AU708224B2 - Ferro-nickel smelting - Google Patents

Description

Page 2 BACKGROUND OF THE INVENTION This invention relates to the smelting of ferro-alloys, particularly for the smelting of nickel-bearing ores of the lateritic type to ferro-nickel alloys.

The well known process for treating ores of this type as described in textbooks, consists of alternating current (AC) electric arc smelting of the ore. The AC electric arc process takes advantage of an electrical resistance heating effect to produce molten slag and metal. A reductant is added to the furnace so that nickel in the ore and some of the iron is 5 reduced to the metallic phase. It then sinks to the bottom of the furnace where it is removed by tapping as molten ferro-nickel. The slag floats on the surface and is also tapped from the furnace.

More recently there have been advances in electric arc furnace technology. The direct current (DC) arc furnace was developed because such a furnace offers certain distinct advantages over the AC furnace.

Some of these advantages are relevant to the production of ferro-nickel.

The DC arc furnace uses a single, hollow graphite electrode (generally the cathode), positioned centrally in a circular refractory lined steel shell above the molten bath of slag and metal. Current passes from the electrode through the molten bath to a second electrode (generally the P.17635/acm Page 3 anode) in the hearth of the furnace. The top electrode is not submerged in the slag but arcs to the slag. Current flows through the slag to the bottom electrode via the conducting metal bath and an electrical conducting system incorporated in the hearth. The furnace operates in an open bath mode with no overlying charge burden.

Some advantages of DC arc furnace operation over AC operation include: more controllable operation; ability to feed fine materials continuously without problems of gas eruptions; .ability to feed through the hollow electrode, or peripherally by side- S"feeders if required; energy is not transferred to the melt by electrical resistance ~heating, as in AC operation, and thus slag composition, which governs slag resistivity, is not as important a consideration; and the centralised energy input allows better control over slag attack on refractories by enabling more efficient formation of a slag freeze lining on the refractories.

However, all electric arc furnaces have the disadvantage of requiring electrical energy, which may be costly in certain locations or countries, to heat the charge materials and produce a melt. This is a particular P.17635/acm Page 4 disadvantage in smelting nickel-bearing lateritic ores because only 1% to 3% nickel is present in the ore and thus a very large amount of slagforming components have to be melted relative to the amount of ferronickel produced.

Despite the advantages of DC arc furnace operation, as already described, the DC arc furnace may be at a disadvantage to the AC furnace with respect to energy efficiency, for certain applications. Its operation in an open-bath mode results in greater energy losses by radiation compared S.'io0 with the AC furnace where the layer of charge above the slag decreases the heat energy losses.

However, both AC and DC furnaces are energy inefficient unless the ore is preheated. This is often practised. The ore can be pre-heated in a ".0*15 rotary kiln so that hot ore enters the electric arc furnace. Fluidized bed roasters have also been used to pre-heat the ore when it is in a finely divided state suitable, for example, as feed to a DC furnace. Gas which is emitted from the arc furnace can be used as fuel for the pre-heating process.

There is another process which has been used to smelt ferro-alloys although, to the applicant's knowledge, and at least in the case of nickel bearing laterite ores, never on a large scale. This process is generally P.17635/acm Page described as the use of oxygen or oxygen-enriched air together with a source of fuel which are injected into a furnace where combustion takes place. This type of furnace is referred to herein as an oxy-air/fuel furnace.

The development of this type of smelting furnace was carried out at the CSIRO in Australia under the name "SIROSMELT". Subsequent commercial development was carried out by two organisations which use the trade names "ISASMELT" and "AUSMELT". The following references describe the development of these furnaces: .10 Floyd J.M. and Conochle "SIROSMELT The First Ten Years".

Extractive Metallurgy Sypmposium. Parkville. Australasian IMM. 1984, pp 1-8 (Australasian IMM Symposia Series, No. 36).

Brew "Status Report on Isasmelt"., Minerals Industry International, March 1994, pp 15-17.

Floyd J.M. and Short "Ausmelt Development of Top-submerged Lance Technology", Minerals Industry International, March 1994, pp 18-24.

If a lance is inserted into a liquid slag bath, and air or oxygen-enriched air and fuel, such as coal, oil or natural gas, are injected through the lance into the bath, then the bath can be supplied with energy to maintain the liquid in the molten state and to melt the incoming ore charge with or P.17635/acm Page 6 without significant reduction. The lance is protected from the molten slag by the cooling effect of the gas passing through the lance.

Reduction of the ore can be undertaken at the same time, and can be achieved by feeding an excess of reductant in the form of coal or other carbonaceous material. A disadvantage of this type of process is that it generates a larger volume of gas, due to the partial combustion of the fuel and the reduction reactions, than what is produced in an electric arc furnace. This also leads to dust losses.

.o SUMMARY OF THE INVENTION The invention provides a method of treating a metal bearing material for ferro-alloy production which includes the steps of producing molten slag by melting the material in an oxy-air/fuel furnace, and producing a ferroalloy by reducing the molten slag in an electric arc furnace.

The operating conditions in the oxy-air/fuel furnace would include the degree of reduction and the temperature of the slag.

The degree of reduction will be very limited and for example may be sufficient to reduce at least 50% of the iron oxide in the ore and preferably about 90% of the iron oxide to the Wustite form (FeO). If a greater ratio P.17635/acm Page 7 of fuel to air is used such that more iron oxide is reduced then excessive gas will be produced.

The slag temperature must be sufficiently high to ensure that the liquid slag can be transferred.

The combination of conditions used for the two furnaces is selected to reduce the total cost of energy consumed in the process. The total energy cost in turn depends on the relative cost of electrical energy and .no coal, and the energy input required from the electrical energy and the t* coal, respectively.

The aforementioned process is particularly suited for the smelting of nickel-bearing ores of the lateritic type, and the production of ferro-nickel.

i: In the oxy-air/fuel furnace, oxygen and coal or any other suitable carbonaceous energy source may be injected into a molten slag bath through a lance.

The oxygen may be carried in air.

The ore, or other form of metal-bearing material, may be fed continuously or in batches into the molten slag bath. Gas and entrained dust, P.17635/acm Page 8 generated in the oxy-air/fuel furnace, may be removed and separated from each other and the dust may be fed to one, or both, of the furnaces.

Energy may be recovered from the gas, for example by cooling the gas in a boiler to produce steam which may then be used to generate electricity.

The electric arc furnace is preferably a DC arc furnace.

BRIEF DESCRIPTION OF THE DRAWING The invention is further described by way of example with reference to the *accompanying drawing which schematically depicts a method according to the invention for the smelting of a lateritic nickel-bearing ore.

DESCRIPTION OF PREFERRED EMBODIMENT The accompanying drawing illustrates a process of heating and smelting according to the invention, for the treating of laterite nickel ore.

The process of the invention includes two main steps namely melting the metal bearing material or ore 14 in an oxy-air/fuel furnace 10 and a reduction step which is carried out in a DC arc furnace 12.

P.17635/acm Page 9 The material 14 is dried or preheated in a preheater 15 and is then fed into the oxy-air/fuel furnace 10. The preheater 15 may be a rotary kiln, a fluidised bed, or other suitable device which, preferably, uses as an energy source gas, such as carbon monoxide, which is produced in the DC arc furnace 12. After having been preheated, the material 14 Is fed to the oxy-air/fuel furnace 10 where it is melted by heat released from the combustion of oxygen-enriched air 16 which is injected together with coal 18 or any other suitable carbonaceous energy source through a lance which is inserted into the liquid slag bath in the oxy-air/fuel furnace .0 The material or ore 14 is continuously fed into the molten slag bath for melting to take place. Very little excess carbon monoxide is produced because the gas is almost fully burnt to carbon dioxide. The extent of reduction which takes place is not significant and is such that about of the iron oxide is reduced to Wustite (FeO). This is beneficial for as the degree of reduction increases, more excess gas is produced.

Gas 20 which is generated in the oxy-air/fuel furnace 10 is extracted and fed to a separator 22 which separates entrained dust 24 from the gas.

The separated dust is fed either to the DC arc furnace 12 through its hollow electrode, or to the oxy-air/fuel furnace, possibly after agglomeration. The separated gas 23 may be fed to a device 25 which contains a boiler which produces steam from the hot gas. The steam may P.17635/acm Page then be used to generate electricity. This energy recovery step increases the economic viability of the process of the invention.

The molten slag 26, with nickel and iron oxides still present in the slag, is transferred to the DC arc furnace 12 continuously or on a batch basis.

Coal 28, or any other suitable reductant, is fed to the molten bath in the DC arc furnace. No combustion takes place since no air is fed to the furnace. Consequently no excess gas is produced and the gas, which has a high content of carbon monoxide, as has already been described, is .:10 suitable for subsequent post-combustion in the preheater 15 to provide energy for drying or pre-heating of the material 14 in a rotary kiln or fluidised-bed or similar pre-heating operation.

Electrical energy 30 to the DC arc furnace 12 is used to maintain the slag in a molten state so that the nickel oxide in the slag and some of the iron oxide may be reduced to the metallic phase. The metallic phase sinks to the bottom of the furnace and is removed by tapping molten ferro-nickel 32 from the furnace. The barren slag 34 floats on the surface of the melt and is also tapped.

The electrical energy 30 fed to the arc furnace is not used to melt the ore.

The melting takes place in the oxy-air/fuel furnace 10 and, for this step, energy in the cheaper form of coal, or any other suitable carbonaceous P.17635/acm Page 11 energy source such as oil or gas is used rather than electrical energy.

The combination of the two furnaces thus optimises the cost of the energy used in the process of the invention. The lower cost energy is used for melting while the electrical energy is used in the reducing step.

The oxy-air/fuel furnace 10 may be of the kind developed by the CSIRO of Australia under the name of SIROSMELT and marketed under the trade names ISASMELT and AUSMELT. This is not limiting and any equivalent oxy-air/fuel furnace may be used in its stead.

Example Of The Method Of The Invention Three tons of a sample of laterite ore from Indonesia were fed to a small oxy-air/fuel furnace built by the AUSMELT company. The furnace was fired with natural gas. The ore was fed to the furnace at 100 kilograms per hour. A small amount of silica sand was added as a flux. The slag was tapped from the furnace at a temperature of 1500°C. The ore and slag were analyzed and the results are given in the following table. The degree of iron reduction is expressed in the form of the valency of the iron. It will be noted that the proportion of the reduced form of iron to the oxidized form is about 53%.

P.17635/acm Page 12 Ni Co Fe Fe 3 Fe 2 SiO 2 MgO CaO Al2 Laterite 1.95 0.10 19.4 18.3 1.1 33.4 24.0 0.08 0.7 Slag 1.73 0.12 17.2 8.0 9.2 42.2 26.2 1.63 1.9 Since the smelting furnace and the DC arc furnace were not at the same location it was necessary to recover the smelting furnace slag by granulation in water.

":The slag was transferred to the DC arc furnace. The DC arc furnace was operated at 200kVA. Slag was fed to the DC arc furnace together with coke obtained from a coke oven. Four separate tests were conducted in which the ratio of furnace slag to coke was varied. The slag and metal recovered from the DC arc furnace were weighed and analyzed and the results are given in the table attached.

Since the AUSMELT furnace was very small and heat losses were large it was not possible to measure the energy consumption during the trial.

It is calculated that the energy which would be required to heat the ore and to melt it to slag for liquid transfer to the DC arc furnace would be 0,4 Mwh per ton of slag transferred. In addition some of the iron was reduced consuming 0,065 Mwh per ton slag. The energy measured during the last three DC arc furnace trials was an average of 0,68Mwh per P.17635/acm Page 13 ton slag transferred. If hot transfer had taken place, the energy consumption would have been 0,215 Mwh per ton slag. This corresponds to 0,2 Mwh per ton ore smelted. Thus the saving in electrical energy by using the oxy-air/fuel furnace (if hot transfer had taken place) was 0,433 Mwh per ton of ore treated. Lower cost fuel energy was used to heat the ore in the oxy-air/fuel furnace.

*o P.17635/acm Page 14

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4 C t C 20 Ausmelt slag, kg 501 401 398 512 Red, kg Cake/t feed slag 26.2 -28.7- 35.6 40.4 Tapping temp,,C 1551 1582 1633 1618 Pouct .masses, k: Spent slag 491 381 357 j441 Alloy 27.5 21.7 J 37.7 j47.2 Sla aalyses, NIO 0.17 0.15 0.09 0.07 coo 0.04 0.04 0.02 0.02 FeO 16.6 14.8 11.6 9.87 42.2 44.7 47.3 49.3 MgO 28.1 29.4 31.1 31.6 CaO 1.93 1.94 2.05 2.18 A1 2 2.87 2.53 2.79 2.69 analy.e.,..

Co -2.5 1.7 1.2 Fe 42.3 64.8 79.6 83.1 NI 55.1 33.5 19.2 15.9 SI 0.09 0.11 0.33 0.26 C 0.0 0.0 0.05 0.26 P 0.18 0.32 0.15 0.33 S 0.13 0.15 0.19 0.25 Eleme t.r. v ryi.lly NI 90.6 92.9 95.5 96.61 Co 54.1 65.4 78.3 84.7 Fe 6.5 18.7 39.3 46.6 Notes: Red Reduct-ant- NEC Net Energy Consumption P. 176351acm Page G6MS The claims defining the invrenon are as fi~ 1. A method of treating a metal bearing material for ferro-alloy production which includes the steps of producing molten slag by melting the material in an oxy-air/fuel furnace, and producing a ferro-alloy by reducing the molten slag in an electric arc furnace.

2. A method according to claim 1 which includes the step of transferring molten slag from the oxy-air/fuel furnace to the electric arc furnace.

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5.5 3. A method according to claim 1 or 2 wherein oxygen or oxygenenriched air, and a carbonaceous energy source, are injected into a molten slag bath in the oxy-air/fuel furnace.

to 4. A method according to any one of claims 1 to 3 wherein oxygen the electric furnace is a DC are furnace.

A method according to any one of claims 1 to 4 which includes the steps of removing and separating gas and entrained dust, generated in the oxy-air/fuel furnace, and feeding the dust to at least one of the furnaces.

6. A method according to claim 5 which includes the step of P.17635/acm

Claims (4)

1. 7. A method according to any one of claims 1 to 6 which includes the step of drying or preheating the metal bearing material prior to feeding the material to the oxy-air/fuel furnace.
2. 8. A method according to claim 7 wherein carbon monoxide, produced in the electric arc furnace, is used as an energy source for the drying or pre-heating step.
3. 9. A method according to any one of claims 1 to 8 wherein the metal bearing material is a nickel-bearing ore of the lateritic type and the ferro-alloy is ferro-nickel.
4. 10. A method of treating a metal bearing material for ferro-alloy *t* production substantially as hereinbefore described with reference to the accompanying drawing. DATED this ELEVENTH day of MARCH 1997 3 s c c BV fea^^^^^pe.a s.v. Z^g"N Patent Attorneys for the Applicant/Nominated Perso' SPRUSON FERGUSON P.17635/acm Ferro-Nickel Smelting Abstract A method of treating a metal bearing material (14) for ferro-alloy production which includes the steps of producing molten slag (26) by melting the material (14) in an oxy-air/fuel furnace and producing a fero-alloy (32) by reducing the molten slag (26) in an electric arc furnace (12). S r ee* oo* e o• [N:\LIBU]11467:JED