Feruza Berdikulova, Abdurasul Zharmenov, Alma Terlikbayeva, Alimgazy Sydykov, Yevgeniy Mazulevskiy

1001

REDUCTION SMELTING OF ANTIMONY CONCENTRATE OBTAINED IN THE COURSE OF LEAD PRODUCTION

Feruza Berdikulova, Abdurasul Zharmenov, Alma Terlikbayeva, Alimgazy Sydykov, Yevgeniy Mazulevskiy

ABSTRACT

The purpose of this study was to investigate the reduction smelting of antimony concentrate obtained in the course of lead production. The blend was prepared by mixing antimony concentrate with coke acting as a reducing agent. The reduction smelting was carried out in alundum crucibles in a shaft furnace with silicon carbide heating elements. Based on the experimental results the antimony concentrate reduction smelting in slag absence had to be carried out at optimum temperatures of 900°С-950°С and сarbon-containing reducing agent of 14 mass % - 16 mass % in respect to the concentrate. The formation of metallic antimony occurred without any decomposition in the course of the direct reduction of sodium metaantimonate. The concentrate reduction smelting carried out in presence of a refining flux provided to obtain rough antimony of low impurities content.

Keywords: antimony concentrate, reduction smelting, rough antimony.

Received 01 August 2017Accepted 10 May 2018

Journal of Chemical Technology and Metallurgy, 53, 5, 2018, 1001-1008

RSE “The National Center on Complex Processing of Mineral Raw Materialsof the Republic of Kazakhstan”, Almaty city 050036, Republic of Kazakhstan E-mail: erkej@mail.ru

INTRODUCTION

Antimony is primarily produced from sulfide ores. It can be also obtained from oxidized ores. The process mainly used is based on sulfide concentrates roasting, which is then followed by antimony oxide reduction with a carbon-containing reducing agent: 2Sb2S3 + 9O2 = 2Sb2O3 + 6SO2 (1)Sb2O3 + 3C = 2Sb + 3CO (2)

The rough antimony is refined from the impurities to give the branded metal. This procedure is employed by the leading manufacturers of antimony. Nevertheless, the process of oxidative roasting of sulphide ores and the reduction smelting of oxidized ores and concentrates aiming to improve the technical and technological parameters are under continuous investigation [1 - 5].

Various antimony-containing secondary materials like alloys, dust of the lead production, slag from the hydrometallurgical processing of zinc raw materials, etc.

are used for antimony productionThere is an industrial interest towards the antimony

concentrate [6] containing 40 % - 45% of antimony. It is obtained as an insoluble residue in the course of lead alkaline refining where liquid rough lead is repeatedly passed through a melt of sodium hydroxide and sodium chloride at temperatures of 420°C - 450°C. The melt is continuously charged with sodium nitrate. Then de-composition releasing oxygen proceeds. The impurities contained react with the refining fluxes to give sodium arsenate, sodium stannate and sodium antimonite:2As + 2NaOH + 2H2O = 2NaAsO2 + 3 H2 (3)2As + 4NaOH +2NaNO3 = 2Na3AsO4+N2+2H2O (4)5Sn+6NaOH + 4NaNO3 =5Na2SnO3 + 2N2 +3H2O (5)2Sb + 4NaOH +2NaNO3 =2Na2SbO4 +N2 + 2H2O (6)

The antimony concentrate is often added to sulfide concentrates as it is not separately processed in view of its small amount. Nevertheless, for countries having no antimony deposits, the development of a technology for

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its separate processing is a topical task. The research in this area is focused at a smelting reduction process to obtain rough antimony and antimony trioxide hydro-metallurgical processing [7 - 9].

Antimony concentrate smelting reduction is per-formed in absence or presence of fluxes. The techno-logical parameters effect on the antimony behavior is not considered in the literature. That of the impurities determining the antimony basic technical and economic indices is also not studied.

In connection with the above, the purpose of this study is to investigate the reduction smelting of antimony concentrate obtained in the course of lead production.

EXPERIMENTALMaterials

An antimony concentrate from lead production served as a feedstock. Its chemical composition is given in Table 1. Sodium hexahydroxoantimonate or mopun-

gite mineral (Fig. 1) was the basis of the raw material.Coke containing 12 % of ash was used as reducing agent.

MethodsThe process of preparing the blend comprised mix-

ing coke with antimony concentrate aiming to receive a high level melt on the ground of uniform particle size distribution and a corresponding chemical composition.

The concentrate reduction smelting was carried out in alundum crucibles in a shaft furnace with silicon carbide heating elements. The temperature was meas-ured by a chromel-alumel thermocouple. The feed and product recovery analysis was performed by atomic absorption using Perkin Elmer 240 FSAA device and photocolorimetry on SPEKOL 2000. The thermal analy-sis was carried out by using Q-1000/D derivatograph system of IOM (Budapest). The method was based on following the changes of the material thermochemical and physical parameters caused by the heating. The

Table 1. Antimony concentrate chemical composition, mass%

Sb Pb Sn As Na K Cu 47.55 0.77 0.73 0.12 4.741 <0.001 0.002

Bi Cd Fe SiO2 Ca Mg Zn <0.001 0.0007 0.34 <0.001 0.34 0.018 0.016

Fig. 1. XRD pattern of antimony concentrate.

Feruza Berdikulova, Abdurasul Zharmenov, Alma Terlikbayeva, Alimgazy Sydykov, Yevgeniy Mazulevskiy

1003

thermochemical state of the sample was described by Т (temperature), DTA (differential thermoanalytical), ТG (thermogravimetric) and DTG (differential thermo-gravimetric) curves. The latter were a derivative of the TG function.The recording was carried out in air in the temperature range of 20оС-1000оС. Heating mode was dynamic (dT dt-1 = 10oС min-1), calcined Аl2O3 was the reference material, the sample weight was 50 mg. The DTA sensitivity of the measurement system referred to 250 μv, while that of ТG – to 500 μv.

RESULTS AND DISCUSSION

The melting of antimony concentrate with coke goes through sodium hexahydroxoantimonate decomposition to sodium metaantimonite followed by its reduction to metallic antimony:NaSb(OH)6 → NaSbО3 + 3Н2О (7)2С+О2 = 2СО (8)2NaSbO3 + 3СО = 2Sb + 3Na2СO3 (9)Sb2O4 + 2С =2Sb + 2CO2 (10)

The solid-phase antimony concentrate reduction is a slow reaction, but the rate increases when it proceeds in a melt.

Experiments carried out were aimed at studying the effect of the temperature, the duration of melting and the coke consumption on the antimony recovery rate and the impurities behavior. The latter refer mainly to lead,

tin and iron. These metals oxides may also be present. The smelting reduction comprises a reaction of

melted antimony with the carbon-containing reducing agent. In fact the melt contains sodium salts of antimony oxide. The antimony obtained is released in the form of droplets and depleted in the form of an antimony slag phase. The latter covers the surface of the metal and prevents its evaporation.

The conventional methods of oxide materials smelt-ing reduction include slag-forming fluxes as additives. The first experiments on antimony concentrate smelting reduction are carried out in absence of slag-forming agents. In this case, the charge preparation is simplified. It consists of melting a mixture of a concentrate and coke and subsequent elimination of the slag. The results of these experiments are shown in Table 2. It is evident that the optimum amount of coke is 12 mass % - 16 mass % in respect to the concentrate. The further increase of coke consumption above 17 mass % - 18 mass % is not economically profitable as it leads to increased heating duration and Sb losses, while the correspond-ing decrease below 12 mass % results in a decrease of antimony crude metal recovery. Under optimum condi-tions, the antimony content of the rough metal is up to 96 %- 97 %, while the recovery rate is 96 % - 98 %. About 2 % - 2.5 % Sb are transferred to the slag, which is 20 %-25 % in respect to the concentrate weight. The

Тable 2. Influence of coke consumption on the recovery rate of antimony (temperature 950°С, duration - 30 min, weight of concentrate - 50 g)

№ Coke consumption,

%

Weight of rough antimony, g

Antimony

content in the

metal, %

Extraction of

antimony metal,

%

Weight of

slag, g

Antimony

content in slag,

%

Extraction of

antimony slag,

% 1 5 14 85.08 50.1 26.2 38.38 42.3 2 10 17 73.14 52.3 16 16.64 11.2 3 12 23.7 96.50 96.2 13 6.40 3.5 4 14 23.8 97.90 98.0 12.1 3.93 2 5 16 25.4 90.05 96.2 12.3 4.83 2.5 6 18 23.2 92.44 90.2 14.6 8.79 5.4 7 20 23.2 86.59 84.5 14.8 9.00 5.6 8 25 24.5 77.63 80.0 14.8 10.92 6.8

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slag content of antimony amounts to 3 % - 5 %. The optimum temperature of antimony concentrate melting is 900°С - 950°С (Table 3). Temperature values below 900°С do not provide a complete melting of the charge, which in turn results in low antimony yield and recovery.

Table 4 shows the content of antimony and main impurities in the most typical products of melting. The numeration of the experiments in Tables 2, 3 and 4 is identical. The concentration of elements such as Cu, Zn, Cd, Bi, K, Se, Te, Ca, Mg, Ni in rough antimony is not indicated because of their low content in the initial concentrate.

One of the main factors determining antimony con-centrate smelting is the rate of heating. The temperature should rise slowly to 920ºC at a rate of 3°С/min - 5°С/min to avoid rapid foaming and large losses of antimony. The duration of melting is 20min - 35min. The end is determined by the melting point.

The thermal behavior of a charge containing a

concentrate + 12 % coke (Fig. 2) is studied. Endo- and exothermic reactions take place in the course of heat-ing from 20°C to 1000°C. The thermogravimetric (TG) curve recorded in the temperature interval of 20°C - 155°C shows a weight loss of the sample proceeding in three stages. This as well as the first two peaks on the differential thermogravimetry (DTG) curve (at 60°C and 75°C) are attributed to the removal of weakly bound wa-ter in an amount of 3.5 mass % in respect to the sample. The next stage of dehydration takes place in the interval of 100°C - 155°C. It proceeds as a monotonic decrease of the sample weight by 1.25 % (Δm2) and is ascribed to the removal of the rest of the adsorbed water.

An endothermic reaction of the concentrate decom-position determines the form of TG curves in the range from 155°C to 185°C. It is followed by a well outlined weight loss amounting to 14.75 % (Δm3) of the heated matter is observed. This major step of the process of concentrate thermal degradation takes place at 200°C -

Таble 3. Influence of temperature on the recovery rate of antimony (coke consumption -14 %, duration - 30 minutes, weight of concentrate - 50 g).

№ Temperature, °С

Weight of rough

antimony, g

Antimony content in the metal,

%

Extraction antimony metal, %

Weight of slag,

g

Antimony content in

slag, %

Extraction of antimony

slag, %

9 850 25.2 83.40 88.4 16.8 14.58 10.3 10 900 24.3 90.31 92.3 14.3 10.81 6.5 11 950 22.5 97.32 92.1 12.3 4.06 2.1

Тable 4. Content of Sb and main impurities in rough antimony and slags, %.

No experi-ment

Rough antimony, % Slag, %

Sb Pb Sn As Fe Sb Pb Sn As Fe 1 85.08 1.55 1.86 0.15 0.67 38.38 0.06 0.25 0.06 0.23 2 73.14 1.58 1.44 0.16 0.76 16.64 0.007 0.21 0.05 0.24 3 96.50 1.57 1.73 0.34 0.72 6.40 0.08 0.24 0.05 0.26 4 97.90 1.62 1.92 0.31 0.82 3.93 0.07 0.26 0.04 0.25 5 90.05 2.42 0.20 0.86 0.38 4.83 0.03 0.29 0.05 0.32 6 92.44 2.44 0.21 0.88 0.41 8.79 0.04 0.27 0.06 0.33 7 86.59 2.52 0.86 0.022 0.44 9.00 0.011 0.28 0.013 0.53 8 77.63 4.85 0.85 <0.001 0.42 10.92 0.006 0.26 0.012 0.37 9 83.40 4.19 1.14 <0.001 0.42 14.58 0.007 0.24 0.120 0.40

10 90.31 2.76 0.56 0.120 0.35 10.81 0.012 0.66 <0.001 0.48 11 97.32 2.19 0.71 0.150 0.18 4.06 0.008 0.59 0.080 0.53

Feruza Berdikulova, Abdurasul Zharmenov, Alma Terlikbayeva, Alimgazy Sydykov, Yevgeniy Mazulevskiy

1005

320° C. It is visualized by a descending TG-line (with a slope of 30 degrees). It is connected the sample weight decrease by 5 % (Δm4) attributed to the removal of the remaining hydroxyl inclusions. The temperature range, coupled with this linear mass loss (Δm4), corresponds to the length of time where the state of the analyzed system should be regarded as established. It is worth noting that the decomposition of sodium hexahydroxoantimonate in this temperature range results in complete removal of physically and chemically combined water.

The concentrate destruction is followed by the thermal oxidation of coke with a clearly pronounced exothermic manifestation in the range of 320°C - 500°C. It is evident that the reducing process of the concentrate starts after the complete decomposition of sodium hexa-hydroxoantimonate to sodium meta-antimonite. The combustion of the organic inclusions present leading to the release of volatile substances in an amount of 10 % is seen on TG curve. A groove is formed on DTG-curve in the same temperature interval. It is attributed to the oxidation of organic carbon to CO2. The amount of lat-

ter released to the atmosphere amounts to 10 mass % in respect to the charge. It corresponds completely to the amount of combustible material (12 %). Such a statement is justified because calcine, i.e. a slag is formed during the combustion of coke requiring control and weight bal-ance prior to and after the heating process. It is also worth adding that the gas emitted by the organic inclusions may contain small fractions of residual water (as OH).

Small amounts of CaCO3 and Na2CO3 dissociate at temperatures of 500°C - 600°C and 600°C - 680°C, respectively. It is reported in the literature that such carbonates may be formed even in the initial stages of heating on the basis of sodium and calcium, and with the involvement of CO2 from the air. The concentrate itself is the source of sodium, while calcium is probably brought to the system by the coke. The smallest weight loss of 0.75 % (Δm8) registered on the thermogravimetric curve occurs in the range from 700°C to 1000°C. It is attributed to sublimation. The changes of the thermo-gravimetric curves form are not identical as they are connected with the release of different constituents of

Fig. 2. Derivation of charge: concentrate + 12 % coke.

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the system (Н2О, ОН, СОоrganic, СО2). Values of the mass loss at each stage of the expan-

sion of the sample match:Δm1 = 3.5 % (Н2О)Δm2 = 1.25 % (Н2О)Δm3 = 14.75 % (Н2О)Δm4 = 5.0 % (ОН)Δm5 = 10 %.0(СОorganic)Δm6 = 1.0 % (СО2)Δm7 = 1.5 % ( СО2)Δm8 = 0.75 % (Sublimation)______________________∑Δm1000°C = 37.8 %

The concentrate smelting product amounts to 74 %.The smelting product is antimony and a slag in the

form of sodium carbonate. The results of the thermal analysis show that the formation of metallic antimony takes place at 700°С, while the laboratory experiments indicate that this process takes place in the range from 900°C to 950°С. This is connected with the heating rate of the charge – under laboratory conditions it is 3°C - 5°C per minute in order to avoid abundant foaming, while it amounts to 10°C per minute in the course of a thermal analysis.

The rough antimony obtained above contains impu-rities whose amount is roughly 2 times than that required to obtain antimony of commercial quality. That is why refining of the rough metal from tin, iron, lead and arsenic is required. The number of refining stages and

the refining flux consumption depends on the amount of impurities in the rough metal. Experiments with soda, sodium sulfate potassium and sodium chloride as addi-tives are carried out aiming melting temperature decrease and partial refining of rough antimony. The results ob-tained are listed in Tables 5 - 7. The results obtained in presence of soda do not differ much from those referring to concentrate melting with the participation of coke only. The rough antimony contains the following metal impurities, %: Pb ~ 1,57; As to 0,34; Sn 1,44 - 1,92; Fe 0,82; Ca < 0,0001; Mg < 0,001; Na 0,096 - 0,59. The slags received in the course of smelting with coke and soda have the following contents of impurities, %: Pb ~ 0,06 - 0,08; As 0,05; Sn 0,24; Fe 0,25; Ca 1,04 - 1,28; Mg 0,075 - 0,13; Na 31,17 - 33,85. The slags received in case of smelting with coke only have impurities content, which does not differ from that obtained in presence of soda. The concentrate obtained in an ore-smelting furnace can contain soda melt but its content should not exceed 8 wt. %. It is so because the increased consump-tion of soda results in antimony losses.

The concentrate smelting with the addition of an equimolar mixture of KCl + NaCl to the charge de-creases the temperature of the process to 880ºС - 900ºС. It provides weak refining in respect to Sn and Fe and increases significantly the slag mass. Due to the refining the antimony content is increased to 98 %.

Melting with Na2SO4, NaOH and NaCl lowers the melting temperature by 50°C. It also decreases the ar-senic content (< 0.001%). This is possible because of

Таble 5. Flux effect on antimony concentrate melting (coke consumption -14 %, weight of concentrate - 100 g).

№ Flux consumption, %

Т,°С τ,min Rough

antimony, g

Content of Sb,

%

Sb extract in metal,

%

Slag, g

Content of Sb,

%

Sb extract in slag, % Na2CO3 Na2SO4 NaOH NaCl KCl

1 8 - - - - 900 20 47.2 96.62 95.91 27.0 4.40 2.50 2 8 - - - - 900 20 47.0 96.63 95.57 28.2 4.27 2.50 3 6 - - - - 880 20 46.7 96.69 94.96 27.7 4.09 2.38 4 8 - - - - 880 25 46.0 96.61 93.46 27.3 5.02 2.87 5 10 - - - - 895 35 46.1 96.45 93.57 27.9 4.56 2.68 6 - - - 6 6 723 30 41.4 96.17 86.87 43.2 3.04 2.79 7 - - - 6 6 740 35 45.4 94.64 92.84 38.6 1.15 0.96 8 - - - 5 5 740 35 45.4 95.50 93.64 38.2 3.54 2.92 9 - - - 6 6 736 35 46.2 91.70 91.54 40.4 3.37 2.94

10 - 6 4 20 - 874 20 47.8 98.44 98.8 48.6 1.83 1.52 11 - 6 4 20 - 865 20 45.8 96.12 95.12 55.6 2.46 2.96 12 - 6 4 10 - 885 20 45.8 97.88 96.86 43.8 3.52 3.33 13 - 6 4 6 6 724 30 44.8 98.52 95.37 45.2 4.52 4.39 14 - 6 4 5 5 724 35 45.6 98.17 96.73 45.8 3.19 3.16

Feruza Berdikulova, Abdurasul Zharmenov, Alma Terlikbayeva, Alimgazy Sydykov, Yevgeniy Mazulevskiy

1007

formation of sodium arsenate which is then transferred to the slag.

Melting with Na2SO4, NaOH, NaCl and KCl de-creases the temperature of the melt by 176ºC (from 900ºC to 724ºC), but the concentration of arsenic in the rough antimony is high. The other impurities content is very high. Therefore, the mixture of fluxes may be considered unsuitable for smelting antimony concen-trate. Melting antimony concentrate with circulating primary smelting slags aims to increase the degree of extraction of antimony from the feedstock. The results obtained are shown in Table 6. A series of 10 experiments

is executed. They refer to the following conditions: 50 % slag from the previous melting are injected, the weight of the concentrate amounts to 50 g, the coke consumption is 14 mass % in respect to the concentrate, the smelting duration is 20 min, while the temperature is 880ºC. Simi-lar experiments are carried out with sodium antimonate reduction with coke in presence of refining salts and circulating slag from previous runs (30 % of slag from previous runs is added). The weight of the concentrate is 50 g, the coke consumption is 14 %, melt duration is 30 min, and the temperature is 725ºC. The presence of Na2SO4 amounts to 6 %, that of NaOH - to 4 %, that of

Table 6. Content of Sb and main impurities in rough antimony and slag, %.

No of experi-ment

Rough antimony, % Slag, %

Sb Pb Sn As Fe Sb Pb Sn As Fe 4 92.73 1.55 1.86 0.15 0.67 5.24 0.06 0.25 0.06 0.23 5 92.78 1.58 1.44 0.16 0.76 4.85 0.007 0.21 0.05 0.24 6 96.62 1.57 1.73 0.34 0.72 4.40 0.08 0.24 0.05 0.26 7 96.69 1.62 1.92 0.31 0.82 4.09 0.07 0.26 0.04 0.25 8 96.12 2.42 0.20 0.86 0.38 3.04 0.03 0.29 0.05 0.32 9 95.50 2.44 0.21 0.88 0.41 3.54 0.04 0.27 0.06 0.33 10 98.44 2.52 0.86 0.022 0.44 1.83 0.011 0.28 0.013 0.53 11 96.12 4.85 0.85 <0.001 0.42 2.46 0.006 0.26 0.012 0.37 12 97.88 4.19 1.14 <0.001 0.42 3.52 0.007 0.24 0.120 0.40 13 98.52 2.76 0.56 0.120 0.35 4.52 0.012 0.66 <0.001 0.48 14 98.17 2.19 0.71 0.150 0.18 3.19 0.008 0.59 0.080 0.53

Таble 7. Results referring to concentrate melting with (I) coke and circulating slag, and (II) coke, refining salts and circulating slag.

No of experi-ments

Rough antimony, % Slag, %

Sb Pb Sn As Fe Sb Pb Sn As Fe

I Melting with coke and circulating slag 1 93.21 2.12 1.42 0.40 0.58 1.45 0.020 0.09 0.027 0.12 9 93.06 2.38 1.44 0.43 0.54 1.77 0.022 0.10 0.049 0.13 10 92.12 2.23 1.45 0.51 0.59 1.65 0.085 0.12 0.051 0.13

II Melting with coke, refining salts and circulating slag

1 96.30 2.31 0.78 0.22 0.15 2.25 0.010 0.54 0.19 0.47 2 96.62 2.29 0.71 0.14 0.15 1.86 0.015 0.48 0.29 0.46 9 95.73 2.44 0.74 0.18 0.14 2.29 0.017 0.53 0.29 0.45 10 96.18 2.52 0.89 0.21 0.18 3.06 0.066 0.52 0.35 0.45

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NaCl to 6 %, while that of KCl to 6 %.The analysis of the results of both series of melting

illustrates the additive effect in respect to impurities content decrease.

CONCLUSIONS

The reduction smelting of antimony concentrate in absence of a slag must be carried out at 900°C - 950°C. A carbon-containing reducing agent of 14 % - 16 % in respect to the concentrate weight is required.

The formation of metallic antimony occurs through direct reduction of sodium metaantimonate without its decomposition.

The reduction smelting of the concentrate with soda is recommended to proceed in an ore-smelting furnace. Molten sodium is the body resistance during melting with slag conductivity. Consumption of soda should not be greater than 8 %.

The reduction smelting of the concentrate with a refining flux provides the production of rough antimony of low impurities content.

Acknowledgements

Research was conducted under the grant funding of the Ministry of Education and Science of the Republic of Kazakhstan for 2015-2017 (No 0831/GF4, contract No 10 dated February 27th, 2017).

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