GB2234528A - Zinc recovery process - Google Patents

Abstract

A pyrometallurgical process for zinc recovery from finely divided flue dust, or the like, involves the use of a high temperature reaction chamber 2 in which zinc oxide is reduced and zinc volatilised. A lance 1 is used to inject oxygen or oxygen-enriched air together with fuel into the reaction chamber, thereby enhancing mass transfer at a gas/slag interface within the chamber. Resulting zinc vapour may be collected as zinc metal by use of a lead/zinc splash condenser 7 and liquator, or as zinc oxide by reoxidation and use of a bag filter. <IMAGE>

Description

ZINC RECOVERY PROCESS The present invention relates to a zinc recovery process and more particularly to a pyrometallurgical process for recovering zinc as metal or zinc oxide from unpelletized, finely divided zinc bearing feeds or flue dusts.

There are many sources of secondary zinc available. Brass works, secondary and primary smelters, and steel mill electric arc furnaces all generate zinc-bearing flue dust. Very often, these constitute hazardous or toxic wastes which cannot be disposed of without satisfying special regulations and incurring significant costs in transport and disposal charges. A typical electric arc furnace dust from a steel mill will have a zinc content of 20% in addition to containing ferrous and non-ferrous metal contaminants. Zinc is generally the most significant value present in the dust.

Many processes have been proposed for recovery of secondary zinc. For example, there are a number of hydrometallurgical processes which involve leaching the flue dust with, for example, sulfuric acid, followed by selective precipitation of other metals by adjustment of pH and then electro winning the zinc from solution. Such a process is not particularly economical unless carried out at a large scale with substantial investment of capital. Other hydrometallurgical processes such as ammonia leaching or ammonium carbonate leaching have been proposed, piloted and operated on a limited scale. Such processes, however, involve many steps of precipitation, filtration, drying and calcining to achieve an end product which is typically zinc oxide rather than metallic zinc.

Various thermal processes are available for processing zinc-bearing dust and upgrading the zinc oxide level to between 50 and 70 percent The use of a Waelz kiln is one example wherein the flue dust is pelletized and mixed with carbon as a reducing agent, processed through the kiln, and zinc is volatilized and subsequently oxidized in the gas stream to produce a more concentrated zinc oxide flue dust.

While this process yields zinc oxide, it does not produce a commodity of high enough grade to be widely saleable, either as zinc oxide or zinc metaG. R A onal process equipment to further upgrade the zinc is required. Another process known as the Imperial Smelting Process takes advantage of the Imperial smelting furnace (a modified blast furnace).

Flue dust is pelletized with coke or carbon, with or without other feed materials, and charged to the top of the furnace shaft Through the combustion of the coke and counterflow of the exhaust carbon monoxide produced thereby, the zinc oxide present in the feed is reduced to zinc vapour. Slag and heavy base metals such as lead are tapped from the bottom of the furnace, and the zinc vapour is carried over into a lead/zinc splash condenser (part of the Imperial smelting process) whereby the zinc is flashchilled and alloyed with the lead. Subsequent separation of zinc from lead is achieved by liquation, and a high-grade zinc metal may be obtained with approximately 1 to 2 percent lead contamination.This process unfortunately requires large scale operation at high capital cost and involves much materials handling, which is not environmentally attractive, to produce an appropriate agglomerated or pellet feed material for the furnace.

Other processes are known for the recovery of zinc from slags and flue dusts involving the use of electric arc furnaces or plasma furnaces in which zinc oxide is again reduced with coke, many other components in the flue dust being tied up in the slag, the reduced zinc oxide being carried over as zinc vapour from the furnace where it is either condensed using a splash condenser (as above) or captured as oxide in a bag filter following oxidation of the zinc oxide and carbon monoxide and cooling of the gas stream. The major disadvantage of this route, if an electric furnace is used, is that only 60 to 80 percent of the zinc can be recovered from the flue dust or the slag (which is the normal feed material) due to the poor mass transfer occurring in the slag phase.In the event that a plasma furnace is used, higher recoveries are obtainable but the excessively high temperatures can lead to tremendous refractory erosion and corrosion. Furthermore, both these processes suffer from high capital costs and are difficult to justify for low throughputs.

We have now found a way of overcoming, or at least reduc=--ingi many of the difficulties associated with the prior art techniques of recovering zinc, either as oxide or metal, in a cost-effective way on a relatively small scale.

According to the present invention there is provided a pyrometallurgical process for recovering zinc as metal or zinc oxide from unpelletized, finely divided zincbearing feeds or flue dusts, which comprises the steps of: providing a single stage; high-temperature reaction chamber having at least one inlet lance; using the lance(s) to inject into said chamber oxygen, or oxygen-enriched air, and fuel; using the inlet lance(s) or another inlet to inject into said chamber finely divided zinc-bearing feeds or flue dusts and silica; providing reaction conditions to favour formation of zinc metal vapour; and collecting the zinc thus formed as zinc metal or zinc oxide. By heating the dust to a high temperature in a reducing atmosphere, the dust being of fine particle size, zinc oxide is reduced and zinc is volatized rapidly in the reaction: ZnO + C = CO + Zn.

The thermodynamics of zinc vapour formation are favoured by high temperatures in the flame zone such as from 1250 to 14O00C. Unlike the electric arc furnace1 mass transfer is significantly enhanced so that the reacted zinc can leave the reaction zone as vapour rather than be tied up in slag, as happens when zinc liberation is thermally limited leading to poor zinc recoveries. The high flame reaction temperatures of the invention lead to vastly improved reaction kinetics.

According to a preferred method of operation, oxygen and fuel in the form of carbon and coal or other hydrocarbon fuels are fed to the reactor in the stoichiometric ratio required for reduction of zinc oxide (ZnO) to zinc according to the above-mentioned reaction.

Sufficient additional carbon and oxygen are added to generate heat to raise the reaction products to at least 12000C. Zinc oxide-containing dust or other finely divided zinc feed material is fed into the bath through the same, or an independent, lance, or in certain circumstances for highgrade zinc directly into the bath. Sufficient silica or other fluxing reagents are added to the reactor to form a fluid slag at the operating temperature range of 1200 to 12500C. This slag may be tapped periodically or continuously from the vessel. Any lead created by reduction processes such as PbO + C = Pb + CO can also be removed periodically or continuously from the bottom of the vessel.

The preferred configuration uses a downy low combustion method in which sufficient reaction time and space is provided above the slag level for adequate development of the reaction and generation of high temperatures. Some residual energy is available in the jet from the lance to create mass transfer at the gas/slag interface.

This is not the only configuration. For example, horizontally fired methods are possible, although the aforesaid mass transfer through impingement is not so prevalent. Supplementary alternative methods of agitating the bath to improve mass transfer are possible, such as sparging with nitrogen or an inert gas or recycling combustion products, at all times taking account of the heat balance requirements to maintain the slag at a temperature at least above 12000C and in the fluid condition.

Zinc oxide, being finely divided, reacts very quickly. A typical particle size in an electric arc furnace dust, which is a common source of such material, is less than 2 microns. In the flame reaction zone, particle temperatures can be rapidly raised to a suitable reaction temperature, and mixing with carbon, pulverized coal and/or other reductants such as hydrogen in fuel can be achieved quickly.

As indicated above, fuels other than coal may be used. These may include carbon, carbon black, acetylene, natural gas, propane, liquified petroleum gases, and hydrogen, among others. It is important that oxygen, or oxygen-enriched air, is used for firing the lance as this promotes very intense reactions, minimizes the quantity of gas for downstream handling and achieves the necessary reaction temperature.

Silica and/or lime may be added as flux to maintain a liquid slag suitable for tapping directly from the furnace. Appropriate selection of reagents here is a function of the presence of alkaline metal oxides in the flue dust, many of which will not require the addition of calcium oxide or calcium carbonate for slag formation.

Another consideration in the selection of flux is to produce a final slag, e.g. a calcium ferro-silicate, which is inert and cannot be leached according to EPA leach tests.

Any residual heavy metals that are contained in this slag will therefore not create an environmental problem.

Typical flue dust may contain in the region of 18 to 25 percent by weight of zinc but present as zinc oxide, iron in the region of 25 to 35 percent by weight as ferric oxide (Fe203) and lead in the region of 1 to 5 percent usually present as lead oxide (PbO) or higher oxides. Additionally, there can be trace quantities of cadmium, arsenic and sulphur.

A typical electric arc furnace flue dust would contain the following: -Component Percent SiO2 0.5 CaO 24.0 MgO 3.0 A12O3 0.6 Fe2O3 29.0 ZnO 22.0 PbO 4.0 C 0.5 S 0.5 It should be noted from the above that the presence of alkaline metals, such as CaO and MgO, is sufficient to provide for a fluid, low melting point, low viscosity slag. A typical slag composition would be: 15% CaO, 30% SiO2, 55% FeO, or 27% CaO, 31% SiO2, 42% FeO.

Such compositions have a melting point range of from 1200 to 13000C. Preferred slag compositions comprise 15-27% CaO, 30-35% SiO2 and 40-55% FeO.

Since the reactions occur fast, it is important to supply oxygen, fuel, reductant carbon, and the flue dust in a carefully controlled ratio. Recognizing the difficulties of feeding very fine-sized material, in the region of 1 to 5 micron particle size, it is important to have a method of introducing such particles evenly and consistently into the reaction zone. A preferred method of achieving this is with a water-cooled, oxy-coal or oxy-carbon lance specifically designed for handling particulate material, which can include the flue dust, through the passageways of the lance. Fluidizing feeders, metering screws or star valves are all potential ways of achieving regular even flow into the lance.

An example of an appropriate lance is the Air Products lance, described in U.K. patent application no.

8715030. Alternative designs which achieve the same ends are also possible. Flux may also be added through the lance or injected directly into the slag bath in the furnace chamber or merely added by some other feed method, such as by a screw feeder above the bath, on a continuous basis. In all cases, the objective is to maintain a liquid slag capable of being tapped from the furnace. Where significant quantities of lead and/or other heavy metals such as antimony capable of reduction are present in the feed these may be tapped periodically from the very bottom of the vessel provided that the carbon tenor is maintained sufficient to selectively reduce these elements. Unlike the situation which may arise in an electric arc furnace or a plasma furnace where reduction potentials are excessively high, metallic iron is not reduced from the flue dust to form a product.This significantly reduces the amount of energy required for the overall reaction.

The thermodynamics of the reaction produce carbon monoxide principally and some carbon dioxide at temperatures in the region of 1200 to 14000C. Under these circumstances, the slags produced are fluid, the reaction kinetics are fast, and zinc oxide present in the flue dust is reacted rapidly, and zinc vaporized. Typical CO/CO2 ratios are in the region of 1 to 2.0 for the above operating temperatures. Benefits derived from using oxygen for combustion, apart from the high temperatures obtained, are the fast kinetics and favourable thermodynamics for zinc oxide reduction. Further, the production of a low flow of exhaust products leads to reduced size of downstream equipment and low heat losses from the overall furnace system. This is important in the overall economics of the process.

The-~économics of any zinc recovery process hinges on the efficient utilization of fuel, power and reductant.

Previous attempts to produce zinc metal cost effectively from flue dust have involved using high quantities of coal or carbon as reductant either in a blast or electric furnace. In addition, both these techniques suffer the aforementioned difficulties of high capital cost, materials handling problems and inefficient recovery of zinc from the feed. Surprisingly, the economics of the present invention are viable for flue dust quantities of the order of 6000 tpa and above approximately 15% Zn content.

Preferred embodiments of the present invention will now be more particularly described with reference to the accompanying drawings wherein: FIG. 1 is a schematic representation of apparatus, including a vertically-fired arrangement of high temperature reaction chamber, for carrying out a first embodiment of the pr-ocess of the present invention wherein the zinc is collected as zinc metal; FIG. 2 shows a horizontally fired arrangement of high temperature reaction chamber for use in an embodiment of the present invention; FIG. 3 shows a part of a second alternative arrangement of high temperature reaction chamber for use in an embodiment of the present invention; FIG. 4 shows a third alternative arrangement of reaction chamber, having an alternative flue arrangement;; FIG. 5 shows a fourth alternative arrangement of reaction chamber, having a further alternative flue arrangement; and FIG. 6 is a schematic representation of apparatus for carrying out a further embodiment of the process of the present invention wherein the zinc is collected as zinc oxide.

Referring to Figure 1, an oxy-fuel combustion lance 1 is fed with gaseous oxygen and coal or carbonaceous fuel premixed with ZnO-containing dust and is fired at high temperature in a refractory-lined furnace chamber, or high temperature reaction chamber 2. Fluxes such as silica (SiO2) are added through a feeder 3 onto the surface 20 of the bath or may be introduced together with the dust through the lance 1. The reaction taking place in the combustion space leads to the formation of a calcium ferro-silicate slag which is tapped periodically from the furnace through a run-off pipe 4. Any lead present in the flue dust is partially reduced and forms a leady metal stream 5 at the bottom of the furnace chamber 2. The exhaust gas leaves the furnace chamber 2 at temperatures in excess of 1200 to 12500C through a refractory-lined flue 6 and enters a splash condenser 7.The splash condenser 7 employs a bath of either zinc metal or lead/zinc alloy and has an agitator 8.

The splash condenser 7 generates a rain of molten metal to flash cool the zinc vapour from the flue 6. Such a condenser design is proprietary to the ISP (Imperial Smelting Process). The lead/zinc metal is circulated through a liquation chamber 9, where is is cooled through a indirect cooling circuit 10. This removes the heat of condensation from the reversion of zinc vapour to zinc metal and the cooled lead alloy stratifies at the bottom of the liquation chamber 9. Any dross formed is periodically removed from the surface of the melt by a run-off pipe 11.

In cooling the lead/zinc alloy, lead separates out at about 4500C and is returned to the condenser 7 via a circulation pipe 12. Product zinc metal, being typically 1% residual lead, is discharged from liquation chamber 9 by outlet 13.

A separate combustion chamber 14 receives the residual combustion products from the condenser which are high in carbon monoxide, and combustion is completed through to carbon dioxide suitable for subsequent downstream gas cleaning and discharge to atmosphere. Heat produced by the process may be removed as steam or hot water from cooling coil 16. Alternatively, this CO-rich gas stream may be recycled, further combusted or applied directly, in other heating sources, e.g. for drying, etc.

An alternative configuration of the reaction chamber 2 is shown in Figure 2 wherein the oxygen-fuel-dust lance 1 is employed extending horizontally. A variety of configurations are possible and can be employed by those versed in the art.

In Figure 3 an alternative method of firing is shown (which again may be either horizontal or vertical, depending on the combustion chamber selected) in which oxygen and natural gas are fired down a first oxy-fuel lance 1, and a second lance 1' is employed for introducing premixed flue dust and carbon, with or without silica flux.

A suitable carrying medium for the flue dust and carbonaceous material (and silica, if used) can be nitrogen, air, oxygen or steam, although preferably nitrogen. The rate of carrier gas must be selected so that the resultant operating temperature in the furnace or reaction chamber 2 is not unduly lowered below 12500C.

The ratio of carbon to oxygen in the lance 1 is always maintained in a sub-stoichiometric range in order to ensure the reducing power of the product gas. A typical reaction is: 2C + 3/2 2 = CO + CO2 Other reactions are possible, depending on the type of fuel selected. Where coal is used, its reaction will be a function of the proportion of carbon and hydrogen in the coal. The resultant exhaust gas CO/CO2 ratio shall be maintained in the region of 1 to 2 or higher.

In the event that inadequate reducing power is available in the combustion products, the exhaust gas may be taken off through a coke bed or carbon bed in a modified flue 6' or 6'', as shown in Figures 4 and 5 respectively.

Coke is fed into the flue 6',6'' by an inlet 17,17' and held in place by water-cooled support tubes 18. In this case, additional carbon present reacts to reduce CO2 back to CO by the reaction of CO2 + C = 2C0. This helps prevent the reversion reaction of zinc with CO2 forming zinc oxide.In other embodiments of the process of the present invention the oxy-fuel lance may be submerged in the bath and the flue dust/carbon and silica may also be injected directly into the bath, provided always that an adequate operating temperature in excess of 12500C for the slag temperature and 1250 to 14000C for the combustion zone are achieved.

The use of the lance as per U.K. patent application no. 8517493, incorporating indirect gas cooling instead of water cooling, may be particularly appropriate for such direct injection of reactant or fluxes into the bath, i.e. under the liquid phase in the reactor. Whatever the configuration, it is important that air inleakage into the furnace is minimized if zinc vapour is to be produced and recovered as metal.

The exhaust gas handling from the furnace can take alternative configurations depending on appropriate uses of the end product. For example, as shown in Figure 6, an alternative exhaust gas handling system may be employed, in which the carbon monoxide and zinc vapour in the gas stream exiting the reactor are further combusted in afterburner 61 with air 65 to zinc oxide and CO2. In this embodiment, downstream of waste heat boiler 62, zinc oxide is collected in a bagfilter 63, together with any other volatile metals and entrained particulate matter.As there is no opportunity to liquate lead from zinc nor to remove a dross before this dust is collected, the purity of the product zinc oxide is less and is generally anticipated to be in the region of 70%-80%. In this case, subsequent upgrading, either of the zinc oxide for chemical use, or by some other treatment process to yield zinc metal, may be required. It is an important feature of the process that the temperatures of operation are high and the particles being finely divided and in unpelletized form, that fusion of the surface of the flue dust is rapid. Accordingly, zinc formation is rapidly achieved at high recovery.Furthermore, particles capable of being slagged are collected and separated rapidly as a liquid phase rather than the dust becoming entrained which can so often happen in al~tguve PI) {allurgical processes.

Example 1 Steel electric arc furnace dust was processed at the rate of 9000 tonnes per annum. The analysis of the dust was Zn 18%, Pb l%-2%, Fe 35%, SiO2 3.0%, CaO 23.3%. Low quantities of cadmium, sulphur and residual carbon were also present.

Zinc was present as the compound ZnO, Fe as Fe2O3, and Pb as PbO. The principal reactions involved in the high temperature reaction zone of the furnace are: ZnO + C = CO + Zn Fe203 + C = 2FeO + CO PbO + C = Pb + CO By addition of approximately 30 percent silica by weight of flue dust, a fluid slag containing approximately 27% CaO, 31% SiO2 and 42% FeO was made. This was fluid at 12500C.

Carbon or coal as a reagent was used for reduction. Using a pulverized coal with an analysis: C 75%, H 5%, S 1%, N 2% and ash 17%, an amount of coal in the region of 9.5% coal by weight of dust was required for reduction reactions and an amount of coal in the region of 21% by weight of dust was required for combustion. Thus the overall coal requirement was 30.5% of the flue dust weight.

Example 2 A furnace was used to process flue dust at a rate of 9000 tonnes per annum (27 tonnes of flue dust per day), the feed rate was 1125 kg/h. The furnace size (in downy low configuration as per Figure 1) was 2m internal diameter by 4m freeboard height and lm operating slag depth. Silica additions were at the rate of 340 kg/h, coal as reductant at 100 kg/h and as fuel at 230 kg/h and oxygen (95% 02) at 570 kg/h. Using a zinc or a lead splash condenser, product zinc metal was obtained with a residual lead content of 1% which was satisfactory for galvanizing metal requirements.

Overall zinc recovery was in the region of 90% from the initial feed material, and this was recovered as zinc metal in a single process under the above circumstances. Thus, it can be seen that zinc of high purity can be obtained at much lower plant capacities than are viable for leach-electrowin hydrometallurgical circuits or for electric arc furnace or plasma furnace recovery circuits.

Claims (11)

CLAIMS:

1. A pyrometallurgical process for recovering zinc as metal or zinc oxide from unpelletized, finely divided zincbearing feeds or flue-dusts, which comprises the steps of: providing a single stage; high-temperature reaction chamber having at least one inlet lance; using the lance(s) to inject into said chamber oxygen, or oxygen-enriched air, and fuel; using the inlet lance(s) or another inlet to inject into said chamber finely divided zinc-bearing feeds or flue dusts and silica; providing reaction conditions to favour formation of zinc metal vapour; and collecting the zinc thus formed as zinc metal or zinc oxide.

2. A process according to claim 1, wherein said fuel comprises one or more of carbon, coke, coal, natural gas, propane, butane, LPG, hydrogen and recycled carbon monoxide in substoichiometric ratio.

3. A process according to claim 1 or 2, wherein the step of providing reaction conditions to favour formation of zinc metal vapour includes providing a gas phase reaction temperature of from 1250 to 14000C.

4. A process according to claim 1,2 or 3, wherein the step of providing reaction conditions to favour formation of zinc metal vapour includes providing a gas ratio within said reaction chamber of CO/CO2 of 1:1 to 2:1 (v/v) or higher.

5. A process according to claim 4, wherein said CO/CO2 ratio is ensured by the provision of a coke or carbon bed through which the zinc metal vapour is extracted.

6. A process according to claim 1,2,3 or 4, wherein the step of collecting the zinc comprises channelling exhaust gas containing the zinc metal vapour from the reaction chamber through a zinc or zinc/lead splash condenser or other suitable condenser for producing liquid zinc, and subsequently separating the zinc as a high-grade zinc metal with approximately 1% lead impurity content.

7. A process according to claim 1,2,3,4 or 5, wherein the exhaust gas from the reaction chamber is further burnt in air or oxygen to produce zinc oxide dust and carbon dioxide, the heat being removed by heat exchanger, waste heat boiler or cooling coil, and the dust subsequently being captured in a bag filter or other particulates collector as a high-grade zinc oxide product

8. A process according to any preceding claim, wherein oxy-fuel is introduced into the reaction chamber by a first lance and a second lance is employed for introducing zinc-bearing flue dust with a reductant and, with or without, a flux, in a carrier gas.

9. A process according to any preceding claim, wherein the zinc-bearing flue dust, coal and oxygen are premixed and injected through the lance directly into a bath within the reaction chamber, the temperature being above 12500C and conditions being kept sufficiently reducing so that zinc vapour is efficiently scavenged from slag formed in said reaction chamber.

10. A process according to any preceding claim, wherein slag formed in said reaction chamber has high fluidity and a melting point of 12500C.

11. A process according to any preceding claim wherein sufficient carbon is introduced into the reaction chamber to reduce any oxidized forms of lead in the flue dust to lead metal or alloy.