Features of Antimonate Concentrate Reduction in Alkaline Melts Under Carbon-Neutral Smelting Conditions

Abstract

In this study, we investigate the reduction smelting of antimony concentrate, where sodium antimonate is the primary antimony-bearing component, in alkaline melts. This study aims to reduce the carbon footprint of metallic antimony production. It is shown that traditional carbon reduction is accompanied by significant formation of carbon-containing gases and sodium losses due to volatilization. Based on thermodynamic analysis and experimental investigations, carbon monoxide is established as the key active gaseous reducing agent for antimonate compounds, predominantly operating in the temperature range of approximately 320–900 °C, which corresponds to the stages of coke oxidation and sodium antimonate decomposition. The authors propose introducing sodium hydroxide into the charge to form an alkaline melt with a lowered melting point when mixed with the antimony concentrate, ensuring the sequestration of carbon dioxide through the formation of sodium carbonate. Experiments confirmed the possibility of chemically fixing up to 75.5% of CO₂ into the slag phase at the laboratory stage and up to 87% of CO₂ during pilot tests of reduction smelting under a flux layer. Crude metal with an antimony content of 94–96.2% Sb was obtained, while coke consumption was reduced by 16–20%. The proposed approach ensures a simultaneous increase in the degree of antimony recovery, the utilization of carbon-containing gases, and the formation of a stable eutectic slag melt. This allows the process to be considered an element of carbon-neutral pyrometallurgical technology for processing antimony concentrates.

1. Introduction

Antimony is a strategically significant and critical metal due to its widespread use in the battery industry and the production of antifriction and radiation-protective alloys, semiconductor materials, specialty glasses, and ceramics [1,2,3]. Given the increasing global demand for and limited availability of high-quality ore resources, the challenges of processing low-grade antimony concentrates and intermediate products, as well as the development of technological solutions that combine industrial feasibility and reduced environmental impact, are becoming increasingly pressing [2,3,4].

Modern methods for processing antimony raw materials are conventionally divided into hydrometallurgical and pyrometallurgical methods. Hydrometallurgical processes based on the acid or alkaline leaching of antimony from stibnite concentrates and secondary materials are actively being developing due to the possibility of selectively extracting the metal at relatively low temperatures [5,6]. However, numerous studies show that such technologies are characterized by multi-stage processes, the formation of significant volumes of liquid waste, the need for reagent regeneration, and complex solution purification, which significantly limits their industrial application for the large-scale processing of concentrates [6,7,8].

A special application of hydrometallurgical processes is the processing of antimony-containing sodium compounds, including sodium antimonate, through dissolution in hydrochloric acid followed by the electrolytic reduction of antimony [7,9,10]. Despite the technological feasibility of hydrometallurgical processing schemes for antimony-containing sodium compounds, including sodium antimonate, these processes are characterized by several significant drawbacks, primarily due to the use of chloride solutions: the presence of Cl⁻ ions in the electrolyte causes intense corrosion of electrode materials and process equipment, promotes side electrochemical reactions (including chlorine release), reduces the antimony current efficiency and selectivity of reduction, degrades the stability of the electrolysis, complicates the control of process parameters, and also leads to the formation of highly reactive chlorine-containing waste, which collectively reduces the efficiency of the process and increases operating and environmental costs. Furthermore, the electrolytic reduction of antimony is energy-intensive but economically feasible primarily due to the production of high-purity metal from pre-purified solutions, but not for processing complex concentrates and process slags [10,11]. The formation of highly reactive chlorine-containing liquid waste also incurs additional costs for neutralization and purification [9].

Industry primarily relies on pyrometallurgical technologies for producing metallic antimony. The pyrometallurgical production of metallic antimony is accomplished by reducing antimony-containing compounds (Sb₂O₃, Sb₂S₃, and antimonates) with carbon, carbon monoxide, or iron at temperatures of 600–1300 °C in a reducing atmosphere with a low oxygen partial pressure, ensuring a negative Gibbs free energy change (ΔG < 0). The process is carried out at atmospheric pressure using reducing agents (coke, CO, Fe) and fluxes (SiO₂, CaO, Na₂CO₃), which promote the formation of a free-flowing slag and the efficient separation of the metallic and slag phases. Under optimal thermodynamic and kinetic conditions, antimony recovery reaches 95–98%, yielding metallic antimony with a main component content of up to 96–99% [12,13,14]. Research shows that the reduction of antimony compounds is accompanied by the intense formation of a gas phase, represented primarily by carbon monoxide and carbon dioxide [13,14,15]. Traditional antimony production technologies have a significant carbon footprint due to the high energy intensity of these processes and significant CO and CO₂ emissions [15,16]. Studies on the reductive and redox roasting of Sb₂S₃ have shown that the formation of CO is an inevitable consequence of carbon reduction, and its subsequent oxidation to CO₂ requires either an additional oxygen supply or special gas exchange conditions [17]. In most existing process flowsheets, carbon monoxide is considered an undesirable by-product subject to post-combustion or removal in gas cleaning systems without its intended use directly in the metallurgical process [14,17].

Against the backdrop of the implementation of global climate initiatives, including the Paris Agreement and IPCC recommendations, interest in the development of low-carbon and carbon-neutral metallurgical technologies is increasing [18,19,20]. Best Available Techniques (BAT) documents emphasize the need to move from simple emission reduction to closed technological cycles that enable the utilization and chemical fixation of carbon-containing gases within a single production process [19,20]. In energy and chemical engineering industries, the high efficiency of alkaline and alkaline earth compounds, including Na₂O, NaOH, and Na₂CO₃, has been demonstrated for binding CO₂ to form thermodynamically stable carbonate phases [21,22,23]. Thermodynamic studies have also indicated the possibility of multi-stage reduction processes in alkaline melts involving both solid carbon and gaseous carbon monoxide, which can act as secondary reducing agents [24,25,26]. However, the application of these approaches in antimony pyrometallurgy has so far been covered only partially in the literature.

Region-specific resources are of significant importance when choosing a carbon-neutral metallurgy strategy. The Republic of Kazakhstan has significant coal reserves and a developed infrastructure for its extraction and use, ensuring high availability of carbon-containing reducing agents for metallurgical processes [27]. Under these conditions, the complete replacement of carbon with alternative reducing agents, such as hydrogen, is economically and technologically unjustified for the processing of mineral concentrates. Analysis of literary data shows that hydrogen reduction technologies are focused primarily on the production of high-purity metals and powders from oxide or pre-purified raw materials and are accompanied by high capital and energy costs [24,25,26,28,29]. Their use for complex antimony concentrates leads to a decrease in process selectivity and an increase in product costs. Several studies have demonstrated the effectiveness of pyrometallurgical methods for the recovery and refinement of antimony from concentrates and intermediate products, including those generated in lead production [30,31]. These studies highlight the technological stability of carbon-based antimony reduction and the need for further process improvements by controlling the composition of the charge and slag phase.

Thus, an analysis of the literature shows that existing hydrometallurgical and electrochemical methods for processing antimony-containing raw materials are primarily focused on producing high-purity metal and tend to be highly complex and expensive. Traditional pyrometallurgical technologies, despite being industrially mature, yield significant emissions of carbon-containing gases and do not provide a closed carbon balance for the process. Given the need to reduce the carbon footprint of metallurgical production and Kazakhstan’s resource base, the development of technological solutions aimed not at eliminating carbon reduction but at increasing efficiency and the intraregional recycling of the resulting gaseous products is particularly relevant.

In this regard, this work investigates the reduction smelting of antimony concentrate, the main antimony-containing component of which is sodium antimonate, in an alkaline environment with the simultaneous utilization of carbon-containing gases, mainly carbon monoxide (CO) and carbon dioxide (CO₂), formed during the oxidation of carbon and participating in the reduction of antimonate compounds and carbonate-forming reactions. This study also aims to explore the mechanisms of antimony reduction, the role of the gas phase in multi-stage redox reactions, and the conditions for the chemical fixation of carbon dioxide needed to form stable carbonate phases. This allows us to consider the antimony production process as an element of a carbon-neutral pyrometallurgical technology aimed at processing concentrates and intermediate products on an industrial scale.

Thus, this study aims to investigate the reduction smelting of sodium antimonate concentrates in alkaline melts with the simultaneous utilization of carbon-containing gases. For the first time, a carbon-neutral pyrometallurgical scheme for antimony production based on internal CO utilization and chemical fixation of CO₂ into stable carbonate phases is proposed and experimentally validated at laboratory and pilot scales.

2. Materials and Methods

The investigated material is an antimony concentrate obtained during the alkaline refinement of crude lead for the removal of antimony, tin, and arsenic. The concentrate predominantly comprises mopungite NaSb(OH)₆ (Figure 1). X-ray diffraction analysis (XRD) was performed with a D8 Advance (Bruker, Billerica, MA, USA) apparatus, α-Cu, with a tube voltage of 40 kV and a current of 40 mA. The obtained diffraction data were processed, and interplanar spacings were calculated using DIFFRAC.EVA software (accessed on 06 June 2012). Sample interpretation and phase identification were performed using the Search/match program and the PDF-2 release 2023 powder diffraction database.

Chemical composition: wt.%: 47.5 Sb; 0.62 Pb; 0.54 Sn; 0.2 As; 10 Na; 0.34 Fe.

The reducing agent was Shubarkul coal coke. The smelting process was carried out in a roasting furnace using silicon carbide heaters, and the reaction zone was an alundum crucible. The process temperature was controlled with chromel–alumel thermocouples connected to a millivoltmeter for temperature measurement and gas sampling tubes for the chromatograph.

The gas phase was analyzed using a Crystal 4000M chromatograph (NPF Metallom LLC, Yoshkar-Ola, Russia, 2003) with a flame ionization detector (FID) to determine hydrocarbon gases and two thermal conductivity detectors (TCDs) to analyze the permanent gases carbon monoxide (CO), carbon dioxide (CO₂), oxygen (O₂), and nitrogen (N₂).

Pilot tests of antimony concentrate with coke were conducted under a flux layer (a mixture of antimony concentrate and sodium hydroxide). The charge was prepared using the following method: 10 kg of granulated antimony concentrate and 1.4 kg of special coke from Shubarkul coal. Flux (granulated): 1.7 kg of antimony concentrate + 1.2 kg of sodium hydroxide. Melting unit: furnace with selite heaters; alundum crucible (H = 300 mm; d = 150 mm with a maximum charge of 4.5 kg of charge). Heating mode: gradual; initial current, 15 A; increase rate, 0.17 A/min.

Charge load: one-time batch of charge, 2.85 kg; flux above the charge, 0.725 kg. A total of 4 smeltings of crude antimony were carried out. The gas phase was captured in the gas cleaning system, with sampling for CO₂ content. Process progress: Upon reaching a temperature of 920 °C, the melting process continued for 25 min, after which the slag was poured into molds; then, the next batch of charge was loaded into the molten metal, followed by the loading of flux.

The charge composition and NaOH addition levels were selected based on thermodynamic calculations, preliminary laboratory experiments, and analysis of phase equilibria in Na–Sb–C–O systems.

3. Results and Discussion

The reduction smelting of the concentrate proceeds through the decomposition stage of sodium hexahydroxoantimonate, Na[Sb(OH)₆], to sodium antimonate, Na₃SbO₄ (Figure 2). TG analysis was performed at a heating rate of 10 °C/min under an air atmosphere.

At the beginning of the coke combustion process at 320–500 °C, CO is formed, which is an active reducing agent for sodium antimonate. Of all possible reduction reactions, only the reduction reaction (

Table 1. Thermodynamic data for the sodium antimonate reduction process.

T, °C	ΔH, kJ/mol	ΔS, kJ/mol·K	ΔG, kJ/mol	K	Log K
200	−316.202	−133.817	−252.885	8.320 × 1027	27.920
400	−320.268	−141.695	−224.89	2.832 × 1017	17.452
600	−328.712	−151.846	−196.125	5.418 × 1011	11.734
800	−296.056	−115.378	−172.239	2.422 × 108	8.384
1000	−209.911	−38.493	−160.9	4.000 × 106	6.602
1200	−216.811	−43.497	−152.733	2.606 × 105	5.416
1400	−227.036	−49.982	−143.407	3.002 × 104	4.477
1600	−240.346	−57.484	−132.67	5.012 × 103	3.700
1800	−256.596	−65.714	−120.361	1.079 × 103	3.033
2000	−275.68	−74.492	−106.349	2.080 × 102	2.444

) involving CO, according to reaction (1), is thermodynamically possible:

2Na₃SbO₄ + 2C + CO (g) = 2Sb + 3Na₂CO₃

(1)

Stoichiometric calculations of the reaction show that 36 g of carbon is required to reduce 509.4 g of sodium antimonate; i.e., 5.6 g of carbon or 6.9 g of Shubarkul coal special coke is required per 100 g of concentrate (of which 80.5 g is anhydrous antimony concentrate). With this special coke consumption, the degree of antimony reduction does not exceed 50–60%.

Experimental work has established an optimal coke consumption of 12–14%. Moreover, the slag yield as soda does not exceed 30%, whereas based on the initial sodium content, the slag yield should be 50–54%. This is explained by the partial volatilization of sodium as sodium peroxide and elemental sodium, formed by reaction (2) at a temperature of 700 °C:

2Na₂O → Na₂O₂ + 2Na

(2)

This effect is explained by carbon oxidation at temperatures of 320–500 °C and subsequent decomposition of the concentrate. This results in the release of CO₂ from the reaction zone and partial loss of sodium oxides.

To address this issue, we propose introducing sodium hydroxide, which has a melting point of 323 °C, into the charge. Partial melting of the charge before carbon oxidation allows for the binding of the released carbon oxides and dissolution of sodium oxides, ensuring carbon neutrality in the process.

The experimental results are presented in

Table 2. Degree of carbon extraction into slag depending on the composition of the charge; smelting duration, 30 min.

№	Consumption from Concentrate Mass, %		t, °C	Results of Recovery Smelting
	Coke	NaOH		Metal Yield from Concentrate Mass, %	Slag Yield from Concentrate Mass, %	Degree of CO2 Binding in Slag Melt, %
1	10	0	900	47.4	26	37.86
2	10	6	900	48.5	30	40.5
3	10	8	900	48.5	32	44.2
4	10	10	900	48.0	36	50.31
5	10	12	900	47.6	40	55.6
6	10	14	900	47.2	44	61.5
7	10	10	800	46.7	42	58.7
8	10	10	700	45.7	45	62.89
9	10	15	800	46.7	52	75.5

and

Table 3. Chemical composition of the obtained slags, %.

№	Sb	Pb	Sn	As	Fe
1	6.4	0.08	0.24	0.05	0.26
2	5.8	0.1	0.30	0.1	0.3
3	5.2	0.1	0.30	0.2	0.3
4	5.5	0.1	0.30	0.3	0.3
5	5.0	0.27	0.30	0.34	0.3
6	4.8	0.28	0.48	0.34	0.3
7	4.8	0.3	0.55	0.34	0.36
8	4.5	0.3	0.68	0.34	0.55
9	4.2	0.4	0.71	0.34	0.67

. The degree of carbon recovery (81% of the carbon is contained in the special coke) into slag was calculated based on the carbon content of the sodium carbonate.

The experimental results demonstrate the ability to bind more than 75.5% of CO₂ into the slag phase melt, forming soda (Table 3).

Thermogravimetric studies on the concentrate’s reduction smelting process in the presence of sodium hydroxide revealed the absorption of the resulting CO₂ in the range of 500–560 °C, forming sodium carbonate (Figure 3).

Thus, the developed process mode for the reduction smelting of antimony concentrate ensures a low-carbon antimony technology: a metal yield of 46–47% with Sb content of 94–96.2%, and a rate of CO₂ conversion to sodium carbonate of 75.5%. A reduction in the carbon dioxide content in the gas phase from 3 to 0.01% was achieved compared to the smelting process without the addition of sodium hydroxide (Figure 4).

To improve process efficiency and utilize the strong reducing agent CO, a process flow is proposed for the reductive smelting of antimony concentrate with coke under a flux layer of antimony concentrate and sodium hydroxide. This approach enables the further oxidation of CO during its interaction with sodium antimonate, yielding an additional amount of metal, and the conversion of CO₂ to sodium carbonate via reaction (3) with sodium oxide from sodium antimonate and sodium hydroxide (

Table 4. Thermodynamic data for the reduction of sodium antimonate (Na₃SbO₄) by CO (g) in the presence of NaOH.

T, °C	ΔH, kJ/mol	ΔS, kJ/mol·K	ΔG, kJ/mol	K	Log K
200	−1011.02	−0.79	−797.05	8.320 × 1027	27.920
400	−1015.20	−0.79	−644.97	5.418 × 1011	11.734
600	−1033.41	−0.82	−316.51	8.353 × 1018	18.922
700	−983.01	−0.77	−237.99	5.730 × 1012	12.758
800	−968.87	−0.75	−161.85	7.494 × 107	7.872
900	−800.65	−0.60	−94.04	1.537 × 104	4.187
1000	−783.64	−0.59	−34.44	2.589 × 101	1.413
1100	−767.76	−0.58	23.80	1.241 × 10−1	−0.906
1200	−751.44	−0.57	80.91	1.353 × 10−3	−2.869
1300	−735.77	−0.56	136.97	2.839 × 10−5	−4.547

). The role of sodium hydroxide in the charge is the formation of a eutectic mixture [20,21] with an optimal melting point for binding with carbon dioxide. The addition of sodium hydroxide promotes the formation of a low-melting eutectic alkaline melt, which reduces slag viscosity, enhances mass transfer, stabilizes sodium-containing phases, and creates favorable conditions for CO₂ absorption with subsequent formation of sodium carbonate. This significantly intensifies reduction kinetics and improves impurity distribution between metal and slag phases.

Carbon monoxide acts as an active gaseous reducing agent predominantly in the temperature range of approximately 320–900 °C, corresponding to the stages of coke oxidation and sodium antimonate decomposition.

The experimental results are presented in

Table 5. Average experimental results on the optimization of metallic antimony smelting modes at a pilot plant; smelting duration, 30 min.

№	Mixture		Flux		T, °C	Results of Recovery Smelting
	Concentrate, kg	Coke, kg	Concentrate, kg	Naoh, kg		Yield of Crude Metal from the Mass of Concentrate		Slag Yield from Concentrate Mass, %		Losses
						kg	%	kg	%	kg	%
1	3.25	0.455	0.2	0.55	900	1.37	30.76	2.47	55.44	0.61	13.8
2	3.25	0.455	0.375	0.375	900	1.36	30.63	2.63	59.13	0.46	10.24
3	3.25	0.455	0.55	0.2	900	1.41	31.55	2.60	58.32	0.45	10.13

,

Table 6. Chemical composition of the resulting rough metal, wt.%.

№	Sb	Pb	Sn	Na	Fe	Cu	Zn	C
1	94.3	3.87	0.32	0.36	0.26	0.11	0.1	0.41
2	94.07	4.03	0.27	0.23	0.33	0.17	0.22	0.29
3	95.1	3.35	0.39	0.33	0.49	0.10	0.18	0.37

and

Table 7. Chemical composition of slag with metal inclusions, %.

№	Na2CO3	NaOH	Sb	Pb	Sn	As	Fe	Cu	Zn	C
1	84.98	not found	12.47	0.43	0.24	0.18	0.045	0.0015	0.002	1.81
2	81.12	not found	16.64	0.34	0.30	0.19	0.13	0.005	0.001	2.04
3	80.82	not found	15.69	0.14	0.30	0.06	0.041	0.0015	0.0025	1.52

.

2Na₃SbO₄ + 5CO (g) + 4NaOH → 2Sb + 5Na₂CO₃ + 2H₂O

(3)

The slag, in addition to soda, contains 12–14% antimony, which consists of small metallic antimony inclusions and unreacted antimony concentrate. The X-ray diffraction composition of the slag was analyzed after the metallic antimony fraction was collected (Figure 5 and

Table 8. Phase composition of slag with metal.

Name of the Phase	Formula	Card Number in the Database	Content, %
Sodium antimonate catena-antimonate	NaSbO3	00-074-0124	12
Sodium hydroxide	Na2CO3	00-072-0628	80
Quartz	SiO2	00-081-0068	8

), and energy-dispersive X-ray spectral analysis was performed to determine the elemental composition (Figure 6,

Table 9. Elemental composition of slag.

Spectrum	O	Na	Al	Si	S	Fe	Sb	Result
Spectrum 2	26.59	40.10	0.74	0.91	0.66	0.73	30.26	100.00
Spectrum 3	37.25	48.18	0.75	1.24	0.77	0.00	11.81	100.00
Spectrum 4	39.41	49.25	0.66	1.74	0.72	0.00	8.23	100.00
Average	34.42	45.84	0.72	1.30	0.72	0.24	16.77	100.00

).

The experimental results demonstrate the possibility of efficiently utilizing CO as a reducing agent for antimony concentrate in a flux, resulting in the production of sodium carbonate. Thus, the process simultaneously increases the degree of antimony reduction, fixes CO₂ in the solid phase, and forms a stable slag melt with a eutectic composition, ensuring a carbon-neutral balance in the system.

Pilot tests of antimony concentrate with coke were carried out under a layer of flux (a mixture of antimony concentrate and sodium hydroxide).

Test results:

-

A total of 4.6 kg of crude metal containing 94.3% Sb was obtained;

-

A total of 8.5 kg of slag containing metallic inclusions was formed, and an additional 0.9 kg of crude metal containing 92% Sb was recovered after its separation;

-

Phase analysis revealed that the slag was 81% sodium carbonate, confirming an 87% conversion and capture rate of CO₂ emitted during concentrate reduction with coke;

-

The CO₂ content in the exhaust gases was 0.001%.

The material balance of the pilot tests for the production of pilot batches of crude antimony is presented in

Table 10. Material balance.

Flow of Materials	Units	Weight, kg	Sb, %	Weight, kg	Na2O, %	Weight, kg	C, %	Weight, kg
Received
Reducing charge
Antimony concentrate	kg	10	47.7	4.77	26.8	2.68	0	0
Coke	kg	1.4	0	0.00	0	0.00	78	1.092
Flux
Antimony concentrate	kg	1.694	47.7	0.81	26.8	0.45	0	0
Sodium hydroxide	kg	1.2	0	0.00	77.5	0.93	0	0
Received
Rough metal	kg	4.656	94.3	4.39	0	0.00	0.8	0.037
Slag consisting of sodium carbonate	kg	7.51	0	0.00	54	4.06	11.6	0.871
Slag consisting of metallic inclusions	kg	1.047	92	0.96	5.5	0.00	2.5	0.026175
Dust–gas mixture	m3	0.3	0	0.00	0	0.00	5.05	0.01515
Mismatch				0.22		0.01		0.142

.

Thus, pilot tests were conducted on a carbon-neutral technology for producing metallic antimony, involving smelting concentrate with coke under a flux layer (a mixture of antimony concentrate and sodium hydroxide), establishing the stability of the process modes and environmental performance. The rate of CO₂ conversion to sodium carbonate was 87%. The resulting CO was used as a reducing agent for additional antimony concentrate, producing metallic antimony. This reduced the coke consumption of metal smelting by 16–20%.

4. Conclusions

This research established the principles of reducing antimony concentrate smelting in alkaline melts and demonstrated the fundamental possibility of reducing the process’s carbon footprint through the regional recycling of carbon-containing gases. It was experimentally confirmed that antimony reduction proceeds through a multi-stage mechanism involving the active participation of carbon monoxide, which effectively contributes to the re-reduction of antimonate compounds in the temperature range of approximately 320–900 °C.

The introduction of sodium hydroxide into the charge promotes the formation of a low-melting alkaline melt, ensuring the chemical fixation of carbon dioxide to form sodium carbonate and reducing sodium losses due to volatilization. A CO₂ binding rate of up to 75.5% was achieved under laboratory conditions, and up to 87% was achieved in pilot tests under a flux layer consisting of a mixture of sodium hydroxide and antimony concentrate while simultaneously reducing the CO₂ content in the exhaust gases to 0.001%. Pilot tests confirmed the stability of the process modes and the reproducibility of the results, ensuring the production of black antimony with Sb content of 94–96.2% and a reduction in coke consumption by 16–20%. The obtained data demonstrate the feasibility of implementing carbon-neutral reduction smelting of antimony concentrates without eliminating the use of carbon reducing agents.

The developed approach is recommended for further optimization and scaling under industrial conditions and is of practical interest for the processing of antimony concentrates and intermediate products in regions with a developed coal base.