The present invention relates to methods for destruction of hazardous silica containing fibers, i.e. generally silica containing fibers with a length of approximately 5 pm to 100 pm capable of deep penetration into the respiratory tract. The present invention especially relates to destruction of hazardous asbestos fibers comprised in constructions materials such as cementitious asbestos comprising materials, for example, panels, tubes, corrugated roofing sheets, flat sheets, roofing tiles and water tubes.

The term asbestos is related to a group of six naturally occurring fibrous silicate minerals. Five of the asbestos minerals (crocidolite, amosite, tremolite, actinolite, anthophyllite) belong to the amphibole group of minerals, one asbestos mineral (chrysotile) belongs to the serpentine group of minerals. Chrysotile, crocidolite and amosite are the most widespread applied types of asbestos and are often referred to as white, blue and brown asbestos, respectively. The color of the asbestos is related to its chemical composition with chrysotile being predominantly a magnesium silicate (Mg ₃ Si ₂ 0 ₅ (0H) ₄ ), crocidolite predominantly being an iron, magnesium, sodium silicate (Na ₂ ((Fe ²⁺ , Mg ²⁺ ) ₃ Fe ³⁺ ₂ )Si ₈ 0 ₂₂ (0H) ₂ and amosite being an iron silicate (Fe _v Si ₈ 0 ₂₂ (0H) ₂ ).

When broken or mechanically bruised, the asbestos finer bundles can release microscopic thin fibrils of < 0.5pm thick. When inhaled, these fibrils are considered dangerous and inhalation of asbestos is related to severe lung diseases like mesothelioma and lung cancer.

Crocidolite and amosite are considered to be more hazardous than chrysotile.

Fibers longer than 100 pm are considered non-respirable as they are mostly filtered out in the upper respiratory tract. Smaller fibers are able to penetrate the lung tissue and cause local inflammation. Fibers smaller than approximately 5 pm are encapsulated by macrophages and effectively shielded from surrounding tissue without causing further damage. It is therefore the 5pm to 100 pm fiber fraction range that is of most concern. Additionally is the ratio between the length and thickness of importance. Generally a length/thickness ratio of higher then 3 is considered hazardous.

Asbestos has been used to reinforce a number of cementitious products like corrugated roofing sheets, flat sheets (roofing slates) and drinking water tubing. Upon weathering these materials will release their fiber content in the environment and pose a long term hazard. It is therefore imperative to dispose of these asbestos containing materials in an effective, cheap and sustainable way. However, the larger part of these products consists of cement minerals which in themselves are harmless and don’t need to be treated with similar precautions as the asbestos minerals. Next to this, these cement minerals have a large neutralizing value and could serve as a substitute for commercially available neutralizing agents (as lime). Accordingly, there is a need in the art for new processes for effectively processing asbestos comprising materials or, formulated differently, there is a need in the art for processes that convert hazardous asbestos comprising materials into non-hazardous materials.

It is an object of the present invention, amongst other objects, to meet the above need in the art.

This object of the present invention, amongst objects, is met by providing methods as outlined in the appended claims.

Specifically, this object of the present invention, amongst other objects, is met by methods for destruction of hazardous silica containing fiber, the method comprises the steps of: a) mixing hazardous silica containing fiber comprising material, which was subjected to mechanical size reduction to provide a powder, preferably a powder with a passing fraction of less than 1 mm, with chloric or nitric acid, wherein the weight ratio of chloric or nitric acid to the hazardous silica containing fiber comprising material is between 2 to 8 and incubating the mixture at a temperature of 15°C to 50°C during 1 to 4 hours until a pH of between 3 and 4 is reached; b) separating the mixture of step (a) into a wet solid fraction comprising the hazardous silica containing fiber and a filtrate not comprising hazardous silica containing fiber; c) adding 20% to 30% (v/v) hydrochloric acid or nitric acid to the wet solid fraction using a wet solid fraction to acid weight ratio of 1:0.3 to 1:1 and milling the mixture for 2 hours to 24 hours; d) incubating the milled mixture at a temperature of 70°C to 90°C while stirring during 1 to 8 hours; e) optionally repeating step (d) until providing destruction of hazardous silica containing fiber. f) optionally additional mechanochemical treatment with dry or liquid alkaline reagent of the silica end product

The present inventors have surprisingly discovered that the above combination of cold acid leaching, milling and hot acid leaching is sufficient to convert hazardous silica containing fibers, i.e. silica containing fibers with a length of 5 pm to 100 pm into non-crystalline silica.

The first cold acid leaching step removes, or dissolves, materials, such as cement minerals, from the ground hazardous silica containing fiber comprising material, utilizing the neutralizing capacity of the material while simultaneously exposing the silica containing fibers. Then an acid milling step is used to mechanically bruise the fibers in order to make them more vulnerable to acid attack. Finally, a hot acid leaching is used to destroy the fiber resulting in an end product substantially free a hazardous silica containing fiber, i.e. a residue with less than 5%, such as less than 4%, 3%, 2% or 1%, of silica containing fibers remaining. For asbestos preferably less than 100 mg/kg dry solids are remaining for chrysotile and less then 10 mg/kg dry solids for the asbestos of the amphibole type. These limits generally designate a product as “asbestos free”.

The present hazardous silica containing fiber comprising material is subjected to a mechanical size reduction step before being subjected to a cold acid leaching step. A suitable mechanical size reduction step is any conventional grinding step using breaking, crushing and milling equipment as used in the demolition industry, preferably with special measures to remove steel, wood and other debris present in the material which could hinder the sequential process steps. The size reduction processing preferably takes place with protection against the hazardous nature of the fibers such as underpressure in a contained environment.

According to a preferred embodiment, the present hazardous silica containing fiber is asbestos, glass wool or rock wool, most preferably asbestos.

The present asbestos is preferably comprised of, are capable of producing or releasing, fibers with a length of between 5 to 100 pm and selected from the group consisting of chrysotile asbestos, crocidolite asbestos, amosite asbestos, tremolite asbestos, actinolite asbestos, and anthophyllite asbestos.

Within the context of the present invention, the term destruction of hazardous silica containing fiber denotes reducing the fiber length of the silica containing fiber to less than 5 pm for substantially all fibers such as more than 95%, such as more than 96%, 97%, 98%, 99% or 100% of the fibers. Within the context of the present invention, less than 5 pm also encompasses the absence of fibers.

The present method is especially suitable for processing cementitious asbestos comprising materials generally used as construction materials, panels, pipes, corrugated roofing sheets, flat sheets, roofing tiles and water tubes.

According to a preferred embodiment, 5% to 25% (v/v) chloric or nitric acid is used in the cold leaching step. By using chloric or nitric acid under relatively mild conditions, i.e. temperature, hazardous silica containing fibers remain largely unaffected while simultaneously non-hazardous materials such as cement dissolve or become suspended facilitating a subsequent separation step and reducing the amount of material to be treated in the hot acid leaching step.

Preferably, 20% to 30% (v/v) further hydrochloric acid, or acids such as phosphoric acid, HF, or combinations thereof, either simultaneously or sequentially, are used in the hot leaching step(s), depending on the starting material, to increase the efficiency of the process.

After completion of the hot acid leaching step, the solid fraction and liquid fraction can be readily separated and reused. For example, the solid fraction is processed into filler for concrete and the filtrate can be regenerated to be used as a recycled acid solution in the present process.

For example, the filtrate can be regenerated using >85% (v/v) sulphuric acid thereby regenerating hydrochloric acid. Three ways of regeneration can be considered:

1) Highly concentrated acid (>85% H ₂ S0 ₄ ) can be added until all cations in solution (Ca, Mg, Al, Fe, Na, K) are balanced by sulphate anions and all chlorine will be present as free HC1. Precipitated gypsum can be separated by filtration. The remaining liquor can be distilled as an HC1 distillate.

2) Highly concentrated acid (>85% H ₂ S0 ₄ ) can be added until all cations in solution (Ca, Mg, Al, Fe, Na, K) are balanced by sulphate anions and all chlorine will be present as free HC1. Precipitated gypsum can be separated by filtration. The remaining liquor can be freeze crystallized. Regenerated HC1 will be decanted and metal sulphates will be centrifuged and/or filtered.

3) Highly concentrated acid (>85% H ₂ S0 ₄ ) will be added until all calcium in solution (Ca) is balanced by sulphate. Precipitated gypsum can be separated by filtration. The remaining liquor can be pyrolysed providing a HC1 condensate.

Another, post-processing step, i.e. after the hot leaching step(s), contemplated is filtration and mixing the wet solids with 60-85% H ₃ P0 ₄ until a final H ₃ P0 ₄ concentration of 40% to 45% is reached. Subsequently the mixture is leached at 90°C for 1 or 2 hours.

Yet another, post-processing step, i.e. after the hot leaching step(s), contemplated is filtration, washing and drying. The dried solids can be ground with calcium rich materials, such as in a ratio of 1 : 1 , during 1 to 2 hours.

Advantageously, the presently used acids are derived from industrial waste streams. Waste acids have to be neutralized before discharge, which is usually done with sodium hydroxide (NaOH) or limestone (CaC0 ₃ ) based chemicals. Neutralizing waste acids with asbestos containing material saves the consumption of limestone (CaC0 ₃ ) based neutralizing agents. Substitution of commercially available neutralizing agents by asbestos containing materials reduces the costs of waste acid discharge and saves C0 ₂ emission related to both production and use of neutralizing agents.

The present invention also relates to installation comprised of at least three vessels, wherein the first vessel is configured for performing step (a), the second vessel is configured for performing step (c) and the vessel is configured for performing step (d) of the above methods and the installation further comprises means for transporting acidic material from and to the vessels and means for mechanical size reduction of hazardous silica containing fiber comprising material. The present invention will be further detailed in an example and appended figures wherein:

Figure 1: represents a SEM image showing shows milled asbestos fibers in cement matrix;

Figure 2: represents a SEM image showing asbestos fiber concentrate in silica matrix after cold leaching (present steps a);

Figure 3: represents a SEM image showing silica residue after hot leaching (present step d).

Example

Asbestos destruction process

Size reduction and cold acid leaching

A mechanical size reduction of the asbestos containing material to < 1 mm can be performed with standard breaking, crushing and milling equipment as currently used in the demolition industry, with special precautions to remove steel, wood and other debris present in the material which could hinder the sequential process steps. The size reduction processing takes place with protection against the hazardous nature of the asbestos material, using underpressure in a contained environment.

In this step, the asbestos containing cement is mixed with strong chloric or nitric acid mixture containing at least 5-25% of these two acids and is used at relatively mild conditions (15 to 50°C ). Sulfuric and phosphoric acid are not used due to precipitation of sulphate (gypsum) and phosphate (calcium phosphates).

The recipe depends on the concentration of the strong acid available, in acids ratio of 2-8 times the amount of asbestos cement expressed in weight and determined by the end pH reached.

The asbestos cement is added to the acid in the reaction vessel. The reaction between finely ground cement minerals and acid is exothermal and fast. The acid will be neutralized until a pH of at least 3 is reached, but not higher than 4. Depending on the acid concentration used and impurities present the slurry has to be diluted with water or diluted acid to prevent gelation (of the silica from the cement) and blockage of the filter.

If the pH drops below 3 extra asbestos cement is added and left to react until pH is equal or above 3. In this step the cement matrix will be destroyed for more than 80%, but with limited or no destruction of the asbestos fibers. This process step takes approximately 1-4 hours depending on the waste acid concentration and particle size of the starting material. After step 1 the fibers are liberated from the matrix and accessible for further mechanical treatment and leaching in acid in the next processing steps. Before the next step solid/liquid separation is carried out.

Size reduction and hot acid leaching of the filtered solids

A waste acid mixture having at least a 20-30 % concentration of hydrochloric or nitric acid is added to the wet solids from step 1 in a wet solids: acid weight ratio ranging from 1:0,3 to 1:1. This slurry is pumped into the milling equipment. The fibers are ground in a wet process using a ball mill filled for 30-40% of volume with ceramic grinding balls in different sizes and 20-35% of volume slurry for 2-24 hours.

In this process the fibers are crushed and their crystal structure weakened. After the milling stage is completed the slurry is partly diluted with cold strong acid and pumped into the next reactor. Any slurry remaining in the ball mill is washed with remaining cold strong acid and added to the next reactor in which hot acid leaching will take place.

The reactor is filled with mixture of the slurry from the ball mill and fresh hydrochloric acid, concentration 20-30% and brought to a temperature between 70-90 °C for 1-8 hours while vigorously stirred. After the leaching step solids in the slurry will be separated from the liquid and washed. The solids will be washed until their chemical composition meets the requirements for end use (i.e. as functional filler in concrete).

Washing will be carried out in a three step counter current washing procedure. The concentrated washing liquor can be reapplied if needed or disposed of. The wet filter cake solids are being dried if required.

Analyzing the solids out of process step d with optical microscope, SEM and XRF will confirm the sufficient destruction of asbestos fibers. The filtrate will be treated in order to regenerate hydrochloric acid. Three ways of regeneration are considered:

• Highly concentrated acid (>85% H ₂ S0 ₄ ) will be added until all cations in solution (Ca ²⁺ , Mg ²⁺ , Al ³⁺ , Fe ³⁺ , Na ⁺ , K ⁺ ) are balanced by sulphate anions and all chlorine will be present as free HC1. Precipitated gypsum will be separated by filtration. The remaining liquor will be distilled. The HC1 distillate will be reused for asbestos leaching, the metal sulphates can be disposed of or further refined.

• Highly concentrated acid (>85% H ₂ S0 ₄ ) will be added until al cations in solution (Ca ²⁺ , Mg ²⁺ , Al ³⁺ , Fe ³⁺ , Na ⁺ , K ⁺ ) are balanced by sulphate anions and all chlorine will be present as free HC1. Precipitated gypsum will be separated by filtration. The remaining liquor will be freeze crystallized. Regenerated HC1 will be decanted and metal sulphates will be centrifuged and/or filtered. The HC1 will be reused for asbestos leaching, the metal sulphates can be disposed of or further refined. Highly concentrated acid (>85% H ₂ S0 ₄ ) will be added until all calcium in solution (Ca ²⁺ ) is balanced by sulphate. Precipitated gypsum will be separated by filtration. The remaining liquor will be pyrolysed. The HC1 condensate will be reused for asbestos leaching, the metal oxides can be disposed of or reused.

Results

Table 1: XRF analysis of solids

AC = asbestos cement, the high calcium ( CaO) content is indicative for the high amount of cement minerals

Cold leaching = asbestos cement with most cement minerals leached and asbestos concentrated. Lower CaO compared to AC is indicative for leaching of cement minerals. Increase of iron (Fe ₂ 0 ₃ ) and magnesium (MgO) is indicative for relative concentration of acid resistant minerals like crocidolite, amosite and chrysotile.

Hot leaching = silica residue remaining after destruction of asbestos. Decrease of iron (Fe ₂ 0 ) and magnesium (MgO) is indicative for leaching of acid resistant minerals like crocidolite, amosite and chrysotile.

The silica concentration increases in each step as in the first step the cement minerals are dissolved. Cement contains high amounts of calcium. In the sequential steps the asbestos minerals are being dissolved and removed from this stream.

Conclusions

1. Destruction of chrysotile asbestos (serpentine type) in a combined mechanical and chemical process, consisting of multiple sequential steps;

2. Destruction of amphibole types of asbestos in a combined mechanical and chemical process, consisting of multiple sequential steps; 3. Destruction of fibers originating from glass wool/rock wool and other isolation material fibers;

4. Neutralization of (waste) acids by the alkaline (Ca, Mg) part of the cementitious asbestos containing material;

5. C0 ₂ savings by using waste cementitious material instead of NaOH or limestone based products to neutralize waste acids;

6. Recovery of hydrochloric acid and production of gypsum by adding sulphuric acid to a calcium-chloride brine;

7. A two-step mechanical size reduction of the asbestos containing cement to improve the neutralization rate of the waste acids. A first stage acid destruction of the alkaline cement matrix in the asbestos containing material in order to release the asbestos fibers. A milling step to induce lattice defects in the asbestos crystal structure. A second stage chemical destruction of the fibers with waste acid. Milling of the end product;

8. Reduction of energy required for grinding by removing the cementitious matrix;

9. New analyzing method based on a combination of SEM, XRF and QEMSCAN.