Chromium wood preservatives

Wood Preservation. The use of chromium compounds ia wood preservation is largely because of the excellent results achieved by chromated copper arsenate (CCA), available ia three modifications under a variety of trade names. The treated wood (qv) is free from bleeding, has an attractive ohve-green color, and is paintable. CCA is widely used, especially ia treating utility poles, bull ding lumber, and wood foundations. About 62% of all the chromic acid produced ia the United States is consumed by the wood preservation industry (77,167) (see Building materials, survey). 

Chromium-containing wood preservatives and their chemical compositions are Hsted ia Table 13 (199). Chromium compounds have a triple function ia wood preservation (200). Most importantiy, after impregnation of the wood the Cr(VI) compounds used ia the formulations react with the wood extractives and the other preservative salts to produce relatively insoluble complexes from which preservative leaches only very slowly. This mechanism has been studied in the laboratory (201—206) and the field (207). Finally, although most of the chromium is reduced to chromium (ITT), there is probably some slight contribution of the chromium (VT) to the preservative value (208). 

Cox, T.R.G. and Richardson, B.A. (1978). Chromium in wood preservation health and environmental aspects. International Research Group on Wood Preservation, Doc. No. IRG/WP 3120. 

Kazi, E.K.M. and Cooper, P.A. (2002). Rapid extraction oxidation process to recover and reuse copper chromium and arsenic from industrial wood preservative sludge. Waste Management, 22(3), 293-301. 

At the Coast Wood Preserving, Inc., Superfund site (Ukiah, California), the technology was used to remove metal contamination to comply with both state and federal cleanup standards [50 parts per billion (ppb) arsenic, 50 ppb chromium, and 1 ppm copper]. The estimated total cost for the source control component of the remedy was 1,000,000, and the estimated total operational and maintenance costs was estimated to be 19,500 for a 20-year period (D16888B, p. 3, Report Documentation p. 2). 

The Lewis ENVIRO-CLEAN process removes and recovers metals such as chromium, copper, nickel, mercury, lead, zinc, iron, and cadmium and has effectively demonstrated that it can treat a matrix of multiple metals in a single stream with positive results. The process treats wastes from wood preserving, metal finishing, mining, surface and groundwaters. The two-step process uses granular-activated carbon and electrolytic metal recovery to yield a salable metallic by-product. 

Metals are major pollutants only at wood preserving plants that treat with water-borne salts of copper, chromium, arsenic, and zinc. 

Chromium compounds, mostly in chromium(III) or chromium(VT) forms, produced by the chemical industry are used for chrome plating, the manufacture of dyes and pigments, leather tanning, and wood preserving. Smaller amounts are used in drilling muds, rust and corrosion inhibitors, textiles, and toner for copying machines. For more information on the physical and chemical properties and on the production and use of chromium, see Chapters 3 and 4. 

British Standard 5666, Methods of Analysis of Wood Preservatives and Treated Timber, Part 3, Quantitative Analysis of Preservatives and Treated Timber Containing Copper, Chromium, Arsenic Formulations, British Standards Institution, 2 Park Street, London W1A 2BS, 1979. 

Wood preserved with Chromated Copper Arsenate (CCA), a chemical mixture consisting of three pesticidal compounds (arsenic, chromium and copper)... 

CCB impregnated wood was pyrolyzed at SOO and 600 °C and the heavy metals copper and chromium were determined in the char and liquid by XRF (see Table 5). The data clearly show that typical heavy metals from wood preservatives are trapped together with the char fraction. If the char is burnt for energy recovery, the heavy metals would remain in the ash. From here they might be recovered or dumped. The oil is practically uncontaminated. These results are consistent with those found already in a laboratory fluidized bed reactor (3). 

Many of the biocides presently used in wood preservation preparations, e.g. pentachlorophenol (PCP), linane (y-HCH), creosote, and copper chromium arsenic-salts... 

Inorganic Salts. A number of the metal salts have fungicidal activity and are used to formulate commercial wood preservatives. The principal metal salts used are compounds of arsenic, chromium, copper, and zinc. In order to provide the desired fungicidal activity, leach resistance, and low corrosivity, combinations of these compounds are used. All of the formulations discussed later are waterborne solutions. Only a brief description of these preservative systems will be presented here more detailed presentations can be found elsewhere (3). 

Inorganic Salt Preservatives. Inorganic compounds used to formulate wood preservatives are normally water soluble. As a result, the salts deposited in the wood are susceptible to leaching unless they are transformed to insoluble compounds or are chemically fixed to the wood substrate. Indeed, such conversions do occur as a result of interactions between some of the salts and the wood substrate. Both single element and multicomponent reactions are involved in the fixation mechanisms copper and chromium are the most reactive of the possible components. 

Chromium is a transitional element with many industrial uses. It is mainly used in imparting a shiny appearance to metal surfaces. In the early 1800s, the mineral, now known as chromite, was widely used in the production of paint as well as in the production of chromium compounds. These compounds can be used in a variety of applications. For example, potassium dichromate is used in the dyeing industry and chromium salts are used in leather tanning and wood preservation. Today, perhaps its most important use is in the production, in combination with iron, of stainless steel. 

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