent Dosage
The results of effect of reductive agent dosage on crude antimony grade and direct antimony recovery rate were illustrated in Figure 9.All experiments were carried out under the following conditions: 100g stibnite, α salt = 1.0,W NaCl :W salt = 40%, α Fe 2 O 3 = 1.0, smelting at 850 ˝C (1123 K) for 60 min.The curves showed that resultant crude antimony grade descending from 97.94% to 88.32%, and direct antimony recovery rate ascended from 50% to 94.99% respectively as coke addition increased from 10% to 50%.Under weak reductive atmosphere, the produce of crude antimony is small because forward reductive sulfur-fixing reaction is not thorough, so that antimony cannot be enriched and recovered completely.Strong reductive atmosphere is beneficial to increase the recovery of antimony.However, the contents of metallic Fe in crude antimony will increase simultaneously.Therefore, 40% coke dosage was the optimum addition for antimony extraction.min.The curves showed that resultant crude antimony grade descending from 97.94% to 88.32%, and direct antimony recovery rate ascended from 50% to 94.99% respectively as coke addition increased from 10% to 50%.Under weak reductive atmosphere, the produce of crude antimony is small because forward reductive sulfur-fixing reaction is not thorough, so that antimony cannot be enriched and recovered completely.Strong reductive atmosphere is beneficial to increase the recovery of antimony.However, the contents of metallic Fe in crude antimony will increase simultaneously.Therefore, 40% coke dosage was the optimum addition for antimony extraction.

Confirmation Experiments
According to results of above tests, the optimum conditions for one-step extraction of antimony in low temperature from stibnite concentrate, using iron oxide as sulfur-fixing agent, were obtained as follow: a smelting temperature of 850 °C (1123 K), 60 min smelting duration, 1.0 time stoichiometric requirement (αsalt = 1.0) of mixed sodium salt (Na2CO3 and NaCl), molten salt composition WNaCl:Wsalt = 40%, 1.0 time stoichiometric requirement of ferric oxide (α Fe 2 O 3 = 1.0),Wcoke:Wstibnite = 40%.These optimum conditions were applied in confirmation experiments to extract

Confirmation Experiments
According to results of above tests, the optimum conditions for one-step extraction of antimony in low temperature from stibnite concentrate, using iron oxide as sulfur-fixing agent, were obtained as follow: a smelting temperature of 850 ˝C (1123 K), 60 min smelting duration, 1.0 time stoichiometric requirement (α salt = 1.0) of mixed sodium salt (Na 2 CO 3 and NaCl), molten salt composition W NaCl :W salt = 40%, 1.0 time stoichiometric requirement of ferric oxide (α Fe 2 O 3 = 1.0),W coke :W stibnite = 40%.These optimum conditions were applied in confirmation experiments to extract antimony from 1000 g stibnite concentrate.The resultant smelting slag was leached in 50 ˝C (323 K) water, L/S = 5:1, for 3 h.The results of ICP analysis of resultants crude antimony, lixivium and leaching slag were given in Table 3. Figure 10   It was observed that 91.48% antimony was directly recovered in crude antimony under the optimum condition.Meanwhile, crude antimony grade could reach 96.00%.Overall, antimony direct recovery attained in confirmation experiments showed a slight decrease around 1%-3% compared to the preliminary test results, but still higher than 91%.The crude antimony purity had an increase of about 1%-10% compared to those obtained during preliminary experiments.The impurities in crude antimony were primarily 2.85% metallic Fe and 0.54% Pb which could be removed easily in following refining process.In addition, 99.55% Au contained in stibnite concentrate enriched in the crude antimony simultaneously.Sulfur contained in stibnite concentrate was nearly fixed (95.31%) in the slag.Furthermore, phase compositions of slag and leaching residue were characterized by XRD.The results were showed in Figure 11.It was observed that molten slag primarily comprised NaFeS2, NaCl, Na2S2O3 and Fe3O4.That indicated fraction of Na2CO3 were involved in sulfur-fixing reaction and generated Na2S.The  It was observed that 91.48% antimony was directly recovered in crude antimony under the optimum condition.Meanwhile, crude antimony grade could reach 96.00%.Overall, antimony direct recovery attained in confirmation experiments showed a slight decrease around 1%-3% compared to the preliminary test results, but still higher than 91%.The crude antimony purity had an increase of about 1%-10% compared to those obtained during preliminary experiments.The impurities in crude antimony were primarily 2.85% metallic Fe and 0.54% Pb which could be removed easily in following refining process.In addition, 99.55% Au contained in stibnite concentrate enriched in the crude antimony simultaneously.Sulfur contained in stibnite concentrate was nearly fixed (95.31%) in the slag.Furthermore, phase compositions of slag and leaching residue were characterized by XRD.The results were showed in Figure 11.
an increase of about 1%-10% compared to those obtained during preliminary experiments.The impurities in crude antimony were primarily 2.85% metallic Fe and 0.54% Pb which could be removed easily in following refining process.In addition, 99.55% Au contained in stibnite concentrate enriched in the crude antimony simultaneously.Sulfur contained in stibnite concentrate was nearly fixed (95.31%) in the slag.Furthermore, phase compositions of slag and leaching residue were characterized by XRD.The results were showed in Figure 11.It was observed that molten slag primarily comprised NaFeS2, NaCl, Na2S2O3 and Fe3O4.That indicated fraction of Na2CO3 were involved in sulfur-fixing reaction and generated Na2S.The residues after leaching of molten slag mainly contained elemental sulfur (S) and Fe3O4.It was found that FeS reacted with H2O and generated S. The reaction equation [20] was shown as follow: FeS + 2H2O = FeOOH + 1.5H2 (g) + S (6) Resultant FeOOH would dehydrate and continually be oxidized into Fe3O4 during drying operation.It was observed that molten slag primarily comprised NaFeS 2 , NaCl, Na 2 S 2 O 3 and Fe 3 O 4 .That indicated fraction of Na 2 CO 3 were involved in sulfur-fixing reaction and generated Na 2 S. The residues after leaching of molten slag mainly contained elemental sulfur (S) and Fe 3 O 4 .It was found that FeS reacted with H 2 O and generated S. The reaction equation [20] was shown as follow:

Conclusions
Resultant FeOOH would dehydrate and continually be oxidized into Fe 3 O 4 during drying operation.

Conclusions
In this study it can be concluded that iron oxides are a kind of high-efficiency alternative sulfur-fixing agent.Thermodynamic analysis and laboratory experimental results also verified the reliability and feasibility of the proposed renovate process.The optimum reductive-sulfur-fixing smelting conditions for one-step extraction of antimony from stibnite concentrate were determined.Under the optimum conditions, the direct recovery rate of antimony can reach 91.48%.Crude antimony with a purity of 96.00% has been achieved.95.31% of sulfur is fixed in form of FeS in the presence of iron oxide, resulting in a process that is free to atmospheric pollution.Furthermore, precious metals contained in stibnite concentrate are enriched and recovered comprehensively in crude antimony.The reaction flux, binary molten slat Na 2 CO 3 -NaCl, can be regenerated and reused.What's more, the iron-containing secondary materials can be recycled environmentally friendly with economic value through this process.

Figure 1 .
Figure 1.Flow sheet of reductive sulfur-fixing smelting of stibnite concentrate for one-step extraction of antimony in low temperature.

Figure 10 .
Figure 10.The distribution behaviors of main elements in products during confirmation experiments.

Figure 10 .
Figure 10.The distribution behaviors of main elements in products during confirmation experiments.

Table 1 .
Chemical compositions of stibnite concentrate and coke used in experiments (mass fraction, %) Flow sheet of reductive sulfur-fixing smelting of stibnite concentrate for one-step extraction of antimony in low temperature.

Table 1 .
Chemical compositions of stibnite concentrate and coke used in experiments (mass fraction, %).

Table 2 .
Main reactions occurred during reductive sulfur-fixing smelting of stibnite concentrate.

Table 2 .
Main reactions occurred during reductive sulfur-fixing smelting of stibnite concentrate.
illustrated the distribution behaviors of main elements in confirmation experiments.

Table 3 .
Chemical compositions of resultants in confirmation experiments.