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General

UNDERSTANDING LEAD AND LEAD BASED ALLOYS

D.K. Biswas and Y.P.S. Nirvan

1. Introduction

Lead, a silvery gray colored element, is one of the softest and heaviest of the common metals. It can be easily rolled and extruded to any form of engineering applications. However, a small amount of such elements as antimony, bismuth, arsenic, copper and alkaline earth metals increase its hardness. It has excellent corrosion resistance properties against attack by many acids, particularly sulfuric, which makes it highly useful in building construction, in chemical process plants and for pipes and cable sheath. Other important uses are for storage battery plates, paint pigments, chemical reagents, shot, solder, type metal and other alloys and compounds.

However, use and handling of lead based compounds and alloys can cause one of the oldest and best known of all occupational diseases. Lead poisoning, "Plumbism" and "Seturnsim" have been described as the oldest known industrial poisoning except for carbon monoxide intoxication. Many Greek, Latin and Arabian writers were aware of the fact that many lead preparations when taken internally can produce lead colic.

This paper overviews briefly the metallurgical aspects of occurrence of lead, its extraction and refining, characteristics and properties of lead based alloys, as well as its applications. It deals in hazards of industrial exposures of lead and in the remedial measures taken to remove or minimize lead intoxication in industrial application.

2. Occurrence and Extraction of Lead

2.1 World Lead Deposits

The geological and geographical distribution of lead and zinc are identical. They occur in mineral deposits as very intimately mixed and are seldom entirely free from the other. Galena, lead sulfide, the common ore minerals of lead contains about 86.4% of lead. Other important ore is cerussite, PbCO3, containing 77.5% lead. Anglesite, PbS04 and Pyromorphite 9PbO.3P2O5.PbCl2 also contain lead. Galena, however, constitutes the most common and important of all lead minerals.

The bulk of the lead ore mined is concentrated in United States, Mexico, Australia and Canada. The important types of lead deposits are found at shallow depth in sedimentary rocks without apparent connection with igneous rocks. The ores of this type, which occur as tabular replacements of receptive strata, usually in limestones and dolomites, contain Galena but seldom gold, silver, copper and antimony to any appreciable extent. These extensive and commercially valuable deposits are of worldwide distribution and are found importantly in Mississippi valley, Silesia and Morocco.

Deposits associated with igneous rocks are characterized by complex ore comprising of vein deposits formed near the surface, veins filled under conditions of intermediate temperature and pressure, disseminated pyritic replacements of igneus rocks and silver-lead replacements in limestone. Deposits of San Juan region and Lake city in Colorado, the Schemnitz deposits in Hungary, the Mapimi and Santa Eulalia deposits in Mexico and the deposits of Insbach and Freiberg in Germany are of the sub-surface vein deposit. Deposits of Bawdwin, Burma and Ridder, Siberia are of pyritic replacement types. The deposits of Leadville, Colorado, Parck City, Utah and Sierra Majoda, Mexico belong to silver lead replacements in limestones.

The Australian production originates from Broken Hill mines, the lake George mine in South Wales and the Mt. Isa mines in Queensland. The Canadian mines are in British Columbia. The Sullivan mine at Kimberely is the most important. Lead mines are also found in the principal countries of Europe, Spain, Italy, Germany and Yugoslavia. In Asia, Burma was the lading lead producing country. Japan, Korea, Turkey followed in order. In Russia around Ridder, Siberian lead has been in production for many years. Africa was, also a leading lead producing country with mines in Rhodesia, Tunis, Algeria, Morocco and two recent developments in Tsumeb in southwest Africa and the Zellidja in East Morocco. In India a small quantity of lead is being produced in Jhawar, Rajasthan.

2.2 Benefication of Lead Ores

Lead ores are concentrated by various processes, some on Jigs and Tables, producing gravity concentrates and others by flotation process. Complex ores containing gold, silver, lead, copper lead, zinc and copper concentrates are drossed by Flotation process.

3. Production of Lead Bullions

3.1 Smelting of Lead Ore

There are two methods presently in practice: the ore hearth smelting and sintering followed by blast furnace smelting.

3.1.1 Ore Hearth Practice:

This process, traditionally known as ‘Scotch Hearth’, was first practiced in Europe. In this process lead concentrates high in lead content (70% or more) are smelted in ore hearth furnace. This is based on the roasting of PbS to PbO in the hearth furnace and reduction by carbon to Pb according to following equation:

(1) 2PbS + 30₂ ® 2PbO + 2SO₂

(2) 2PbO + C ® 2Pb + CO₂

(3) PbS + 20₂ ® PbSO₄

(4) PbS + 2PbO ® 3Pb + SO₂

 

The modern hearth furnace consists of a cast iron basin of about 10-12 ft. long, 20 inches wide and 10 inches deep, and so supported that the top is 26 inches above the level of the working floor. The bottom of the basin is horizontal through 10 inches, one half the distance from back to front and then slopes up at approx. 45 to the lip of the basin. A cast iron water jacket forms the back of the hearth. The water jacket is cored to accommodate the tuyers, through which air blast passes to the charge. A steel hood supported by steel framework extends over the length of the furnace and is connected to a balloon flue. The charge consists of 60% fresh ore (concentrate and flotation slimes), the rest being fumes of lead sulfate which when mixed with the concentrate and returned to the hearth in the form of ore mix, hastens the roast reactions to such an extent that the hearth handles its own dust burden.

Coke breeze is preferably used as fuel and constitutes about 10-12% of the ore mix. The normal blast is 7 oz/sq.in, and once the furnace is in operation the smelting is continuous. Each furnace delivers approx. 1000 cu.ft. flue gas per minute, the temperature of the charge is 930-980 C when the furnace is in blast. The charge is floated on a molten bath of lead and built-up by alternatively spreading thin layers of ore mix and coke breeze. At the working temperature of the charge, the slag forming constituents fuse as they are turned over repeatedly during rabbling. They coalesce to form a clinker, like a snowball and assume a spherical shape. This product, the heath slag, is composed of the slag forming constituents cementing together semi-smelted particles of the charge. The assay percentage of lead of the conglomerate is approximately one half of the ore mix charged.

The main hood conducts the fume to a balloon flue. There is also a ventilating hood, on natural draft, which clears any fumes arising from material on the apron and maintains a curtain of air in front of the working face of the ore hearth.

3.1.2 Sintering and lead blast furnace smelting

Usually complex lead ores containing gold, silver, lead, copper and zinc are first sintered to eliminate the sulfur from the sinter charge as sulfur dioxide and also to agglomerate or condition the charge and make a product for the blast furnace charging. Present day sintering operations are carried out on Dwight-Lloyd sintering machines, which consists of a strong frame of structural steel track or guide. A continuous succession of pallets to which interlocking grate bars are fitted to allow for the passage of gases travel at an average rate on the steel track. Underneath the train of pallets are wind boxes which are connected to a down draft fan discharging to the bag house or Cottrell precipitators. The reaction that takes place in sintering is very similar to that taking place during roasting in ore hearth furnace. Excess air is drawn down through the charge by means of the continuous down draft fans, which quickly removes the sulfur dioxide formed and thus prevents formation of too many sulfates. In operation of a sintering machine the ore mix is crushed and screened to minus 3/8 inch, moistened and well mixed before roasting. The charge is ignited by oil or gas fired brick lined stove extending across the width of the machinery which varies from 42 to 63 inches with a length of 22 to 46 feet. The stove is set above the edge of the first wind box. Once the charge is ignited, the reaction is exothermic and the product of combustion are drawn off by the down draft fans. Sufficient sulfur is kept in the first over machine to provide fuel over the second over machine. The first over sinter is crushed and resized for the feed to the second over machine. The low sulfur sinter from the second over machine becomes the ore charge to the blast furnace. The lead content of the sinter will vary from 25-50%. The silica, ferrous oxide and calcium oxide content of the sinter calculated at the time of bedding and all additions of flux are added to the ore beds prior to sintering. Recirculation of wind box gases from the sinter machine produces gas containing approx. 5-7% sulfur dioxide gas for introduction to a contact acid plant. This plant can produce 40-50 tons of 98% sulfuric acid per day as a by-product.

3.2 Blast Furnace Smelting

The charge for blast furnace smelting consists of approx. 80% sinter and 10-15% coke, which is spread evenly over the sinter contained in the skip hoist or charge car. About 1-3% scrap iron is also added to the charge for arsenic control in blast furnace bullion. The balance of 8-10% is the lump of lead that have solidified at the bottom of ladles. The prime function of the blast furnace is to reduce the metallic oxides contained in the sinter.

The two processes involved in blast furnace smelting are reduction and precipitation at temperature which ranges from 1000-1400 C. The carbon supplied by the coke acts as a reducing agent for the metallic oxides. As the charges descends through zones of gradually increasing temperature, the following chemical reactions take place:

C + O₂ ® CO₂

CO₂ + C ® 2CO

Pbo + Co ® Pb + CO₂

AS₂O₃ + 3 Co ® 2AS + 3 CO₂

Sb₂O₃ + 3CO ® 2Sb + 3 CO₂

Cu₂O + CO ® 2 Cu + CO₂

4 AS + 2 Fe ® 2 FeAS₂

Cu+S ® CuS

2Cu+AS =® Cu₂AS

PbS+Fe ® Pb+FeS

PbO + Fe ® Pb+FeO

2PbSO₄+SiO₂+C ® Pb₂SiO₄+CO₂+2SO₂

The products formed because of the above reactions tend to separate into layers, because of their different specific gravities. The products and their specific gravities are as follows:

Slag 3.6

Matte 5.2

Spetss 6.0

Lead Bullion 11.0

The blast furnace is of a vertical rectangular section shaft of brick construction from bottom to top having a crucible at the bottom, the water cool smelting zone or bosh with tapered sides and usually straight ends. The top of the furnace contains the charging arrangement. The general practice is to employ a crucible lead well and siphon for handling the lead production. The lead well over flaws to bullion ladles for transferring to adjacent drossing plant. Speiss and matte are withdrawn from the furnace through a tap hole. Any of these materials entrained in the lead are removed subsequently during the drossing operation. In some smelting practice lead bullion, speiss, matte and slag are all removed through the top hole. The separation then takes place in the first settler and the slag overflows to a second settler. In either method the slag is removed to slag dump as the lead content in slag is negligible. The species and matte are sent to copper plants together with the speiss and matte from copper drossing.

3.3 Drossing Bullion and Smelting Copper Dross

The term dross refers to any solid scum floating on top of a molten bath. In molten lead bath oxidation will cause the formation of a scum of lead oxide. Blast furnace bullions before being transferred from the smelter to the lead refinery undergoes the operation of "drossing" which removes any lead dross and also the copper content as a dross of copper oxide or sulfide. The process of drossing consists of stirring of hot blast furnace lead in a cast iron kettle of approx. 100 tons capacity equipped with a large mechanical stirrer. The mixture is then removed and the dross is skimmed off which collects on the bath. The bath is then cooled down to 340-350 C. Two pounds of ground sulfur per ton charge of bath is then added and mixing is started again. Vortex caused by the stirring action causes the sulfur to be immediately drawn under the bath of lead and therefore, vary little burning takes place. The charge is stirred for 20-30 minutes, the mixture is removed and the dross is skimmed off. The copper drosses are then melted in the reverberating furnace with pig iron and silica sand. In the soda ash process high copper-lead ratios in the matte and speiss are obtained by adding soda ash (Na₂Co₃), lithiarge or baghouse fumes (PbO), coke and sulfur.

3.4 Refining of Lead

The refining operation connected with chemical lead is very simple. Copper, nickel in lead is brought out by first drossing the bullion and then very carefully cooling the bath to liquidation point where copper and nickel are removed with the dross which forms at this freezing temperature. However, the productions of fully refined lead where the impurities like AS, Sb, Bi are controlled to a very low limit is an entirely different operation. There are two basic processes of lead refining, e.g. Electrolytic Refining and combined Furnace-Kettle refining. In the latter removal of hardening elements is carried out either in reverberating type, furnace or in large steel kettles. De-copperizing, de-silverizing and final removal of zinc, bismuth are carried out in kettles. In electrolytic refining of lead commonly called the Betts process, refining is carried out in electrolytic tanks or cells in which anodes of lead bullions are electrolyzed in a solution of lead flu-silicate and free flusilic acid. The starting sheets for the cathodes are made from electrolytic lead. The tanks are usually of concrete construction and lined with asphalt. The anodes are made of bullion lead usually 30-36 inches long and 26 inches wide with a thickness of about 7/8 inch to 1¼ inches. The anodes are cast on regular anode casting machines. Usually 24 anodes are set per cell with a spacing of approximately 4 inches center to center. Thus, 25 cathode starting sheets will be required. The refined lead is cut mechanically into bars for marketing.

3.5 Softening of Lead

Softening of lead is carried out in continuous batch reverberating process and the kettle process. The Harris process is an oxidizing process whereby the lead, in the presence of sodium hydroxide becomes oxidized by additions of sodium nitrate (Niter). The niter also oxidizes the impurities arsenic, antimony and tin which are subsequently collected as sodium arsenate, antimonate and stannate. The softening equipment consists of large steel kettle holding 100-270 tons of lead with a reagent cylinder supported in the kettle. The process is carried out in batches.

4. Properties

Lead is a soft, high density metal having a relatively low melting point and a high boiling point. Its easy pliability allows handling during fabrication and installation; low elastic limit, high co-efficient of thermal expansion and excellent antifrictional properties are physical characteristics of lead. Because of its low strength, ductility of lead is poor. Depending on its impurity content the lead is classified in various grades, un-desilverised lead, which contains a small concentration of copper (0.04-0.08%), silver (0.002-0.020%) and less than 0.005% bismuth is known as chemical lead. The impurities present enhance the corrosion resistance of chemical lead and therefore this lead has been widely used in chemical industry. Corroding lead is a very pure form of lead, which corrodes into white lead. There are other varieties of lead like copper lead, antimonial lead, acid lead and tellurim lead which are also used in chemical industries in specific environments. Tellurium lead is fine grained and has higher tensile strength, greater fatigue strength, the ability to work harder and excellent corrosion resistance to chemical solutions. Antimonial lead has better mechanical properties.

5. Application of Lead

Lead is largely used for linings, coils, pumps, valves, etc, in the manufacture of a number of chemicals. Lead has excellent corrosion resistance to brine solution, salt spray and seacoast atmospheres, and its corrosion rate in seawater being less than 0.0003 inch per year. Lead is used for waste pipe carrying seawater aboard ships, for lining refrigeration rooms and for large aquariums. Lead has given good service life in the use of wide line of equipment in the manufacture of dye intermediate in processes like halogenisation, sulfonation, hydrolysis, oxidation, esterification, reduction, extraction and condensation.

Lead resists corrosion to sulfuric acid at concentration upto 95% due to virtual insolubility of the hard, impervious coating of lead sulfate, which forms on its surface. It is used in the manufacture of sulfuric acid by chamber process and in such processes as the sulfuric acid pickling of steel, and the manufacture of nitroglycerin, titanium dioxide and ethers. It finds application in lead covered anodes and for the lining of electrostatic precipitates used to remove sulfuric acid mist for sulfur dioxide gas. It is also used in refining of petroleum where the sulfuric acid treatment is followed by a caustic wash. Lead is used in the manufacture of phosphoric acid. The pulp and paper industry use good amount of lead pipe in cooling systems of sulfur dioxide gases in bleaching stock with hydrogen peroxide or zinc hydrosulphite and for conveying the discharge liquors from the pulp digestors. Lead is also widely used for manufacture of rayon and nitroglycerine and for the handling of photographic solutions.

5.1 Other uses of Lead

High-density (11.3) of lead makes it a very desirable material for X-ray protection. It finds wide use for completely lining the room containing the X-ray equipment, to protect personnel outside. Extruded lead blocks of concave and convex shape are used for walls to confine hard rays from nuclear fission and radioactive isotopes. The high density of lead coupled with its ease of casting and its salvage value makes it ideal counterweight material in machinery's and equipment's and in the keels of ships.

Soft properties of lead are taken advantage of in its use as impression lead for half tone plates and types.

Lead can be easily extruded in pipes in long lengths. Its pliability permits such pipe to be bent and curved in construction work without necessitating many joints. Another advantage of its pliability is that it permits water service pipes to accommodate themselves to ground subsidence without damage.

Lead is also used as a vibration damping material under machines and buildings. Other uses of lead include gasket material because of its pliability and low creep strength, hammer where care must be taken not to injure the work. Large tonnage of lead is used for caulking the joints of water and gas pipes made of iron. Sheet lead is used as a flooring material in electroplating and chemical plants, where acid spillages go to concrete floors, and in the manufacture of explosives where sparking will be disastrous.

5.2 Use of Lead Alloys

Lead, which has relatively low melting point, alloys with all the other elements in that melting range to give a series of alloys that find wide use in industry. Lead alloyed with varying amounts of antimony finds wide application. This lead, commercially known as hard lead, has excellent strength and corrosion resistant properties.

Lead containing 3-9% antimony is used for storage batteries for grids, lead oxide paste within the grid structures, battery plugs and terminals. Because of its good strength and corrosion resistance properties antimonial lead is widely used in the chemical industry as pipe, sheet, lead-lined steel valves and pumps, solid antimonial-lead valves and pumps and fittings. Lead containing 13% antimony, together with 1% tin, 0.5% arsenic and 0.1% copper is widely used for casting toys and ornaments. An alloy of similar composition is used for manufacturing slush castings, which produces in detail replica of the mold. A great variety of lead based and tin based alloys are used for die-casting, storage battery inserts and for many other uses. Antimonial lead is used in thin walled pipes and lining of tanks. Another use of antimonial lead is in buildings for flashings and roofs.

Terne plate with a nominal composition of 10-25% tin and 90-75% lead is applied to sheet iron by hot dip process in manufacture of containers. The lead alloy coating not only is corrosion resistant but it also facilitates its soldering and drawing. Lead and lead alloys are used in flame sprayed metallising processes in the form of wire or powder for building up and reclamation of worn out machinery components. Cable industry consume huge tonnage's of lead alloys for sleeving and sheathing as a flexible covering to prevent the entrance of moisture and oil. Copper bearing lead and antimonial lead are the principal materials for covering electric power and communication cables. Another alloy that has shown promise in cable industry is the lead containing 0.04% calcium. Calcium increases the strength without impairing the ductility and pliability of the alloy. Lead alloys with tin content are extensively used for manufacture of collapsible tubes.

5.3 Lead compounds

Large amounts of inorganic and organic lead compounds are also used in the industry. Tetraethyl lead is produced in large quantities and is used as chemical intermediate for the manufacture of bromine, ethyl chloride and ethylene dichloride and sodium producing facilities. Nearly all the gasoline contains tetraethyl lead as antiknock agents. It also gives economics of gasoline consumption in higher compression engines, ships and pigments for paints and glass industries.

6. Fabrication and Welding of Lead and its Alloys

Lead and its alloys can be easily cast in iron or steel moulds without iron contamination. Although for general melting purpose no covered flux is required, for certain classes of alloys fluxes based on zinc chloride, sodium chloride or halides of the alkali or alkaline earth metals is used as an advantage to retard dross formation. Completely finished products of lead and lead alloys may be manufactured by die casting, gravity casting or centrifugal casting. Because of its malleability, lead can be easily cold rolled to sheet form slabs. Because lead has three times the coefficient of thermal expansion than steel, lead linings on steel or wood vessels are required to be properly fastened to avoid buckling. Lead and lead alloys can be extruded into a wide variety of shapes including pipes.

Welding lead, commonly called lead burning, is different from the practice used for other metals. No flux is used and the welding rod is the same as the metal being welded. Oxyacetylene or oxyhydrogen gas welding is an accepted process. A natural flame is recommended since reducing it will deposit carbon, and an oxidizing flame will promote oxidation preventing the metal flow. Because of the toxic nature of the lead components, adequate ventilation should be provided.

7. Control of Health and safety Aspect of the Use of Lead

In the production of lead, health and safety control is as important as metallurgical controls. It has become established policy to provide safeguards for the health of the employees against occupational hazards of lead production. Trained and experienced hygienists work in the closest co-operation with ventilation engineers and safety medical practitioners in order to evaluate the hazards and provide for improved working conditions. A number of occupations like lead mining, lead smelting or refining, handling and fabrication of lead articles, handling of lead in hot processes, manufacture of lead salts and organic lead compounds can cause lead poisoning. Manufacturing processes in which lead compounds are used like storage battery, paint, glass manufacturing, rubber compounding and chemical industries, spray painting with lead application and removal of lead containing paints, flame cutting of painted metals typographic trades are also hazardous occupations.

Lead exposures may arise from any of the above process in which sufficiently high atmospheric contamination with lead compound is brought about. Whenever adequate amounts of lead are present in the atmosphere, lead absorption by the worker will be inevitable unless suitable measures are instituted for their protection. The degree of absorption will be directly related to the amount of the lead compound, to the physical state of the compound and to the length of time over which exposure occurs. Inorganic lead absorption can occur either by inhalation or by ingestion. The former route is more common in the industry where the lead compound is distributed in the atmosphere of the work place. The lead absorption will be produced by inhalation of fumes or dust. Inorganic lead compounds cannot be readily absorbed through the skin. Accumulation of lead compounds on the hands, however, frequently will result in the ingestion of this substance of the organic lead compounds. Tetraethyl lead, because of its extensive use as an antiknock material in motor, is the most hazardous. Acute poisoning may result from the exposure to a high concentration of vapor for several hours.

Dust concentrations in the atmosphere are monitored by the use of an impinger – a small portable suction device where solids from a measured quantity of atmosphere are collected on a filter paper and analyzed for metal content by use of a polarographic procedure. Analyses of blood samples of the employees is also made to determine the absorbed lead content. In the lead industry, ventilation is of utmost importance. Operations connected with sampling, ore beds, sinter plants, blast furnace and reverberations and welding are carefully studied and proper ventilation provided. Wetting down of dust in open areas and wet rotoclons are used where necessary. Kettles used for drossing are provided with high velocity slot type exhaust hood. Baghouse dust and flu dusts are handled mechanically and by judicious use of wetting, dusting is kept under control. Well-regulated lead installations can operate very satisfactorily if proper safeguards and working conditions are met.

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