GB2104882A - Briquette for smelting process and method of its production - Google Patents

Abstract

A briquette for use in a smelting process comprises a smeltable metal compound in a crystalline lattice. The briquette may be made by mixing dross from Pb/Zn smelter and optionally coke dust with an ionic solution, for instance sulphuric acid, to form a paste. The acid reacts with the dross to form metal salts. The paste is briquetted and the briquettes are allowed to dry. On drying the metal salts form a crystalline lattice binding the unreacted dross into a briquette. The briquette may also include a fluxing agent such as sodium chloride. The briquette may be formed from fine ore from a crusher or material collected in a cyclone separator downstream of a smelter.

Description

SPECIFICATION Briquette for smelting process and method for its production This invention relates to a briquette for use in a smelting process, and to a method for its production. In particular, but not exclusively, the invention relates to a briquette for use in recycling lead/zinc dross to a lead/zinc smelting furnace.

Lead and zinc occur naturally as their sulphides in various ores. The ores are sintered and concentrated before being fed with coke to a smelting furnace maintained at 7000C and operating under reducing conditions. In the furnace, the lead and zinc compounds are reduced to the metal and sulphur dioxide and carbon oxides are released. Metal oxide dusts, ash and coke particles are also formed. Lead is formed as a liquid and is tapped off, but zinc is formed as a vapour which is removed from the furnace with the off-gases. Air is introduced into the furnace gas above the charge, and the temperature of the off-gases is therefore raised to about 1 0000C by the partial burning of carbon monoxide.

The furnace off-gases are led through a spray of liquid lead which condenses the zinc and also collects other material in the off-gases. The spray produces a semi-solid material containing lead, zinc and other metal values. This material is usually known as dross.

After passing through the lead spray, the off-gases are further purified in a cyclone separator which removes any remaining particulate material therefrom. The sulphur dioxide in the off-gas stream is conventionally used in the production of sulphuric acid.

In order to increase the recovery of metal values from the ore it is the current practice to re-cycle the dross to the smelting furnace. The dross as collected is not suitable for feeding directly to the furnace as it is in the form of a fine powder which will be blown out of the furnace before it can be smelted. Therefore the dross is formed into briquettes using binders such as bitumen or pitch. It has been shown that these binders will produce a briquette with adequate strength at room temperature so that it can be handled without breaking before introduction to the smelting furnace. However, the briquettes quickly lose strength on heating, so that on introduction to the furnace they are likely to break up and the dross is at least partially swept out of the furnace before it can be smelted.Moreover, as the binder is consumed, it gives off tars and organic vapours and gases which are not removed from the offgas stream, and which therefore cause contamination in the sulphuric acid plant.

It is therefore an aim of the present invention to provide a briquette and a method of its production which at least in part overcomes the disadvantages of presently used briquettes.

Therefore, according to the present invention, a briquette for use in a smelting process comprises a smeltable metal compound bound in a crystalline lattice.

Preferably the compound is a metal oxide or sulphide and advantageouly the compound is dross from a lead/zinc smelter.

According to a second aspect of the present invention, a method of forming a briquette for use in a smelting process comprises mixing a smeltable metal compound with an ionic solution to form a paste, briquetting the paste and drying the briquette to cause the metal compound to be bound in a crystalline lattice.

Preferably, at least one of the ionic species in the solution reacts at least in part with the metal compound so that it becomes chemically linked into the lattice. The ionic solution may be acidic or basic. In the former case, the acid function will react with the metal compound to form soluble salts which, on drying, will form a lattice binding the unreacted metal compound. Preferably the acid is sulphuric acid which may be obtained from the sulphuric acid plant normally associated with a smelter.

In the latter case, the basic function will react with amphoteric metals to form complex metal anions. For instance if sodium hydroxide is used with a metal compound including zinc, complex zincates will be formed, and, on drying, a lattice of sodium zincate will be formed.

A neutral salt may also be incorporated into the lattice. The metal compound may not react with the neutral salt. However the salt may perform a useful function. For instance sodium chloride may be incorporated as a fluxing agent in some smelting processes and may therefore facilitate the smelting of the briquette.

The neutral salt may also be formed during the production of the briquette. A first part of the metal compound may be treated with a basic solution and a second part with an acidic solution. If the two pastes are mixed together, briquetted and dried, the briquette will comprise a lattice including salts of the metal, salts of complex metal anions and salts formed by the neutralisation of the acidic and basic solutions.

The briquette may also contain a proportion of coke dust mixed with the metal compound. Coke is usually supplied in lump form, but during handling coke dust is formed. If this were included in the briquette it would increase the economic efficiency of the process. It is also possible to buy coke dust relatively cheaply from coke producers and users. Its incorporation into briquettes could reduce the amount of lump coke needed, thereby increasing the economic efficiency of the smelting operation.

The amount of solution used is conveniently in the range 2 to 10% by weight of the material to be briquetted. The exact amount chosen will need to balance the needs for a briquettable mixture and a short drying time. The concentration of the ionic solution will also need to be selected so that sufficient binder is present in the dry briquette. Moreover, the concentration of the solution should be chosen so that any activity of the solution is substantially neutralised, either by reaction with the metal compound or by addition of a neutralising agent. For instance, sulphuric acid may be used in concentrations from 10 to 30% by weight in water, and sodium hydroxide may be used in concentrations from 25 to 35% by weight in water.

The briquetting step may be carried out in any conventional type of press, for instance a mould and plunger, roll or extrusion press.

The metal compound may comprise dross from a smelter, material collected in cyclone separators downstream of the smelter, fines from ore crushers for preparing ore for smelting, or mixtures of both in any proportions. It is envisaged that the present invention will be of particular but not exclusive use in the briquetting of dross from zinc/lead smelting.

The briquettes of the present invention do not have equivalent crushing strength at room temperature to those made using pitch or bitumen as binder. However their crushing strength is sufficient for them to be able to withstand their charging to a smelting furnace without disintegration.

Moreover, the briquettes retain much of their crushing strength at temperatures up to about 3000C and do not produce any volatile material which can contaminate downstream operations. This is in contrast to briquettes made using pitch or bitumen as the binder.

The present invention will now be described, by way of example only, with reference to a process for the production of zinc, lead and sulphuric acid from a lead/zinc ore.

The ore is sintered and concentrated before being crushed and screened to a suitable size for introduction to a smelting furnace with coke. The furnace is maintained at 7000C and is operated under reducing conditions. Air is fed to the furnace gas above the charge and partial combustion of carbon monoxide in the gas raises its temperature to about 10000C.

The lead is formed as a liquid which is tapped off. Zinc is formed as a vapour which is removed from the furnace with the off-gas. The off-gas is passed through a liquid lead spray which condenses the zinc and produces a semi-solid material known as dross containing metal values.

Off-gases from the lead spray are passed to a cyclone separator wherein fine particles are separated from the gases. The sulphur dioxide in the gases is used in a plant for the separation of sulphuric acid.

Briquettes of dross were prepared using a pitch binder and also using sulphuric acid or sodium hydroxide as the binding agent. The latter two are methods according to the invention, whereas the first represents a prior art process.

In the prior art process a mixture comprising 92% dross and 8% pitch by weight was prepared and briquetted at 1 000C. There was no need to dry the briquette. The pitch was a coal tar pitch having a Ring and Ball softening point of 800 C. The briquettes were tested for crushing strength at room temperature and at elevated temperatures.

The briquettes according to the present invention were made by mixing dross and an ionic solution in predetermined proportions and concentration, briquetting the mixture at room temperature in a mould and plunger press at a pressure of 6 tpsi, and drying the briquettes at 11 OOC. The briquettes were about 1 inch in diameter, 0.8 inch long and weighed about 409. They were tested for crushing strength at room and elevated temperatures. The conditions of briquetting and the results are shown in the Table below which also shows the comparable results for the pitch bound briquettes.

It is apparent from these results that the pitch bound briquettes have superior strength at room temperature. However on heating they rapidly lose strength. In contrast, lattice bound briquettes have lower but adequate strength at room temperatures and much of this strength is retained at higher temperatures.

The briquettes made using sulphuric acid solution were tested for residual acidity by immersion in de-ionised water for about 1 6 hours. It was found that the vast majority of the sulphuric acid had reacted with the dross, thereby showing that the dross was at least in part chemically linked into the briquette.

Thus the present invention provides a briquette for use in smelting processes which allows the efficient recycling of dross or addition of fines to the furnace, thereby increasing the recovery of metal values from the ore. Moreover, the use of the briquettes does not lead to contamination of downstream operations as found to occur with pitch-bound briquettes.

PROPERTIES OF DROSS BRIQUETTES

Sodium Binder Pitch Hydroxide Sulphuric Acid Level of addition %w/w 8 5 5 7* 5 6 7* 7 5 Concentration %w/w - 29 10 10 15 15 15 20 30 Crushing Strength (mean. Kg) Room Green 189 4.9 13.5 18.4 12.3 18.5 26.3 24.2 10.8 Temp. Dry - 55.0 31.0 31.6 50.6 61.8 50.3 81.6 35.2 100 C 66 87 200 C 5 79 Less than 300 C 1.5 32 900 C - 14 * Dewatered during pressing

Claims (21)

1. A briquette for use in a smelting process comprising a smeltable metal compound bound in a crystalline lattice.

2. A briquette according to Claim 1, wherein the compound is a metal oxide or sulphide.

3. A briquette according to Claim 1, wherein the compound is dross from a lead/zinc smelter.

4. A briquette according to any one of the preceding claims, wherein the lattice is formed by action of an ionic solution with at least part of the compound.

5. A briquette according to any one of the preceding claims, and including a neutral salt.

6. A briquette according to Claim 5, wherein the neutral salt is a fluxing agent.

7. A briquette according to claim 5, wherein the neutral salt is sodium chlorine.

8. A briquette according to any one of the preceding claims, and including a proportion of coke dust.

9. A briquette for use in a smelting process, substantially as herein before described with reference to the Examples.

1 0. A method of forming a briquette for use in a smelting process, comprising mixing a smeltable metal compound with an ionic solution to form a paste, briquetting the paste and drying the briquette to cause the metal compound to be bound in a crystalline lattice.

1 A method according to claim 10, wherein at least one of the ionic species in the solution reacts at least in part with the metal compound.

12. A method according to either one of claims 10 and 1 wherein the ionic solution is acidic.

13. A method according to claim 12, wherein the ionic solution comprises sulphuric acid.

14. A method according to either one of claims 10 and 1 wherein the ionic solution is basic.

1 5. A method according to claim 14, wherein the ionic solution comprises sodium hydroxide.

1 6. A method according to any one of claims 10 to 15, wherein a neutral salt is incorporated into the lattice.

1 7. A method according to claim 16, wherein the neutral salt is a fluxing agent.

18. A method according to claim 16, wherein the neutral salt is sodium chloride.

19. A method according to claim 16, wherein the neutral salt is formed by mixing a paste including an acid species with a paste including a basic species such that the neutral salt is formed by neutralisation of the acid and basic species.

20. A method according to any one of claims 10 to 19, wherein coke dust is incorporated into the mixture.

21. A method of forming a briquette for use in a smelting process, substantially as hereinbefore described with reference to the Examples.