Processo per la trasformazione termica di lastre di cemento-amianto utilizzando un forno industriale continuo

This patent describes an industrial process for the direct complete temperature induced transformation (decomposition and recrystallization) of packages of plastic material containing roofing cement-asbestos slates or other asbestos containing wastes such as friable asbestos. The industrial firing process ensures a complete transformation of both the package and the asbestos containing material via a decomposition reaction of the organic compounds and a decomposition and recrystallization of all the inorganic mineral phases including the asbestos minerals.The firing process which ensures the total transformation of the packages and the mineral assemblage including the asbestos phases is accomplished using an industrial continuous tunnel kiln, with a technology comparable to that employed for the production of bricks or sanitary-wares. The length of the industrial kiln should be within 100 and 150 m to guarantee the proper firing cycle and total decomposition and recrystallization of both the hazardous serpentine and amphibole species of asbestos. The transformation via decomposition and recrystallization of these minerals leads to the formation of non hazardous crystalline Si-, Ca-; Mg-rich silicates. To this aim, the steps of the firing cycle which are required are:Step 1: a heating non-isothermal run up to the maximum firing temperature of 1200-1300 \ub0C not shorter than 8 h;Step 2: an isotherm at the maximum firing temperature of 1200-1300 \ub0C not shorter than 12 h;Step 3: a slow cooling non-isothermal run down to room temperature not shorter than 6 h.During the firing cycle, the industrial kiln must ensure complete isolation of the packages to be treated from the environment outside the kiln. All the volatile compounds and the asbestos fibres eventually released during the preliminary stage (Step 1) of the firing process inside the kiln will be conveyed to a post-combustion system (a supplementary heater at the top of the kiln) operating at temperature not lower than 1100 \ub0C, to guarantee their complete thermal transformation and decomposition. The hot smokes and particulate from the post-combustor system will be successively cooled down to a temperature lower than 150 \ub0C to allow the flow through double absolute HEPA filters before they are released in air.This industrial process is highly innovative and unique because it makes possible a safe direct thermal transformation of the packages containing the asbestos materials. The packages are not opened nor mechanically ground before the firing process. In this way, such pre-firing operations, highly dangerous because of possible dispersion of asbestos fibres in air, are avoided.The energy to the kiln will be supplied using natural gas.This industrial system will be capable to operate either with cement-asbestos sheets and friable asbestos and/or synthetic glass/wool fibers.As consequence of the thermal treatment, in agreement with the actual laws concerning the treatment and recycling of transformed asbestos containing wastes (see Table B Italian Minister Decree nr. 248, July 29th 2004), the product of transformation, composed of a mixture of innocuous non-fibrous crystalline silicates can be milled and recycled as a raw material for the formulation of a number of different industrial products such as traditional ceramics, pigments, cements and bricks