Asbestos, in various types and forms, is still widely used in many countries for many applications, including uses in building materials, various processes, transport devices, etc. However, many countries have banned the use of asbestos in areas where it can be replaced with other materials, for good reason. The basis for such bans was precipitated through published medical data on the carcinogenicity of asbestos dust. Due to the above-mentioned issues, the problem of processing millions of tons of waste, accumulated earlier and still emerging today, has become even more urgent. In this article, we will address this problem for one type of asbestos, chrysotheil, which is still mined in large quantities in a number of countries.
A dry gravitational method is used to enrich chrysothemum asbestos from ore. Enrichment occurs due to the difference in density and hovering velocity in the air environment of asbestos fibers and rocks. The enrichment process involves multi-stage crushing of ore with the extraction of asbestos by the separation of fibers from the rock in the air stream. The result is obtaining a commercial asbestos fiber (4-6 percent from the ore) and the waste is in the form of shredded rock (the remaining 94 percent). It should be noted that asbestos waste is composed of minerals (serpentinites, dunits and garzburgites) that are very close in chemical composition to asbestos and differing almost exclusively by its structural condition. The main chemical components of waste are the magnesium oxides (39-40 percent) and silicon oxides (36-38 percent) present in the form of magnesium hydrosilicates, where water content is 11-14 percent. Iron (5.19-7.41 percent), nickel and chromium (0.12-0.19 percent and 0.28-0.53 percent, respectively) impurities are present also.
Based on the chemical and mineralogical composition of the asbestos production waste, there are two main areas of their processing that appear to be most promising:
- Obtaining a synthetic asbestos, as the chemical and mineral composition of the asbestos waste almost completely coincides with the composition of the asbestos itself.
- Decomposition of waste by chemical methods through the extraction of magnesium and other components. The silicates with molecular ratio MgO/SiO2 ≥1.5 are relatively easy to leach with mineral acids.
In this present article, we will discuss implementation of Item 2 above, namely; a chemical decomposition.
A lot of waste disposal work has been carried out in the UK, the U.S. and Canada (See, for example, References1-4). Similar work has been, and continues to be, performed on a small scale in areas like Kazakhstan (See References 5-7). Practically all of the cited authors used hydrochloric acid waste treatment to produce MgCl2 magnesium chloride, which serves as a raw material for generation of metallic magnesium.
Such approaches have not yet paid off either technically or economically. Costs are too high, as the products that are being obtained are low price. In addition, the vast majority of the waste, namely the entire silicate part and impurities, remain unused.
The authors, based on the analysis of the composition of raw materials coupled with their technological experience, have applied completely different approaches to the disposal of asbestos waste, by applying the following three (3) main principles:
- Apply stronger chemical reagents, based on fluorides and sulfates;
- Use the separation of raw material components between different phases (solid, liquid and gas); and
- Include in the overall processing scheme of the operation the complete regeneration of the chemical reagents used.
As a result of such an approach, the authors managed to combine a completely closed-by-reagent scheme of waste processing with a zero residue of the substance; namely, almost waste-free. See Figure1for an abbreviated version of the chemical reactions.
If one sums up all of the basic chemical reactions (there are about 11 of them), it will yield the following final chemical equation:
Mg3Si2O5(OH)4 = 3MgO + 2SiO2 + 2H2O
In other words, hazardous asbestos waste is being decomposed into two valuable and expensive safe products: high-purity magnesium oxide and silicon oxide with a regular water remaining. According to our approximate estimates, one ton of raw materials through the proposed chemical processes could produce the commercial magnesium and silicon oxides for an amount over $900. Conversely, according to the current mechanical technology used by mines producing asbestos fiber, they only retrieve $12 from one ton of ore.
As one can clearly see, this new scheme allows a mine to generate almost 100 times more revenue, compared to current traditional asbestos fiber production.
- UK patent №2033364; C 01 F 5/00; 21.05.1980.
- USA patent №5980854; С 01 F 5/30, С 22 В 26/22, 09.11.1999
- R.W. Stanley, M. Berube, C. Ctlik-Noranda Technology Center, Y. Osaka-Aisin seiki Co.Ltd., Avedesian-Noranda Metallurgy Inc.: Magnola, The Magnola process for magnesium production. 53rdAnnual International Magnesium Association Conference, June 2-4, 1996, Ube, Japan, pp.58-64
- Bedard M. The Production of Magnesium by Noranda.58thAnnual World International Magnesium Association Conference. May 20-22, 2001, Brussels Belgium, p.p.57-64
- RF patent №2118406, С 25 С 3/04; 27.08.89
- RF patent №2244044, С 25С3/04, С 22 В 26/22, 10.01.2005
- RF patent № 2241670, C01F5/30 C25C3/04 C01B7/01 C01B33/1, 21.07.2003