Application of side-submerged combustion smelting(SSC) process in recovery of lead smelting by-products

The treatment of lead smelting by-products,such as anode slime and antimony-containing dust and slag is an important process, which is not only related to improvement the rate of comprehensive recovery of resources, but also plays a very important role in improving the economic benefits of plant. The existing by-product treatment technology and current situation were introduced in first. Combined with the current treatment process and characteristics of by-products, this paper focuses on the characteristics of the oxygen-enriched submerged combustion side-blown smelting technology and the application of by-products treatment. Based on the engineering practice and relevant technical and economic indicators of treating 5812t anode slime annually, compared with the conventional technology, this technology has obvious advantages in operation cost, treatment efficiency and environmental protection.


Introduction
Lead resources are mainly distributed in Australia, China, Russia, Peru, etc.China's lead reserves account for nearly 20% of the total, and like other important metals, there is a problem of excessive exploitation and utilization of lead.China is the largest producer of refined lead, with a production of approximately 60% of the world refined lead [1,2].Under the tight supply of lead concentrate in China, smelter is seeking complex resources, imported lead concentrate, silver concentrate, etc. to expand the source of raw materials.In recent years, lead smelters often get low TC/RC (Treatment and refining charges for Processing concentrates), thus the smelters are on the edge of loss.Facing this situation, smelters need to compensate for their benefits by increasing the income of by-products.The revenue from rare and precious metals has always been the main source of profit for lead smelter, especially the recovery of precious metals such as gold, silver, antimony, and bismuth.
In order to improve the resource recycling efficiency and benefits, lead smelters are maximizing the amount of valuable metals associated with raw materials, resulting in a sharp increase in the output of anode slime, copper dross, antimony dust, etc. during the lead refining process.Although there has been a significant improvement in economic benefits due to the increase in by-products, the impact on existing comprehensive recovery systems is gradually becoming prominent.Mainly reflected in the outdated equipment, insufficient processing capacity, deteriorating indicators, and backlog of precious metals in the anode slime treatment system.
The yield of lead anode slime is usually 1~3%.For a lead smelters with an annual output of 100,000 tons, the annual output of anode slime is 3000 tons at a yield of 3 %.At present, the yield of anode slime in lead smelter enterprises is generally 3 ~ 6 %.For smelter that deal with low lead-containing materials in large quantities, the yield of anode slime is greater.The conventional pyrometallurgicalhydrometallurgical combined process is widely used in most lead smelter, includes two steps: pyrometallurgical reduction, oxidation refining and electrolytic refining [3,4].There are some main advantages of the process: wide adaptability of raw materials, simple and reliable equipment, moderate processing capacity, and low technical requirements.
However, the processes still have shortcomings, such as long working period, producing excessive slag, more precious metal backlog, large dust collection systems, dust pollution and poor working conditions.The main purpose of the pyrometallurgical process is to produce qualified silver alloys for further refining in the electrolysis system.After the successful application of bottom blown smelting and side blown smelting in lead smelting, Chinese metallurgical researchers have developed new processes for treating lead anode slime using oxygen bottom blown smelting and oxygen-enrich side blown smelting.In this paper, the treatment process widely used in China is briefly analyzed, the characteristics of the new treatment process are introduced, and the application prospect of the new process is prospected.

Conventional smelting process
The conventional pyrometallurgical treatment process is the wet anode slime is piled up for oxidation for 7~10 days, then add reducing coal and flux, and add to the reduction smelting furnace in batches.Heat is provided in the natural gas or diesel burner to maintain the furnace temperature above 1100 °C.PbO, CuO, Bi 2 O 3 , part of Sb 2 O 3 and As 2 O 3 in anode slime were reduced by reducing coal to obtain molten metal lead and enrich gold and silver.Flux is usually soda, which can form stable slag with oxides such as arsenic and antimony in anode slime, and reduce the melting point of slag, improve the fluidity of slag, and make it easy to separate from silver-rich lead.During the reduction, some Sb and As volatilize into the dust, which can be used as raw materials for recovering Sb.The silver-rich lead produced by the smelting furnace is transferred to a converter where it is blown with air to further oxidized and refined to produce qualified gold and silver alloys.
The smelting process is carried out in two special converters.There are many procedures and long operation period in converter operation, which are divided into feeding, melting, precipitation, slag discharge, antimony blowing and lead discharge.Usually, the operation period of the smelting furnace is generally 15~24h, and the operation period of the refining furnace is generally 60~78h.During operation, the feeding and discharge needs to be completed manually.
The natural gas or oil burner is used to heat the smelting furnace.The heat transfer mode is mainly radiation, which is low efficiency, and most of heat loss taken away by the off-gas.Usually gas consumption of 1t anode slime smelting section is about 100~200 m 3 /t, and gas consumption of 1t silverrich lead refining section is about 1500~2500 m 3 /t, which equivalent to gas consumption of 1t anode slime smelting section is about 1000~2000 m 3 /t.

Bottom blowing smelting process
A lead smelter in Henan Province built a new anode slime process system with a capacity of about 6000 tons in 2006, which is the first oxygen bottom blowing smelting system.The new process system consists of 5 smelting furnaces, which including an oxygen bottom blowing smelting furnace, an oxygen bottom blowing refining furnace, two refining furnaces, and a slag smelting furnace.After a period of operation and modification, the indexes of the new smelting system have reached or exceeded the design indexes.Practice has proved that the process of oxygen bottom blowing smelting anode slime is feasible, and the energy consumption and cost of anode slime smelting are significantly reduced [5,6].
The bottom blowing smelting process is mainly divided into three stages, and the process flow is shown in Figure 2: Figure 2. Process flow diagram of bottom blowing smelting process First stage: The anode slime is continuously fed into the oxygen bottom blown smelting furnace with coal and other return.Under the heating of the natural gas burner, and the oxidation and stirring of the bottom oxygen lance, most of the arsenic, antimony and some lead volatilized into the off gas, and a small amount of antimony, lead and quartz react to form slag, and bismuth, copper and some lead were reduced to form molten lead, which enriches a large amount of rare and precious metals such as gold, silver and tellurium.The smelting slag is sent to the recovery system to further recover antimony.
Second stage: the preliminary oxidation refining of silver-rich lead in the bottom blowing refining furnace to improve the grade of gold and silver in lead, and prepare for the final refining.In the refining furnace with bottom oxygen supply, arsenic, antimony, lead, bismuth and other impurities in silver-rich lead are oxidized.The refining furnace with oxygen supply from bottom has strong oxidation ability and has a good removal effect on impurities such as arsenic, antimony and lead in precious lead.
Third paragraph: It is difficult to oxidize impurities such as bismuth, tellurium, copper in the alloy, and the final impurity control is relatively low.So Intermittent refining is adopted to remove all impurities in gold-silver alloy, meeting the requirements of subsequent electrolytic refining.
Process characteristics:  Chutes are used to connect different melting furnaces in different heights, reducing the workload of melt transfer. The processing capacity is greatly improved, more than doubled. The bottom blowing has strong impurity removal ability and high precious metal grade in lead, which is beneficial to improve the efficiency of refining process.Arsenic and antimony are enriched in dust, which is beneficial for the recovery of arsenic and antimony. Fresh and wet anode slime can treated directly, which can shorten the operation period and reduce the intermediate occupation.
 The energy utilization efficiency is improved, the heat transfer efficiency is improved by bottom blowing stirring, and the metallurgical process reaction is strengthened.The natural gas consumption of the bottom blowing smelting process is only 60~70 % of that of the conventional converter smelting process.

Technical description
Side-submerged combustion bath smelting technology(SSC) is a kind of enhanced bath smelting technology for treating non-exothermic materials, which inject oxygen-enriched air and fuel ( natural gas, producer gas, pulverized coal ) into the molten bath with the multi-channel side-submerged lance at high speed and pressure.The materials on the molten bath are rapidly mixed into the bath due to the strong agitation of the injected gas, and the physical and chemical reactions are completed in bath.
This SSC technology is particularly suitable for the treatment of non-exothermic materials, and also suitable for the treatment of small-scale materials such as anode slime.The blowing injected gas in the molten bath and the rolling molten slag create a huge reaction interface, where the material is heated, melted and reduced, and the reduced metal settle and separate from slag.Under the action of gravity, a relatively static metal layer below the lance is formed.Thus, a layer of slag is formed on the upper part of the metal layer, which collects some impurity elements and gangue.

Technical feature
1) Simple preparation process and environmentally friendly.The basic characteristic of the molten bath smelting is that the preparation is relatively simple, and the raw material does not need drying, and batching processes, which reduces the dust pollution.
The conventional converter smelting process has strict requirements on preparation.The wet anode slime needs to be stored and naturally oxidized for 7-10 days, which is convenient for removing impurities during the smelting process.At the same time, the converter smelting needs to limit the moisture of the material to avoid causing the furnace accident.
2) High temperature smelting, high heat utilization rate, and stable heat balance in the furnace.The oxygen-enriched air concentration reaches 60~80%.The amount of smelting off gas is small, which take away the less heat loss.Through the side blowing lance directly inserted into the molten bath, the fuel is injected into the bath to replenish heat.The fuel burns in the bath directly, and the heat is released fully and absorbed by the bath.The heating transfer is fast and the heat utilization rate is high.The temperature of the molten bath can be adjusted quickly by change fuel, and the heat balance can be controlled.
3) Control oxygen potential flexible in furnace By controlling the amount of oxygen and fuel, the redox atmosphere of the molten bath in the furnace can be flexibly adjusted, and the on-line control of the furnace can be realized.

4) High recovery rate and less backlog of precious metals is
The submerged bath smelting technology injects and blows the slag layer of the molten bath, and the position is higher than the bottom blowing smelting process.So less base metal in the silver-rich lead can be oxidized and the base metal content in the silver-rich lead is higher, then the ability to enrich precious metals is stronger, and the loss of precious metals in dust and slag is less, which is beneficial to improve the yield of gold and silver in the silver-rich lead.The silver content of side-submerged smelting dust is less than 1000g/t, and less than 700g/t in the slag, while the silver content of bottomblown smelting dust is less than 1600g/t, and 1300g/t in the slag.
The discharge frequency of SSC technical for silver-rich lead is higher, once every two hours; While the bottom blowing smelting technology has a long period of discharge, which usually takes 12~24 hours to discharge once, and there are many precious metals in the furnace.

Process flow
Submerged combustion smelting anode slime system mainly includes side blowing reduction smelting and slag settlement, preliminary refining and final refining.And the process flow is shown in Figure 3.
Anode slime, returned high-silver dust and a small amount of reduced coal were blended and mixed, then added to the reduction furnace continuously.Under the action of oxygen-rich air and natural gas supplied by the side-blown lance, violent combustion and agitation, part of the arsenic and antimony in the anode slime volatilized into the dust as low-valence oxides, and some of them reacted with lead oxide and gangue components to form slag. Most of bismuth, copper and some lead are reduced and form the alloy in the molten state, and the rare and precious metals such as gold, silver in the anode slime are enriched in the alloy.
The slag produced by the reduction furnace enters the slag converter for insulation through the chutes, further settling and separating the silver-rich lead mixed in the slag.The discharged slag is sent to the antimony system for further recovery of valuable metals.
The refining process is the same as the traditional smelting process, that is, the oxidation is used to remove the impurities except gold and silver in the alloy, and the gold and silver alloy with a gold and silver content of more than 98 % is obtained for further electrolytic refining.The oxidation order of metals in alloy is: Sb, As, Pb, Bi, Cu, Te, Se, Ag.
The silver-rich lead produced by the reduction furnace is initially removed in the preliminary refining furnace, mainly to remove arsenic, antimony, and some lead, which is preparing for further refining and reduce the period of final refining.By blowing air to the surface of silver-rich lead, the easily oxidized arsenic, antimony, lead are oxidized and volatilized into dust, and the grade of gold and silver in the alloy is improved further.The high-grade silver-lead alloy, which removes arsenic, antimony and some lead, complete the final refining in the refining furnace and produces qualified anode alloy.From the perspective of comprehensive recovery, the refining process is divided into two sections, which can not only improve the efficiency of the refining furnace, but also preliminarily separated arsenic and antimony dust as raw material for comprehensive recovery of antimony.

Application of SSC in anode slime smelting process
Taking the project with an annual processing capacity of 5812 t anode slime as an example, the operation of the project is briefly introduced [7].The lead anode slime produced by lead smelter, and copper smelter anode slime (copper removal ) are raw materials.The main components are shown in Table 1.The moisture of anode slime is about 30 %.The main smelting equipment of the anode slime smelting project is shown in Table 2.The main technical indexes of treating anode slime by side-submerged combustion bath smelting process are shown in Table 3.

Application of SSC in the smelting of antimony-containing materials
Antimony-containing dust and slag produced in the process of anode slime smelting are used as raw materials for comprehensive recovery of antimony.Taking the project with an annual processing capacity of 4685 t antimony-containing materials as an example, the operation of the project is briefly introduced.The main components antimony-containing dust and slag were Sb 35.5 % and Pb 7.9 %.The antimony comprehensive recovery process is shown in figure 4.  4. The main technical indexes of antimony comprehensive recovery by side-submerged combustion bath smelting process are shown in Table 5.

Conclusion
Side-submerged combustion bath smelting process has many successful practices in the non-ferrous metal smelting, and also has been applied in the field of anode slime and antimony-containing material smelting, which has showed the special advantages and characteristics.At present, after years of technical improvement and upgrading, the overall technical and equipment of main non-ferrous metals smelting in China have been significantly improved.However, the technical and equipment of rare and precious metal comprehensive recovery plants are uneven.The new technology is suitable for lead and copper anode slime smelting projects, other rare and precious metal smelting and comprehensive recovery, has broad market and application prospects.

Figure 1 .
Figure 1.Process flow diagram of conventional anode slime smelting

Figure 3 .
Figure 3. Process flow diagram of side-submerged combustion bath smelting process

Figure 4 .
Figure 4. Process flow diagram of antimony comprehensive recovery processThe main smelting equipment of antimony comprehensive recovery project is shown in Table4.Table4Equipment table of antimony comprehensive recovery project

Table 1
Chemical composition of anode slime (Dry basis, Wt % )

Table 3
Equipment table of anode slime smelting project Main technical and economic indicators of anode slime smelting