Description

The present invention relates to methods for processing cement comprising construction or building materials such as panels, pipes, corrugated roofing sheets, flat sheets, roofing tiles and water tubes and especially to methods for processing cement comprising construction or building materials in combination with, or containing fibers, cellulose fibers asbestos fibers, glass wool, glass fibers, rockwool or rockwool fibers whereby the processing results in complete destructions of both bound and unbound fibers.

Cement is a substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement can be used in combinations such as in combination with sand or gravel. Cement mixed with fine aggregate is used to produce mortar, or with sand and gravel, to produce concrete. Asbestos cement, also designated as eternit, fibro or fibrolite, is a building material in which asbestos fibers are used to reinforce rigid construction or building materials such as cement sheets, panels or pipes.

The term asbestos is generally used to designate 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 (Mg3Si2Os(OH)4), crocidolite predominantly being an iron, magnesium, sodium silicate (Na2((Fe ²⁺ , Mg ²⁺ )3Fe ³⁺ 2)Si ₈ O22(OH)2 and amosite being an iron silicate (Fe ₇ Si ₈ O22(OH)2).

When broken or mechanically bruised, asbestos finer bundles can release microscopic small fibers of less than 0.5pm thickness. When inhaled, these fibrils are 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 5 pm to 100 pm fiber fraction range that is of most concern. Additionally, the ratio between the length and thickness is of importance. Upon weathering, asbestos cement 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, affordable, 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.

As alternatives for asbestos cement, other reinforcing fibers such as glass, mineral and cellulose fibers are used in combination with cement as construction or building materials. Rock wool, glass wool and mineral wool are also commonly used in construction and building materials. However, all these materials suffer to a certain extent from similar hazardous properties as asbestos.

Accordingly, there is a need in the art for new processes for effectively processing construction or building materials and especially fibers such as asbestos, glass fiber, rock wool, glass wool and/or mineral wool comprising materials or, formulated differently, there is a need in the art for processes to recycle fiber comprising construction or building 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 processing cement comprising construction or building materials, the methods comprise the steps of: a) suspending cement comprising construction or building materials with a particle size of less than 1.5 mm in water wherein the mass ratio cement comprising construction or building materials to water is from 1:1 to 1:50 and subsequently mechanically reducing the particle size of the slurry to less than 100 pm; b) heating the slurry obtained in step (a) to a temperature between 60°C to 99°C and, simultaneously, or sequentially, mechanically reducing the particle size of the slurry to less than 10 pm.

The present inventors have surprisingly discovered that the above methods suffice to convert, or recycle, cement comprising construction or building materials. For example, the slurry obtained after step (b) can be further processed into calcium carbonate (chalk) and/or calcium sulphate (gypsum) from the liquid phase obtained after step (b) and silica comprising material from the semi solid phase obtained after step (b) can be processed into geopolymers. A liquid phase and a semi-solid phase can be readily obtained by phase separation, or sedimentation, of the slurry of step (b). It is noted that according to the present invention, no pH increasing agent such as KOH and NaOH are required to be added in steps (a) and (b).

The present methods are especially suitable to process, or recycle, construction or building materials comprising, or in combination with, fibers, cellulose fibers, silica containing fibers such as asbestos, glass wool, glass, or rock wool since the process allows complete destruction of these fibers.

The term “fiber” according to the present invention denotes particles with a length/thickness ratio of higher than 3.

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 10 pm, preferably 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 or elongated particles.

Within the context of the present invention, the terms “fiber length” and “particle size” are used interchangeably to designate the longest dimension of an (elongated) particle or fiber. Thus, a particle size of less than 100 pm can similarly be used to designate a fiber with a length of less than 100 pm.

The present methods are especially suitable for processing cementitious asbestos, glass wool or rock wool, preferably asbestos, comprising materials generally used as construction materials, panels, pipes, corrugated roofing sheets, flat sheets, roofing tiles and water tubes.

According to an especially preferred embodiment, the present methods are used to process, or recycle, construction or building materials comprising fibers of chrysotile asbestos, crocidolite asbestos, amosite asbestos, tremolite asbestos, actinolite asbestos, or anthophyllite asbestos.

According to the present invention, mechanically reducing the particle size in step

(a) results in particles with an average size distribution D95 between 10 pm to 80 pm, preferably a D95 of 20 pm to 50 pm. Dxx denotes that the portion of particles with a length smaller than this value is xx%, for example D95 denotes that the portion of particles with a length below this value is 95%. As noted, the length of the present particles relates to the longest dimension of the particles.

According to the present invention, mechanically reducing the particle size in step

(b) results in particles with an average size distribution D95 between 0.5 pm to 5 pm, preferably a D95 of 1 pm to 3 pm, more preferably a D95 of 1 pm to 2 pm. Again, Dxx denotes that the portion of particles with a length smaller than this value is xx%, for example D95 denotes that the portion of particles with a length below this value is 95%. As noted, the length of the present particles relates to the longest dimension of the particles. In the present processing or recycling methods, step (b) can, but not necessary, further comprise the addition of one or more compounds selected from the group consisting of pH modulating compounds, metal precipitating or binding compounds and carbonate bound alkali or earth alkali metals. Generally, these compounds are added to remove impurities or other unwanted compounds in the final product or to optimize the present methods.

For example, metal precipitating or binding compounds such as sulphur, citrates or chelates can be used to remove (heavy) metals from the slurry or final product.

According to the present invention, after step (b), the slurry can be subjected to phase separation, or sedimentation, and/or further processing yielding, for example, calcium carbonate (chalk) and/or calcium sulphate (gypsum) and silica bound material which, for example, can be used for geopolymers.

Present step (a) preferably comprises mechanically reducing the particle size or length of the slurry to less than 100 pm and is performed in, for example, a ball mill for 0.5 to 30 hours, preferably 2 to 30 hours and/or present step (b) preferably comprises mechanically reducing the particle size or length of the slurry to less than 10 pm and is performed, for example, in a bead mill for 0.5 to 10 hours such as 2 to 10 hours.

According to an especially preferred embodiment, the present methods comprise the steps of: al) mechanically reducing cement comprising construction or building materials to pieces of 5 to 20 cm; a2) mechanically reducing the pieces of step (al) to a powder with a particle size of less than 1.5 mm; a3) suspending the powder of step (a2) in water, wherein the mass ratio cement comprising construction or building materials to water is from 1 : 1 to 1:50; a4) mechanically reducing the particle size of the slurry of step (a3) to less than 100 pm; bl) heating the slurry obtained in step (a4) to a temperature between 60°C to 99°C during 1 to 5 hours; b2) mechanically reducing the particle size of the slurry of step (bl) to less than 10 pm.

The present methods as detailed above are especially suitable for destruction of hazardous silica containing fibers selected from the group consisting of asbestos, glass wool, rock wool, chrysotile asbestos, crocidolite asbestos, amosite asbestos, tremolite asbestos, actinolite asbestos, and anthophyllite asbestos, preferably chrysotile asbestos and/or crocidolite asbestos. The present invention will be further detailed in an example. In the example, reference is made to the appended figures wherein:

Figure 1: shows an SEM (Scanning Electron Microscopy) image of asbestos fibers in a cement matrix. Top image is chrysotile asbestos and bottom image is crocidolite asbestos;

Figure 2: shows particle size or length distribution after present step (a);

Figure 3: shows an SEM image of fibers after present step (a). Top image is chrysotile asbestos (magnification 3700x) and bottom image is crocidolite asbestos (magnification 3000x);

Figure 4: shows particle size or length distribution after heating in present step (b);

Figure 5: shows an SEM image after heating in present step (b). Top image is chrysotile asbestos (magnification 3500x) and bottom image is crocidolite asbestos (1900x);

Figure 6: shows particle size or length distribution after present step (b);

Figure 7: shows SEM images after present step (b). Top image after 60 minutes mechanical size reduction (magnification 3300x); middle image after 180 minutes mechanical size reduction (magnification 3000x); and bottom image after 300 minutes mechanical size reduction (magnification lOOOx).

Example

Method and Results

Pretreatment

Asbestos cement roofing sheets (AC) were mechanically broken into pieces of about 10 cm. These pieces were further broken in a shredder and the broken material was ground into a powder in a pen mill.

The AC used was analyzed using Scanning Electron Microscope - Energy Dispersive X-ray spectroscopy (SEM-EDX) - on asbestos fibers (EDX spectrum not shown). Both chrysotile and crocidolite were found (10-15% chrysotile, 2-5% crocidolite, bounded, Figure 1). In Chrysotile, the woolly curved morphology was visible, in crocidolite straight crystals with a lot of fission. In the EDX spectrum (not shown) of chrysotile it was observed that the Mg peak was similar in size to the Si peak which is typical for chrysotile. In the EDX spectrum of crocidolite (not shown), an Fe peak and Na peak (at 1.0 keV) were observed.

Step (a) Powder material from the pen mill (33 kg) was collected in a reactor vessel and mixed with water (88 kg) and thereafter the mixture was homogenized. The slurry obtained had a pH of 12.21. Subsequently, the slurry was pumped to a ball mill (300L, 30 rpm, 40% filled with aluminum oxide balls) and ground for 20 hours.

After grinding, the particle size was measured (laser diffraction). The particle size distribution observed is shown in Figure 2 and Table2. Almost all material is smaller than 100 pm. 90% of the material was smaller than 25 pm.

The pH and dry residues were determined before and after grinding (Table 1). The pH had increased to 12.6. This is slightly higher than the expected pH of 12.3 of a saturated calcium hydroxide solution. The dry residue was slightly decreased which was probably because some water had been added to pump the slurry.

Table 1: pH and dry residue before and after step (a).

To analyze the mixture after step (a), an SEM-EDX measurement (EDX spectrum not shown) and particle analysis were performed and numerous fibers were observed in the material (Figure 2 and Figure 3 and Table 2 below).

Table 2: average particle size (length) distribution after step (a) To determine whether material is dissolved, a sample of the slurry was filtered (0.45 pm filter) and analyzed with ICP (Inductively Coupled Plasma atomic emission spectroscopy). Only calcium was found in significant amounts (0.21%). With the ICP method, no silicon can be determined, although this is an important component of the matrix. Furthermore, it is striking that there was no iron and magnesium in solution, while this is amply present in the matrix and in the asbestos fibers. This can be due to the relatively high pH of the slurry causing iron and magnesium hydroxides to be insoluble.

Step (b)

The slurry from step (a) was pumped into a reactor. After adding additional water (68 kg), the mixture was homogenized. The mixture was then heated and stirred for 2 hours at - 90°C and cooled overnight to 53 °C.

The particle size distribution after the reaction step was determined (Table 4, Figure 4) and the particle size was decreased as compared to step (a).

The pH and dry residue were also determined (Table 3) and the pH remained stable around 12.5. To determine whether material is dissolved, a sample of the slurry was filtered (0.45 pm filter) and analyzed with ICP. Only calcium was found in significant amounts (0.26 - 0.27%).

Table 3: pH and dry residue before and after step (a).

To characterize the heated mixture, an SEM-EDX (EDX spectrum not shown) measurement and particle analysis were performed (Figure 5). Fibers can still be observed. However, bundles appear to be split into more loose fibers.

Table 4: average particle size (length) distribution after heating in step (b)

Subsequently, the slurry was ground in a bead mill for 5 hours. Samples were taken during grinding and analyzed to assess asbestos degradation.

The particle size (Table 5 and Table 6 and Figure 6) decreased sharply during the first 60 minutes. From 180 minutes, no significant change in particle size can be seen. The pH remained constant at 12.6 during grinding, the dry matter content was also stable (Table 5). Table 5: particle size and other observations during grinding a bead mill Table 6: average particle size (length) distribution after step (b)

The samples were viewed with a light microscope (lOOOx magnification). In the first sample, many fibers were visible. After 120 minutes of grinding, it became difficult to see any fibers. In the 240- and 300-minute samples, no fibers were visible.

The solutes were analyzed with ICP. Only calcium was found in significant amounts (0.25 - 0.28%).

In the analysis with SEM-EDX (EDX spectrum not shown) after 0, 30, 60 minutes, it was observed that the fibers became smaller and fractured. An example of crocidolite can be seen in Figure 7 (see also Table 5). In the analysis after 120 minutes, structures can be seen that look like fibers, but these could no longer be characterized as asbestos. After 180, 240 and 300 minutes of grinding, no asbestos fiber can be observed.

An EDX analysis of a point in the amorphous matrix was performed (EDX spectrum not shown). This is the same for the samples of 180, 240 and 300 minutes. In this EDX analysis, the elements can be found from both the cement (Si, Ca, Al) and the elements that make up chrysotile (Si, Mg) and crocidolite (Si, Fe). The material obtained is a thick beige-gray slurry. The density is 1,08 g/L.

Conclusion

Asbestos-containing roofing sheets were successfully reduced and treated into an asbestos-free material. This was done by breaking the roofing sheets and grinding the powder obtained in water. Then, the slurry was heated to 90°C and ground again resulting in complete breakdown of asbestos fibers.