PRIMARY EXTRACTION OF LEAD
The most important lead ore is galena (lead sulphide). Other important ores such as cerrusite (lead carbonate) and anglesite (lead sulphate) may be regarded as weathered products of galena and they are usually found nearer to the surface.
Lead and zinc ores often occur together and for most extraction methods they have to be separated. The most common technique is selective froth flotation. The ore is first processed to a fine suspension in water by grinding in ball or rod mills - preferably to a particle size of less than 0.25 mm. The dilution of this suspension (or pulp) can vary from 5 to 40% solids by weight. Air is then bubbled through this pulp contained in a cell or tank and due to the previous addition of various chemicals and proper agitation, the required mineral particles become attached to the air bubbles and are carried to the surface to form a stable mineralized froth which is skimmed off. The unwanted or gangue particles are unaffected and remain in the pulp.
The chemicals added include frothing agents to produce the stable froth and collecting or promoting agents to give the desired mineral the right kind of surface - for example non wetting - for collection. Modifying agents are also added, notably depressants, which prevent collection of certain minerals, and activators which remove the effects of depressants. Thus, for example, with lead-zinc sulphide ores, zinc sulphate, sodium cyanide or sodium sulphite can be used to depress the zinc sulphide, while the lead sulphide is floated off to form one concentrate. The zinc sulphide is then activated by copper sulphate and floated off as a second concentrate. The froth is broken down by water sprays and the resulting mineral suspension is dewatered by appropriate filtration equipment.
The first stage in smelting is to remove most of the sulphur from the concentrate. This is achieved by roasting in a Dwight Lloyd Sintering Machine in which a layer of a mixture of concentrate, flux and some returned sinter fines is moistened and spread on the continuous grate of the sinter machine and ignited. Combustion is rapidly propagated by a current of air blown upwards through the ore mixture by wind boxes.
The sulphur in the mixture provides the fuel for the exothermic reactions which take place; the returned sinter fines are added to dilute the fuel content to prevent overheating. As the charge on the continuous grate moves forward, the sulphide is largely converted to oxide and the fine powders are agglomerated into lumps, which are broken up as they leave the machine, to a size convenient for use in a blast furnace - the next stage in the process. The sinter plant gases are routed to gas cleaning equipment for recovery of fume and the removal of sulphur-containing gases to form sulphuric acid.
The graded sinter is mixed with coke and flux such as limestone, and fed into the top of the blast furnace, where it is smelted using an air blast (sometimes preheated) introduced near the bottom, The chemical processes taking place in the furnace result in the production of lead bullion (lead containing only metallic impurities, usually including gold and silver, hence the use of the name bullion) which is tapped off from the bottom of the furnace and either cast into ingots or collected molten in ladies for transfer to the refining process.
An almost identical process, sintering followed by reduction in a blast furnace, is an integral part of the more complex Imperial Smelting Process for the simultaneous production of zinc and lead. In this blast furnace, a mixed lead/zinc sinter is added and the lead bullion is tapped conventionally from the bottom of the furnace but the metallic zinc is distilled off as a vapour and captured in a shower of molten lead. This is allowed to cool and zinc can be floated off, while the lead is recirculated to the collector.
These traditional two stage processes offer a large number of opportunities for hazardous dust and fume to be released. This necessitates the use of extensive exhaust ventilation and results in large volumes of lead-laden exhaust gases which must be cleaned before they can be discharged to atmosphere. The collected dusts are returned to the smelting process.
The environmental problems and inefficient use of energy associated with the sinter/blast furnace and Imperial Smelting Furnace processes have provided the incentive for much research into more economical and pollution-free processes for the production of lead. Most of this research has been aimed at devising processes in which lead is converted directly from the sulphide to the metal without the need to produce lead oxide in an initial step and then reduce it to the metal in a separate operation. As a result a number of such direct smelting processes are now in existence, though at varying stages of development.
Direct smelting processes offer several significant advantages over conventional methods. The first and most obvious of these is that sintering is no longer necessary and a major environmental problem, i.e. the creation of dust, is avoided. Moreover, the heat evolved during oxidising the ore (sintering in the two-step processes) is no longer wasted but is put to direct effect in the smelting operation, thus providing a considerable fuel saving. The volumes of gas requiring filtering are much reduced, and at the same time the sulphur dioxide concentration of the off-gases is greater and therefore more suitable for sulphuric acid manufacture.
The major difficulty in all direct smelting processes lies in obtaining both a lead bullion with an acceptably low sulphur content and a slag with a sufficiently low lead content for it to be safely and economically discarded. In several cases further treatment of either the crude bullion or the slag (or both) is required in a separate operation. There are several direct smelting processes which come close to meeting the desired criteria - the Russian Kivcet, the QSL (QueneauSchuhmann-Lurgi), the lsasmelt and Outokumpu processes are examples.
The use of these newer processes is likely to increase but at the current time, the relative importance of the different smelting methods in terms of metal produced is as follows:
* Conventional blast furnace 80%
* Imperial Smelting process 10%
* Direct processes 10%
The Kivcet process which originated in the USSR is a direct smelting process in which zinc can be recovered simultaneously with lead as a saleable product. Consequently it has been developed specifically with the object of treating complex ores with high zinc contents.
Sulphide concentrate and flux are dried and ground and injected with technically pure oxygen through a burner into the top of a smelting shaft. The material is roasted and smelted while in suspension forming a mixture of lead and zinc metals and oxides which enter the melt at the bottom of the shaft. Provided the sulphur content of the feed is greater than 18%, smelting is autogenous and no additional fuel is required. The process waste gas containing 40% or more sulphur dioxide passes up a gas cooling shaft, is de-dusted by means of electrostatic precipitation and fabric filters and is then used for sulphuric acid manufacture. The recovered dust, mainly lead sulphate, is recycled as smelter feed.
The molten metals and slag pass from the smelting shaft under a water cooled partition into an adjoining electric furnace where the oxides are reduced to metal by adding coke through gas-tight feeders in the roof. Metallic or oxidic materials unsuitable for injecting through the burner can also be introduced directly into the electric furnace in this way. Lead bullion and slag are tapped from the furnace. Total lead recovery can be up to 99%.
The zinc contained in the slag is vapourised at the temperature involved and, together with a smaller amount of lead vapour, can be recovered either by burning to zinc oxide and reduced to the metal in an electrolytic zinc plant or by condensing as the metal in a lead-splash condenser. Zinc recovery from the furnace is about 85%.
The potential environmental advantages of the Kivcet process are many. The handling of dried lead-bearing charge and hence the formation of dust is kept to a minimum and the furnace, which is a closed vessel, operates at a slight negative pressure relative to the outside atmosphere in order to eliminate fugitive emissions. Process off-gas volumes are relatively small since pure oxygen is used instead of air. Sulphur dioxide concentrations are high and hence eminently suitable for sulphuric acid manufacture.
The QSL Process was invented by P E Queneau and R Schuhmann Jr and developed in Germany by Lurgi. Sulphide concentrate, return flue dust and flux are continuously mixed with a little water and compacted into pellets which are dropped directly into the oxidation zone of the reactor. The pellets dissolve rapidly in the resulting molten bath and are partially oxidised to lead and lead oxide by submerged injection of oxygen. Oxidation is autogenous at the operating temperature of 950 - 1000OC and the evolution of lead fume is low.
Metallic lead containing copper and silver sinks to the floor of the reactor and the bullion is tapped continuously. Lead oxide and other metal oxides form slag which flows to the opposite end of the vessel where it is continuously discharged. On the way it passes over a series of submerged injectors through which powdered coal is blown. This reduces the lead oxide to metal which sinks to the floor and flows counter current to the slag back to the oxidation zone where it is tapped together with the directly produced primary bullion. The lead content of the slag decreases from about 55% in the oxidation zone to less than 2% in the slag leaving the reactor. The sulphur content of the lead bullion is about 0.3%.
The process off-gases, containing up to 15% sulphur dioxide, are cooled in a waste heat boiler and an evaporation cooler and then cleaned in a hot electrostatic precipitator. The sulphur dioxide is then converted to sulphuric acid. The precipitated flue dust, mainly lead sulphate and comprising on average about 20% of the original lead feed, is returned directly to the charge pelletizer by means of sealed conveyors.
Environmentally the QSL process is very clean. Since all raw materials are handled in a moist state there is little opportunity for dust evolution. Smelting takes place in a single reactor operated under negative pressure so dust and fume emissions are minimal. Volumes of process gas are relatively small owing to the use of oxygen rather than air and dusts recovered from the gas are recycled under sealed conditions. Sulphur dioxide concentrations are convenient for sulphuric acid manufacture.
The lsasmelt process for lead is a fully continuous two stage process which is based on gas injection into melts via a top entry submerged Sirosmelt lance. Submerged injection produces turbulent baths in which high intensity smelting or reduction reactions may occur. In the first stage of the process, lead concentrate is added directly to a molten slag bath and is oxidized by air injected down the lance. Simultaneously, the high lead slag from this furnace is continuously transferred down a launder to a second furnace and reduced with coal. The crude lead product and discard slag are tapped continuously from the reduction furnace through a single taphole and separated in a conventional forehearth.
The innovative feature of the Sirosmelt lance is the use of helical vanes to impart a swirling motion to the process gas stream, leading to an increased heat transfer rate from the lance to the process gas and the formation of a frozen slag layer on the outer surface of the lance. This slag coating protects the lance, so that it can be immersed in the bath for extended periods without excessive wear. The swirling motion of the gas stream also aids in the dispersion of gas bubbles into the melt at the lance tip.
Submerged gas injection via a Sirosmelt lance provides an alternative to tuyeres in metallurgical processes, greatly simplifying vessel design and eliminating refractory problems associated with tuyeres. The lsasmelt process provides the following advantages:
The Isasmelt process can also be used for secondary lead production.
The Outokumpu Flash Smelting process consists of drying, flash smelting, slag cleaning and gas handling equipment. The lead concentrates and fluxes are mixed and dried to a moisture content of less than 0.3% so the feed will readily ignite in the Flash Smelting Furnace. The feed mixture is fed into the concentrate burner located on the roof of the reaction shaft. This specially designed burner mixes the feed materials with process oxygen or oxygen enriched air. The concentrate is oxidised and smelted directly into lead bullion and slag - the thermal energy for the smelting of the charges is provided by the exothermic oxidation reactions of the concentrate. The degree of oxidation can be well controlled and thus the quality of the lead bullion, especially the sulphur content, can be regulated over a wide range from 0.1% upwards. The use of oxygen produces low gas volumes and a high concentration of sulphur dioxide in the furnace gas which results in a small gas cleaning plant. This consists of a waste heat recovery boiler, an electrostatic precipitator and a sulphuric acid plant.
The flash smelting furnace slag is treated continuously in a separate electric furnace where coal or other reluctant injection is used for reducing the lead in the slag. The waste slag contains about 1.5% lead.
With the prospect of even tighter environmental controls the possibility of hydrometallurgical techniques for the treatment of primary and secondary sources of lead are being researched. Several processes have been described in the literature but most are still in the development stage and probably not yet economic in comparison with the pyrometallurgical processes. The object of the processes in most cases is to fix the sulphur as a harmless sulphate and to put the lead into a suitable solution for electrolytic recovery. Most processes recirculate leach solutions and produce a high purity lead. Examples of these include the Ledchlor process, which can be used on primary materials. Others developed by Engitec (CX-EW), Ginatta and RSR are more concerned with recovery of lead from secondary sources and in particular from battery scrap.
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