Asbestos Destruction with CO2

Asbestos was once a very attractive material for industrial and civil applications, such as thermal insulation and roofing. Once it became known as a potent carcinogen, its use was largely replaced. However, much of this material still remains where it was originally applied, and even more exists stored in hazardous landfills and mine tailings storage sites. A way of turning this dangerous compound into a safe material is urgently needed. Hazardous asbestos can be made harmless via biological deterioration, thermal break-down or by chemical alteration. The biological destruction proceeds rather slow and depends on specific environmental conditions. Melting requires excessive amounts of energy. Traditional chemical routes require either strong acids or weaker acids combined with intense process conditions. Hence, all of these approaches have major drawbacks. In this work, we investigated the destruction of asbestos by applying carbonic acid (derived from CO₂) as the reagent.

Carbon dioxide is a suitable feedstock for the chemical attack route as this produces carbonic acid in aqueous solution, which reacts with the alkaline asbestos minerals. Another aspect that makes CO₂ an attractive feedstock for this process is that the carbon becomes inherently sequestered, and a safe end-product is produced. The proposed process has been tested at lab-scale under various process conditions and proofed with an array of characterization techniques.

The chemical process was accelerated in an autoclave reactor operating at elevated temperatures (150°C and 220°C) and pressures (50 and 80 bar,CO₂). The foreseen full-scale implementation of this process would be in a Gravity Pressure Vessel (GPV) and therefore the tested process conditions were selected to match the GPV’s operating range. In addition, select ‘catalytic’ additives and pre-treatment of the material (heating to break OH-bonds) were used to enhance the destruction degree of the asbestos.

Three different asbestos-containing materials were tested: (i) pure chrysotile (used to assess the highest possible destruction rate); (ii) corrugated asbestos cement roofing sheets (a combination of cement and chrysotile), and (iii) asbestos pulp with chrysotile and amphibole minerals, a waste product from asbestos manufacturing). The latter two materials cover the majority of the Dutch asbestos waste streams.

An array of characterization tests were performed of the original materials and the reaction products, which include: optical microscopy, stereo/polarization microscopy, quantitative XRD, and SEM-EDX.

It was found that after treatment for 1 and 5 hours, without catalysts and without pre-heating, a clear destruction of the chrysotile minerals is visible. The XRD analyses showed decreasing content of asbestos, chemical alteration of asbestos minerals, and the formation of new minerals. Using the cement-containing products, the asbestiform fibers became dislodged from the matrix, facilitating the reaction of the fibers with the liquid medium. Morphologically, however, this material retained asbestiform characteristics. In particular, it was observed that amphibole minerals are hardly affected.

The treatments of the chrysotile with catalysts and/or pre-heating showed a positive effect. The remaining fibrous minerals, when treated with catalyst, had chemically altered surfaces and amorphous and rounded tips. According to the XRD analyses, the combination of pre-heating and catalysts provided the highest conversion, yielding material that would no longer be classified as asbestos. Both the chemical and mineralogical properties and the texture were changed, differing substantially from the original material. Close inspections revealed some acicular minerals; however, these minerals had a different chemical composition and did not possess the morphological properties that make asbestos harmful. The typical brittle, respirable fibers were bundled and their ends cemented and re-crystallized. As a result, these needle shaped minerals are not, according to the current standards for SEM investigation, classified as “asbestos”.