Flash Smelting of Lead Concentrates

Esko O. Nermes and Timo T. Talonen

SUMMARY Oxygen-autogenous flash smelting of lead concentrates

followed by slag reduction by injection coal in an electric furnace has been developed and is ready for commercial application. Pilot-plant studies demonstrate that the proc­ess works. Pilot studies have established process charac­teristics .. The process is easily controlled. Process equip­ment and operation are based on the extensive experience with Outokumpu flash smelting technology in smelting copper and nickel. The process equipment is small, even for high capacities .. Flash smelter and electric furnace equipment are designed for close fit in order to meet the environmental control requirements.

INTRODUCTION Outokumpu flash smelting of copper concentrates was

introduced in 1949 and flash smelting of nickel concen­trates in 1959, both at HaIjavalta in southwest Finland. The first flash smelter outside Finland was started up in 1956 in Ashio, Japan. Between 1956 and the present, about 25 flash smelters for copper and nickel concentrat­es have been put into operation in different parts of the world.

Outokumpu's research organization has been highly involved in the development of flash smelting. For ex­ample, a straight biister flash smelting process was developed in 1975 and introduced in Gl'og6w, Poland, in 1978, and another has been designed for Zaire .. This straight metal process is based on low iron content of the concentrate, which reduces the amount of recycling slag copper. Additionally, the concentrates are rather low in impurities.

Some pilot work on flash smelting of lead concentrates was performed at Outokumpu in the 1960s and 1970s. Four years ago, the latest development work on flash smelting of lead concentrates was started, resulting in the process now being ready for commercial application.

GENERAL ASPECTS OF FLASH SMELTING OF SULFIDE CONCENTRATES

Flash smelters are commercially operated in two dif­ferent process ways:

1. The matte process, which comprises drying, two-step oxidation in the flash smelting furnace and the con­verter, followed by slag cleaning.

2. The straight metal process, which includes drying, one-step oxidation-smelting in the flash furnace fol­lowed by slag cleaning.

The matte process is commonly used to smelt copper and nickel concentrates. It is considered that a two-step oxidation process results in more effective elimination of impurities and a lower load of circulating slag metal. Scrap and reverts are remelted in converters.

The straight metal process is used for copper and lead concentrates. The high-strength sulfur dioxide gas and waste heat as high-pressure steain are recovered totally from the flash smelting line instead of the two streams

JOURNAL OF METALS· November 1982

of gas and dust required in the matte process. Both investment and operation costs are reduced and more effective environmental control is made possible.

Drying of Concentrates

The concentrate is dried to 0.1-0.3% moisture in a direct-dired rotary kiln or indirect steam drier. The drier is provided with gas-handling equipment and pneumatic conveyor, which takes the dried concentrate into the feed­ing system of the flash smelting furnace. Separate low­temperature drying of concentrate is the cheapest and most direct way to get rid of the feed moisture. All water evaporated or generated, e.g., from fuel in the high­temperature smelting unit, reports in weak acid at the sulfuric acid plant. Since the weak acid is usually satu­rated with such impurities as arsenic, cadmium, and mercury, it cannot be used or discharged into the. envi­ronment; because of this, concentration of the weak acid by evaporation and salting out the impurities has been introduced. In order to avoid production of large amounts of weak acid and eventually double evaporation of the feed moisture, separate drying of concentrates is recommended, using process waste steam or carbon mon­oxide from the electric furnace.

Flash Oxidation-Smelting

Dried feed is fed pneumatically at measured rates into the flash distributor in the roof of the reaction shaft or combustion chamber. The feed is efficiently distributed into flash suspension by air, oxygen-enriched air, or oxygen. The oxidation is rapid, easily controlled, and results in the desired matte grade or straight metal and simultaneous smelting of the materials. A certain over-oxidation of iron in reference to the matte-slag or metal-slag equilibrium normally occurs. Sulfur elimina­tion is partial or complete, depending on the oxidation degree and whether the operation is matte or straight metal. Sulfur dioxide strength is high, e.g., 10-70%, depending on sulfur content and combustion value of the concentrate, oxidation degree, and whether the heat bal­ance is achieved by oxygen or fuel. Matte or metal and slag are tapped separately or together, continuously or batchwise, from the settler. The gas with dust enters the waste heat boiler, electrostatic precipitator, and sulfuric acid plant. Dust is separated and recycled pneumatically.

Flash suspension is the most favorable condition to make heat exchange due to good mixing conditions and the large surface area of the feed. This shows up in the small temperature difference between gas and molten products, which decreases the corrosion and erosion of bricks. Controlled over-oxidation of iron has the same effect; the magnitude slag phase solidifies on the brick lining.

The degree of oxidation is easily controlled by the oxygen-to-concentrate ratio. The heat balance is con­trolled by oxygen, oxygen-enriched air, air preheated air, coal, sulfur, pyrite, oil, etc. The most feasible way is to

55

FEED

FLASH BULLION

DUST

CO

Figure 1. Flowsheet.

SLAG BUL LION

run autogenously by oxygen enrichment or oxygen, which increases the heat efficiency, smelting capacity, and sulfur dioxide strength, while decreasing the gas load per ton of feed. Also, process equipment size is decreased, which means lower investment and operating costs. Pro­duction of weak acid is less compared to use of hydrogen­containing fuels, e.g., coal.

Gas Handling

Waste heat, dust, and sulfur dioxide gas are recovered from the flash furnace gas. The high-pressure waste-heat boiler comprises radiation and convection sections. Dust is partly sulfatized when cooled from 1250°C to 350°C. It is taken from the boiler and precipitator by drag convey­or and pneumatically recycled to the feeding system of the flash furnace. Sulfur dioxide gas is used for produc­tion of sulfuric acid, elemental sulfur, or liquid sulfur dioxide.

Slag Cleaning

Two alternative slag-cleaning methods are commercially used: reduction by coke, coal, or gas in an electric fur­nace, or flotation of slowly solidified slag. Both are suit­able for matte and metal process. The choice between them depends on local conditions and cost.

FLASH SMELTING OF LEAD CONCENTRATE

Based on experiences with commercial Outokumpu-type flash smelters and numerous pilot plant runs, the proc­ess to be tested at the pilot was chosen. It is a direct metal process comprising continuous, oxygen-autogenous flash smelting, followed by continuous slag cleaning by coal reduction in.a separate electric furnace (Figure 1). Thus the process is analogous to the straight blister flash smelting which was commercially introduced in 1978.

The following main process characteristics were to be checked at the pilot plant:

• Total elimination of sulfur • Total oxidation and slagging of iron • Amount of lead in slag and of sulfur in bullion

56

VOL-%

80

OXYGEN 60

40 SULFUR DIOXIDE

20

40 50 60 70 % Pb

Figure 2. Oxygen concentrate In combustion air and sulfur dioxide concentrate In outlet gas for the flash furnace.

• Fuming of lead and lead compounds • Amount of recycled dust • Kinetics of slag reduction and settling • Corrosion of bricks There was no doubt about being able to eliminate

sulfur and iron during flash smelting. The main ques­tions were lead content in the oxidized slag, fuming, amount of dust, slag reduction, and corrosion problems. According to calculations, it appeared certain to be able to oxidize sulfide and produce metallic lead at gas phase oxygen pressure of 10-5-10-7, which also corresponds to

JOURNAL OF METALS· November 1982

the minimum content of lead compounds in the gas as well as low states of fuming and dusting.

Corrosion of bricks in the flash furnace was estimated to be less if low lead content (20-40%) in the slag is achieved and small quantities of lead oxide and lead sulfide are present in the gas. Corrosion problems in the electric furnace should not be severe because of the rela­tively constant 1-3% lead content in the slag. To be sure of continuous slag reduction, pretesting of coal injection was made and found successful.

Nm3 It CONC.

800

600

400

200

40 50 60 70 % Pb

Figure 3. Amounts of air and oxygen in combustion air and the amount of outlet gas for the flash furnace.

tIt CONC.

0.8

0.6

0.4

0.2

40 50 60 70 % Pb

Figure 4. Amounts of slag and bullion from the flash furnace.

JOURNAL OF METALS • November 1982

Pilot Flash Smelter

A pilot flash smelter is one of the basic research u~its at the Outokumpu Metallurgical Research Centre. Durmg the past 25 years, numerous different ~ash process mod~­fications have been tested. The capacIty of the plant IS 2-10 tons concentrate per hour, which corresponds to 10,000-50,000 tons of lead per year. The pilot comprises all essential smelter equipment: rotary drier, dry concen­trate bins (1,000 tons), pneumatic conveyors for concen­trate and dust, flash furnace (15 m3), waste heat boiler

tIt CONC.

0.8

0.6

0.4

0.2 DUST

40 50 60 70 % Pb

Figure 5. Amounts of steam production and recycled dust from the flash furnace.

80

60

40

20

0/0

I 40

TOTAL RECOVERY

I 50

I 60

I 70 % Pb

Figure 6. Lead recovered In the flash furnace and electric furnace.

57

(3 tons steamlh), electrostatic precipitator (2,500 Nm3/h) , electric furnace (2 MV A, 250 V). The flash furnace gas is scrubbed with caustic and the electric furnace gas with water. To adapt the pilot plant for lead concentrat­es, a new type of concentrate burner was developed, the flash furnace was provided with a special melt and dust separator, the dust recycling equipment was modified, and the plant was made tight in order to meet the environmental requirements for a lead smelter.

The pilot has been operated at a level of 3-5 tons concentrate per hour ranging 20-75% lead. Bullion and slag were continuously tapped from the flash furnace into the electric furnace, from which lead was siphoned and waste slag overflow granulated. The pilot process is computer-controlled, with 80 measured process values. Several thousand tons of concentrate have been treated.

Results

The analysis of materials with approximate limits of variation is shown in Table 1. The main results have been collected as graphs in Figures 2-10, which are mod­ified for a capacity of 100,000 tons of lead per year for concentrates ranging 30-75% lead and lead content of the flash slag 20-40%.

Table I: Analysis of Materials, wt.%

Pb Fe Cu Zn S Concentrate 47-75 6-2 0.8-1.5 2-4 14-15 Flash slag 20-40 8-13 0.6-1 5-7 0.1-0.3 Flash dust 66-68 0.1 0.5-1.5 10-11 Waste slag 1-3 12-22 0.1-0.2 3-6 0.1-0.2 Bullion 1-2 0.1-0.5

IN CONCLUSION

The process works very well. Flash oxidation and slag reduction are easy to control independently. Thus, lead content of flash furnace slag of 20-40% and sulfur con­tent of bullion of 0.1-0.5% is achieved by changing oxygen-to-concentrate ratio. Independent of the oxidation degree of 20-40% lead in the flash furnace slag, the oxygen potential of gas is in the range of 10-5-10-7, which results in minimum possible dust amount, when con­densed lead and molten material are separated in the flash furnace.

The reduction of slag is performed continuously with two coal injectors in the electric furnace. The reduction and settling of slag is rapid and easily controlled to result in 1-3% lead in the waste slag. Temperatures in the flash and electric furnaces were varied between 1200-1300°C. Lime and pyrite were used as flux.

Pb % IN SLAG

40

30

20

10

0.5 1.0 S % IN BULLION

Figure 7. Dependence of the lead In the slag upon the amount of sulfur In b.ulllon.

58

According to the results in Figures 2-10, a flash smelt­er producing 100,000 tons of lead per year would con­sume the materials and energy given in Table II. The steam production is 3-20 tons per hour. If carbon monox­ide is recovered, as commercially introduced at the Outokumpu ferro chrome plant, an additional energy source corresponding to 10-20 tons steam per hour is available. Waste steam and carbon monoxide can be used, for example, for drying concentrates.

For concentrates 40-75% lead, flash oxidation produces straight metal (= flash bullion) 60-95% of the lead con­tent of the concentrate feed. The rest is recovered by

100

POWER

80 kWh It CONC.

60

40 COAL

kg It CONC. 20

40 50 60 70 % Pb Figure 8. Consumption of power and coal In the electric furnace.

tit CONC.

0.8

0,6

0.4

0,2

40 50 60 70 % Pb

Figure 9. Amounts of waste slag and bullion In the electric furnace.

JOURNAL OF MET.A:LS • November 1982

400

300

200

100

COMBUSTION HEAT / MCAL/t CONC

CO JGAS Nm3/tCONC

40 50 60 70 % Pb

Figure 10. Amount of carbon monoxide gas and its combus­tion heat in the electric furnace.

reducing slag in the electric furnace. Total recovery of lead is 97-99%. The amount of flash furnace gas is small, 3,000-15,000 Nm3/h, gas strength 40-65% S02, and dust in recycle 1.5-5 tons per hour, which is pneumatically fed back into the flash furnace for decomposing the lead sulfate.

The process equipment at the pilot plant has operated satisfactorily in spite of some freezing problems in the launder between the flash furnace and the electric fur­nace. This is due to the bad layout at the pilot plant. In commercial plants, there is no horizontal melt launder; slag and bullion are tapped directly into an electric fur­nace through the shortest possible vertical channel.

The new "heavy duty" concentrate burner has worked properly and the new lead and melt separation construc­tion in the furnace has performed quite well. The sepa-

Table II: Material Consumption Based on Production of 100,000 tons of Lead per Year

Lead in Concentrate Concentrate, metric tons Oxygen, metric tons Coal, metric tons

40%

250,000 70,000 16,000 23,000

60% 170,000 40,000 6,000

10,000

75% 130,000 23,000 2,000 4,500 Power, MWh

ration degree of the device is high, so only gaseous com­pounds enter the waste-heat boiler. The lead sulfate dust pneumatically recycled has not caused blocking problems in the boiler, electrostatic precipitator, or conveying equipment. Corrosion of bricks in the flash furnace and electric furnace has been measured after each pilot run. It is obvious that the brick lining in the flash smelting furnace and electric furnace has a life equal to that in a blister flash smelter.

ABOUT THE AUTHORS

Esko O. Nermes, Manager of Research, Metallurgical Research Centre, Outokumpu Oy, Pori, Finland.

Mr. Nermes received his MSc in chemical engineering. He joined Outokumpu in 1956 where he has worked as superintendent, technical manager, and resident manager for the company's copper and nickel refinery and flash smelters in Harjavalta and Kokkola;

Finland. He was· named manager of research in 1979. He is a member of The Metallurgical Society of AIME.

search Centre.

Timo T. Talonen, Chief, Lead Flash Smelting Project, Metallurgical Research Centre, Outokumpu Oy, Pori, Finland.

Mr. Talonen received his MSC in process engineering and joined Outokumpu in 1966. Since that time, he has worked as plant met­allurgist, Flash Smelting and later as senior metallurgist. He currently serves as chief of the lead flash smelting project at the Re-

Contains 60 papers summarizing the most important developments of the last decade in applications, extractive metallurgy, and environ-

------::

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