Application Study on Technology of Reducing Copper Content in Discarded Slag

Abstract

Effects of flux , vulcanizing agent and reductant on slag cleaning in new-type slag cleaning furnace (Φ3.6 × 8.1 m) were investigated under the conditions including smelting temperature of 1200 and 1250 °C, settling time of 30 and 60 min, and Fe/SiO₂ of 1.7–1.9. The results show that addition of SiO₂ , B₂O₃ or CaF₂ into original copper smelting slag can improve settling separation effect. Though no effect on copper recovery , addition of FeS can provide heat required by reaction with oxygen and reduce copper dissolution in spinel phase and slag . Copper content in discarded slag can be controlled at 0.26% under the optimum comprehensive conditions with ratio of oxygen to natural gas of 1.6.

Introduction

With the sustainable development of China copper smelting industry , copper ore resource is declining gradually. At present, copper mine containing 0.2–0.3% Cu has been exploited. However, Cu content in slag cleaned is above 5%. Copper smelting slag is the important secondary resource of copper . The increment of copper smelting slag reaches up to 15 million tons per year and in which copper content is above 750 kt. Therefore, the technology of reducing Cu content in discarded slag was researched and developed in order to realize the further cleaning of copper smelting slag , reduce Cu content in discarded slag substantially and improve the copper recovery , which had great significance to push the sustainable development of China copper smelting industry.

Due to the limitation of conventional copper smelting process, Cu content in slag are increasing. How to recover the copper resources efficiently has become an important measure to improve the recycling of resources and the intensive development of copper production.

At present, the main processes used for slag cleaning at home and abroad are dilution furnace and mineral separation process [1–6]. Dilution furnace uses AC power generally and has the advantages of small footprint and so on. However, the Cu content in discarded slag is 0.6–0.8% generally. Copper recovery is low and electricity consumption is large. The technology has not been able to break the bottleneck of average Cu content in discarded slag of 0.6% in the industrialization production. As far as mineral separation process, it covers a large area and is suitable for processing sulfide slag . Copper in slag can’t be recovered fully. Experts and scholars from universities, research institutes and enterprises involved in copper smelting in the world are concentrating on the technology of reducing Cu in discarded slag . But none of them has developed the advanced and mature technology.

Dongying Fangyuan Nonferrous Metals Co ., Ltd. has carried out the application study on the technology of reducing Cu in discarded slag with the University of Queensland, Australia. The submerged combustion process was applied in the process of treating copper smelting slag effectively. A series of measures such as the reasonable control of conditions like temperature inside the furnace , addition of vulcanizing agent into burdening, addition of combined flux in the smelting process and the injection of gaseous reductant into furnace are taken to lower the temperature and viscosity of slag , optimize the cohesion and growth mechanism of copper particles, improve the settling and separation conditions of copper and achieve the further cleaning of slag . The main technical indexes like Cu content in discarded slag achieved a historic breakthrough and reached the international advanced level of less than 0.3%.

Main Equipment, Raw and Auxiliary Materials and Technical Route

Main Equipment and Raw and Auxiliary Materials

Equipment: new-type slag cleaning furnace (Φ3.6 × 8.1 m)and EMPA.

Raw and auxiliary materials: copper smelting slag as raw materials, borax (B₂O₃), fluorite (CaF₂) and quartz sand (SiO₂) as flux , FeS as vulcanizing agent and natural gas (CH₄) as reducing agent.

Technical Route

Oxygen, natural gas , air and nitrogen are blown into new-type slag cleaning furnace by lances. Fuel is heated inside the melt and four kinds of gases are switched in proportion automatically according to process requirement. Addition of vulcanizing agent can provide heat required by reaction with oxygen and reduce copper dissolution in spinel phase and slag . Gaseous reductant are blown into furnace in the smelting process to accelerate the reduction reaction and reduce the content of magnet effectively. Addition of combined flux can form good slag type and keep the viscosity and temperature of slag within a reasonable range In the meanwhile, slight stirring generated by submerged combustion is transmitted to settling zone, which can promote the impact, cohesion, growth, settling and separation of matte particles. Copper content in discarded slag can be controlled at 0.26% and reach the international leading level through controlling the ratio of oxygen to natural gas under the optimum comprehensive conditions.

Methods and Conditions

In this study, the single furnace operation mode was adopted, which meant that slag cleaning are carried out by batch and capacity of processing copper smelting slag was 80t/batch . At the beginning of experiment, we must preheat the furnace firstly. The heating curve of the refractory was in strict accordance with the parameters provided by the manufacturer. When the temperature inside the furnace reached to the design requirements, molten bath making started. After that, oxygen, natural gas and air were blown into furnace for heat supply by oxygen lance. At the same time, the furnace was rotated to production position and the ratio of oxygen to natural gas ratio was controlled at 1.6 and oxygen concentration 60%. After the production was stable, oxygen, natural gas , air and nitrogen were switched in proportion according to the process requirement. The effect of flux , vulcanizing agent and reducing agent on the slag cleaning was tested under the conditions including melting temperature of 1200 and 1250 °C, settling time of 30 and 60 min and Fe/SiO = 1.7–1.9 , in order to find the optimum comprehensive technical conditions.

Main Research

Effect of Flux on Cu Content in Cleaned Slag

Effect of SiO₂

It is observed from Fig. 1 that matte droplets gather and grow up gradually in slag with the increasing of SiO₂ at the smelting temperature of 1200 and 1250 °C and settling time of 30 and 60 min, which has good settling effect. With the rising of smelting temperature and extension of settling time, Cu content in cleaned slag will be lowered. It can be seen from Table 1 that Cu₂O in cleaned slag is decreased to 0.45 and 0.36% with addition of 3 and 10% SiO₂ into the original copper smelting slag respectively, which indicates that the increasing of SiO₂ and smelting temperature are possible to lower the solubility of Cu₂O in slag .

../images/468727_1_En_38_Chapter/468727_1_En_38_Fig1_HTML.gif — Fig. 1
Microstructure of quenched samples with different SiO₂ addition

Table 1

Components of smelting slag and cleaned slag with SiO₂ (%)

Samples	“FeO”	Cu₂O	CaO	SiO₂	Al₂O₃	MgO	S	ZnO	Fe/SiO₂
Smelting slag	57.6	3.0	1.0	25.4	4.0	0.7	1.1	3.0	1.8
Figure 1a	58.9	0.45	0.9	26.7	4.5	0.8	0.7	2.7	1.8
Figure 1b	59.6	0.36	0.8	28.1	5.5	0.6	0.6	2.6	1.7

Effect of B₂O₃ or CaF₂

It is observed from Fig. 2 that matte droplets fade away on the surface of slag with the increasing of B₂O₃ or CaF₂ at the smelting temperature of 1200 and 1250 °C and settling time of 30 and 60 min. It indicates that the interfacial tension of slag and matte is decreased by addition of B₂O₃ or CaF₂. The melting point of B₂O₃ and CaF₂ is low, so it is possible to lower the melting point and viscosity of slag to promote the settling separation of matte. Cu₂O in cleaned slag decreases from 0.49 to 0.40% with addition of 3 and 5% B₂O₃ (CaF₂) into smelting slag respectively as seen in Table 2. This probably is due to the decreasing of Cu solubility in the slag with addition of B₂O₃ (CaF₂).

../images/468727_1_En_38_Chapter/468727_1_En_38_Fig2_HTML.gif — Fig. 2
Microstructure of quenched samples with different B₂O₃ (CaF₂) addition

Table 2

Components of smelting slag and cleaned slag with B₂O₃ (CaF₂) (%)

Sample	“FeO”	Cu₂O	CaO	SiO₂	Al₂O₃	MgO	S	ZnO	CaF₂/B₂O₃	Fe/SiO₂
Smelting slag	57.6	3.0	1.0	25.4	4.0	0.7	1.1	3.0		1.8
Figure 2a	58.3	0.49	0.9	24.5	4.1	0.7	0.7	2.9	5.6	1.9
Figure 2b	57.8	0.40	1.0	26.7	5.0	0.6	0.6	3.0	6.7	1.7

Effect of Vulcanizing Agent on Cu Content in Discarded Slag

Addition of FeS can provide heat required by reaction with oxygen and reduce copper dissolution in spinel phase and slag . Also FeS is dissolved into the slag and matte. As seen Fig. 3, spinel phase in the slag is reduced with the increasing of FeS at the smelting temperature of 1200 °C for 30 min. From Table 3, Cu₂O in cleaned slag decreases from 0.75 to 0.73% when addition of FeS increases from 3 to 9% and no obvious change of Cu content in slag is observed. However, Fe and S are increased because FeS could be dissolved into slag . It is found through research and analysis that addition of FeS has little effect on lowering Cu content in cleaned slag .

../images/468727_1_En_38_Chapter/468727_1_En_38_Fig3_HTML.gif — Fig. 3
Microstructure of quenched samples with different FeS addition

Table 3

Components of smelting slag and cleaned slag with FeS (%)

Sample	“FeO”	Cu₂O	CaO	SiO₂	Al₂O₃	MgO	S	ZnO	Fe/SiO₂
Smelting slag	57.6	3.0	1.0	25.4	4.0	0.7	1.1	3.0	1.8
Figure 3a	63.8	0.75	1.1	26.7	5.4	0.6	1.7	2.9	1.9
Figure 3b	63.1	0.73	1.0	27.9	4.8	0.7	1.9	3.0	1.8

Effect of Reducing Agent on Cu Content in Discarded Slag

Under the optimum and comprehensive conditions including smelting temperature of 1250 °C, settling time of 60 min and Fe/SiO₂ = 1.7, 10% SiO₂, 5%B₂O₃ (CaF₂) and 9% FeS are added respectively with adjusting the ratio of oxygen to natural gas in order to research the effect of reducibility of natural gas on Cu content in cleaned slag . According to the result of preliminary experiments, experiments on reduction effect of natural gas are conducted when the ratio of oxygen to natural gas is controlled at 1.6 and 1.8 respectively. As seen Fig. 4, when the ratio of oxygen to natural gas is 1.8, spinel and matte phases coexist in the slag , in which spinel phases are partly reduced and the settlement of matte on the bottom is observed. No spinel is observed and all matte droplets are almost settled on the bottom when the ratio of oxygen to natural gas is controlled at 1.6. It can be seen from Table 4 that Cu content in discarded slag is 0.33% at the ratio of oxygen to natural gas of 1.8 and it is decreased to 0.26% at the ratio of 1.6.

../images/468727_1_En_38_Chapter/468727_1_En_38_Fig4_HTML.gif — Fig. 4
Microstructure of quenched samples with different ratio of O₂/CH₄

Table 4

Components of smelting slag and cleaned slag with natural gas with different ratio of O₂/CH₄ (%)

Sample	“FeO”	Cu₂O	CaO	SiO₂	Al₂O₃	MgO	S	ZnO	O₂/CH₄	Fe/SiO₂
Smelting slag	57.6	3.0	1.0	25.4	4.0	0.7	1.1	3.0		1.8
Figure 4a	59.5	0.33	1.0	25.1	5.4	0.6	1.7	2.5	1.8	1.9
Figure 4b	58.9	0.26	0.9	25.9	6.6	0.6	1.8	2.6	1.6	1.7

The technology of reducing Cu content in discarded slag has been applied to the project of producing 100kt/a blister of Dongying Fangyuan Nonferrous Metals Co ., Ltd. Industrialization demonstration project of processing 250 kt/a copper smelting slag has been constructed. At present, the project run with stabilization , reliability, safety, environmental friendly, high production efficiency and advanced technical indexes. The average Cu content in discarded slag has reached the international leading level of 0.28%. In accordance with processing 250 kt/a copper smelting slag , Fangyuan can recover more 750 t/a copper and newly increased sales revenue RMB 40,000,000. If the technology is used to process 10 Mt/a copper smelting slag in China, 3000 tons of copper can be recovered and revenue of 1.56 billion yuan will be increased.

Conclusion

1.
Addition of SiO₂ into the original copper smelting slag can improve the settling separation effect of matte droplets in the slag . With the increasing of SiO_2, improvement of smelting temperature and extension of settling time, Cu content in cleaned slag is decreased obviously.
2.
Addition of B₂O₃ or CaF₂ can reduce the interfacial tension of slag and matte and is possible to lower the melting point of slag , and make slag diluted to promote the settling separation of matte.
3.
Though no effect on copper recovery , addition of FeS can increase Fe and S in slag .
4.
Under the optimum and comprehensive conditions including smelting temperature of 1250 °C, settling time of 60 min and Fe/SiO₂ = 1.7, addition of 10% SiO₂, 5%B₂O₃ (CaF₂) and 9% FeS and the ratio of oxygen to natural gas of 1.6, Cu content in discarded slag can reach 0.26%.

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