Lead Bullion Fire-refining and Electrorefining Process

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Common lead smelting process usually produces lead bullion containing 2-4% impurities such as copper, arsenic, antimony, tin, bismuth, indium, silver and gold, etc. Those impurities are valuable metals but have negative effects on lead&#;s functional performance in terms of hardness, toughness and corrosion resistance. The process of lead refining is to improve lead purity and recover these valuable metals.

Lead refining mainly falls into two groups, i.e. fire-refining and electrorefining process. Characteristics of both approaches are listed as follows:

1.    Fire-refining can produce fine lead of different grade and lead alloys by using several kettles, requiring simple equipment, less investment and land space. Lead bullion containing less bismuth and precious metal is suitable to fire-refining. However, full-step fire refining has relative low lead direct recovery, worse working environment, long list of working steps and large treatment capacity required by intermediate product.

2.    Electrorefining enriches bismuth and precious metal into anode slime for comprehensive recovery. It results to high metal recovery rate and high purity lead and the working environment is relatively better. Before electro-refining, lead bullion has to be pre-refined to reduce copper and tin content. After electro-refining, lead cathode should be further fire-refined before ingot casting. This process generates, except copper slag and tin slag, especially lead anode slime that contains valuable metals.  

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Full process of lead fire-refining includes several steps to remove Cu, Te, As, Sb, Sn, Ag, Zn, Bi in sequence. In actual operation, the steps are simplified according to the kind of impurities and its content.

The lead-refining process is described as, &#;After copper drossing, generally Te, As, Sn and Sb are removed substantially by the Harris Refining Process using fused NaOH. A certain selectivity during refining is achieved by controlled addition of NaNO3. Another refining step is decopperizing by means of elemental sulfur. A Howard Press is used in the Parkes Process for desilverizing. After vacuum dezinking in kettle, Bi is removed, if required, by means of Ca and Mg. In an end refining step Ca, Mg and Zn are removed by having the lead melt treated with NOH and NaN03. The refined lead as such, or in the form of special alloys, is cast by an ingot casting machine into 50-kg ingots.&#; (Excerpt from the book Extractive Metallurgy of Lead and Zinc)

lead processing

lead processing, preparation of the ore for use in various products.

Lead (Pb) is one of the oldest metals known, being one of seven metals used in the ancient world (the others are gold, silver, copper, iron, tin, and mercury). Its low melting point of 327 °C (621 °F), coupled with its easy castability and softness and malleability, make lead and lead alloys especially suitable for a wide range of cast products, including battery grids and terminals, counterweights, plumbing components, and type metal. With a specific gravity of about 11.35 grams per cubic centimetre, lead is the densest of the common metals, except for gold; this makes it a good shield against X-rays and gamma radiation. Its combination of density and softness make it an excellent barrier to sound. Compared with other metals, lead is a poor conductor of heat and electricity, although it has excellent corrosion resistance when it can form an insoluble protective coating on its surface. The metal has a face-centred cubic crystal lattice structure.

Approximately 30 percent of all lead consumed is in the form of lead compounds, such as oxides, tetraethyl and tetramethyllead, lead chromates, sulfates, silicates, and carbonates, and organic compounds. These lead compounds have been used in paste mixtures in storage batteries, in cements, glasses, and ceramics, as pigments in paints, and as an antiknock agent in gasoline.

History

Lead has been mined and smelted for at least 8,000 years. This is confirmed by artifacts in various museums and by ancient histories and other writings, including the biblical Book of Exodus. Lead beads found in what is now Turkey have been dated to about bce, and the Egyptians are reported to have used lead along with gold, silver, and copper as early as bce. In pharaonic Egypt, lead was used to glaze pottery and make solder as well as for casting into ornamental objects. The British Museum holds a lead figure, found in the temple of Osiris in the ancient city of Abydos in western Anatolia, that dates from bce.

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One of the most important historical applications of lead was the water pipes of Rome. Lead pipes were fabricated in 3-metre (10-foot) lengths and in as many as 15 standard diameters. Many of these pipes, still in excellent condition, have been uncovered in modern-day Rome and England. The Roman word plumbum, denoting lead water spouts and connectors, is the origin of the English word plumbing and of the element&#;s symbol, Pb.

Marcus Vitruvius Pollio, a 1st-century-bce Roman architect and engineer, warned about the use of lead pipes for conveying water, recommending that clay pipes be used instead. Vitruvius also referred in his writing to the poor colour of the workers in lead factories of that day, noting that the fumes from molten lead destroy the &#;vigour of the blood.&#; On the other hand, there were many who believed lead to have favourable medical qualities. Pliny, a Roman scholar of the 1st century ce, wrote that lead could be used for the removal of scars, as a liniment, or as an ingredient in plasters for ulcers and the eyes, among other health applications.

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Many churches and major buildings constructed in the 15th and 16th centuries provide examples of lead employed as a roofing material and for water conveyance. Indeed, the stained-glass windows of many cathedrals and castles of this period were made possible by the use of lead cames that held the glass elements together in a magnificent unity of colours and shapes.

In a French physicist, Gaston Planté, discovered that pairs of lead oxide and lead metal electrodes, when immersed in a sulfuric acid electrolyte, generated electrical energy and could subsequently be recharged. A series of further technical improvements by other investigators led to commercial production of lead-acid storage batteries by . The huge growth of battery markets in the 20th century (eventually consuming about 75 percent of the world&#;s lead production) largely paralleled the rise of the automobile, in which batteries found application for starting, lighting, and ignition. Another prominent lead product was tetraethyl lead, a gasoline additive invented in in the United States to solve &#;knocking&#; problems that had become commonplace with the development of high-compression engines operating at high temperatures. Soon after reaching its peak 50 years later, the use of this lead compound declined in the United States as the installation of catalytic converters became mandatory on the exhaust systems of all American passenger cars.

By the early 21st century, China was leading the world in both primary and secondary lead refining. Other top lead refiners include the United States, the United Kingdom, Germany, and India.

Ores

Of the more than 60 known lead-containing minerals, by far the most important primary ore of the metal is the lead sulfide galena (PbS). Galena often contains silver, zinc, copper, cadmium, bismuth, arsenic, and antimony; in fact, the value of the silver content often exceeds that of the lead, in which case it is deemed a silver ore. Other commercially significant lead-containing minerals are cerussite (lead carbonate) and anglesite (lead sulfate). These are known as secondary minerals in that they derive from galena through natural actions, such as weathering. Cerussite, for instance, is formed by the action of carbonate groundwater on galena, whereas anglesite is formed when galena is subjected to sulfate solutions generated from the oxidation of sulfide minerals.

More than 95 percent of mined lead comes from these three ores. Ores of commercial importance may range from 2 to 20 percent lead or more, even though galena itself contains 86.6 percent lead. This seeming disparity is due to the fact that galena is usually found mixed with other minerals, such as the zinc sulfide zinc blende and the iron sulfides pyrite and marcasite. Consequently, the percentage of recoverable lead in ores is typically about 4 percent, and nearly 90 percent of primary lead ores come as a by-product of zinc and silver mining. More than half of the total lead refinery demand is met by the recycling of spent lead, mostly from reclaimed batteries.

Significant deposits of lead ores are located in Australia, Canada, China, Mexico, Peru, Kazakhstan, Russia, and the United States.

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