Chemical risks in Metallurgy
In order to well define the metallurgy, it is first of all necessary to make the distinction with the mineralurgy, linked to the mining industry
The mineralurgy: all mechanical, physical or chemical methods set-up to separate minerals from their gangue and to partially purify them by concentration. It can be done by gravimetry, magnetic separation or flotation, which is based on processes that lead mineral or metallic particles to float to the surface of a liquid with a variety of additives such as surfactants or solvents. Various compounds are used to modify the properties of minerals to optimize their concentration.
The metallurgy: it has been organized into three main specialties. Each of the three requires a different specialization of the other two. There is, on the one hand, the metallurgy of iron (steel) and, secondly, that of non-ferrous metals, which are divided into precious metals, such as gold, and non-precious, such as aluminium :
The metallurgy covers a wide range of industrial activities including metal recycling, the foundry (in foundries, mills and smelters), the manufacture of raw products through rolling mills, the processing of raw products into semi-finished products, and finally, equipment manufacturing and finished goods for industry (automotive, aerospace, information systems and telecommunications, food packaging, microelectronics, printing, industry and transportation). Finally, a quick overview of the steel industry worldwide helps to day major world leaders of this sector : Alcoa, Rio Tinto (buying out of the group Alcan in late 2007 after a first attempt of takeover bid from Alcoa), Hydro and BHP Billiton (proposal of a buying out of Rio Tinto finally denied by the group in February 2008).
The metallurgical sector uses many chemical agents on a regular base in its daily manufacturing processes : paints, solvents, resins, polyurethanes, oils but mainly for surface treatments (mechanical, chemical, electrochemical or physical operations aiming at changing / improving the appearance or the function of materials’ surface to adapt them to specific conditions of use : corrosion protection or improvement of physical characteristics for example) and cleansing operations. Because of the high diversity of treated metals, these operations are very varied, hence the presence of many products (surfactants ; complexing agents as EDTA ; acids – hydrochloric acid, nitric acid, phosphoric acid, sulphuric acid, acetic acid ; bases – sodium and potassium hydroxides, lime ; solvents ; colorants and wastes) and a major chemical hazard.
A few examples :
Electropolishing : commonly used electrochemical method for smoothing, polishing, deburring and cleaning various metals among which aluminium and its alloys. Electropolishing removes a fine surface layer electrolytically. In these electropolishing processes, different electrolytes are used. They are usually mixtures of various acids (sulphuric acid, chromic acid, citric acid, and/or phosphoric acid) and sometimes organic compounds are added.
Aqueous cleaning : the workpieces are placed in this process solution for several minutes. The solution is usually alkaline or neutral but may be acidic and usually working at increased temperatures (40-90°C) because of the improved cleaning effect.
Pickling : pickling is a chemical procedure used to brighten and/or remove oxides from the metallic surface prior to other surface treatment processes. During the pickling processes, adhering layers, oxide films and other corrosion products are removed by chemical reaction with an acid-based pickling agent. Hydrochloric acid (18-22%) or sulphuric acid (25%) are normally used. In special cases, nitric acid, hydrofluoric acid (20-25%) or phosphoric acid or mixtures of acids are used.
Solutions containing fluorides are necessary for reliably pickling certain alloys.
Anodising : the anodising of metals is an electrolytic surface oxidation process which enhances the natural aptitude for the metal to oxidise ; coatings can be 1000 times thicker than the natural layer. Aluminium is the most important material to be anodised, with alumina (Al2O3) formed at the surface. Aluminium is normally (90% of the cases) anodised in sulphuric acid electrolyte. For special applications, aluminium may be anodised in many different types of process solution : phosphoric acid, sulphuric/oxalic acids, sulphuric/salicylic acids and chromic acid electrolytes.
Phosphating : the phosphating is a chemical reaction made with a phosphorous agent. The treatment of a surface with a diluted solution of hot phosphoric acid allows its passivation (state of metals or alloys in which their corrosion rate is significantly slowed by the presence of a natural or artificial passive film), and improves the resistance to corrosion of coatings of paint. The artificial passivation is done on a metal previously deoxidized (pickling : dissolution of unwanted oxides) by an acid (hydrochloric or sulfuric acids are the most frequently used) or an acid mixture (complementary presence of nitric acid, or HF).
NB : A major challenge for the metallurgical industry is to replace in the short term chromium VI (Cr6+ – hexavalent), under pressure from the legislation for reasons linked to hygiene, safety and the environment (if it is not completely banned today, this agent remains highly toxic).
On line 01/24/2011
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