Process for the Synthesis of a Layered, Clay-Like Material as Well as Its Use

The invention relates to a process for the preparation of a layered, clay-like material using a silicon oxide containing material in a finely divided state as a starting material.

It should be noted that the clay minerals occurring in nature are generated through the erosion of rocks and sediments. Such erosion processes appear to be accellerated under hydrothermal conditions.

Thus, such a hydrothermal process is known from GB-A-1- 560 504, wherein by hydrothermal treatment of igneous rocks, clay-like materials are obtained.

On the other hand, the synthesis of a layered material under hydrothermal conditions is also known. The Derwent abstract AN 91-183100, week 9125 & JP,A,3 112 808, disclose the hydrothermal synthesis of a layered silicate. More specifically, an amorphous silica-containing material, an alkali metal source, water and alcohol are according to these references reacted under hydrothermal conditions at at least 150.degree. C. A source of alumina or of magnesia can further also be present.

Hydrothermal processes nevertheless require special equipments.

Clay materials are responsible for many natural conversion processes, including reactions involving oxidation and hydrolysis. They can bind heavy metals because they have a net negative charge, which is compensated by interchangeable cations. In nature these are mainly the ions of alkaline earth metals.

On the other hand, it is possible to modify clay minerals such that their original affinity will change from hydrophilic to hydrophobic. Because of that the clay becomes suitable for the adsorption of organic compounds which dissolve poorly in water.

At the moment a very important application of clay, whether it has been modified or not, lies then in the field of the immobilization of polluted industrial waste material, in which the waste material is adsorbed by the clay and then isolated by incorporation into a cement matrix.

Due to the growing amount of industrial waste material as well as the requirements, which are becoming more and more strict, concerning the amount of waste material allowed to be discharged into the environment, there is an increasing need for a clay-like material that also has the capacity to absorb different substances that clay inherently possesses.

Surprisingly, a process has now been found for the preparation of a layered, clay-like material in which it is not necessary to appeal to natural sources of clay, nor to appeal to a hydrothermal process.

More specifically the process according to the invention comprises the formation of a layered, clay-like material by reacting a glass phase containing material at a pH from 5 to 8, in the presence of water with a source of layer-forming metal ions, and an activator for the formation of clay, under atmospheric conditions to obtain a material having a stable structure.

It should be noted that the fineness of the various constituents may vary within a wide range of limits. However, it will be obvious that the smaller the size of the particles is, the faster the velocity of conversion will be. Actual practice has shown that a particle size of approximately 100 microns is very suitable.

Preferably aluminium ions are employed as layer-forming metal ions. The source concerned of such ions may in that case consist of aluminum oxide. However, it may also be appropriate to employ magnesium ions as layer-forming metal ions. Advantageously, magnesium oxide is employed then.

Preferably a clay-like material is formed using boron glass or waste glass as starting material. Boron glass has the advantage that in addition to boric oxide (B.sub.2 O.sub.3), both silicon dioxide and aluminum oxide are present, which also form the structural elements from which clay is built up. Waste glass is known in English-speaking countries as cullet. It is employed as a fluidizing agent in the manufacture of glass because it lowers the temperature at which the mixture employed for the manufacture of glass becomes fluid. Although the mechanism underlying the present invention is not entirely clear, it is assumed that it is this property of cullet that makes it possible for the conversion into a stable ordered structure, consequently the clay-like material, to take place relatively quickly.

Preferably the transformation that occurs in the present process is allowed to take place at a pH of approximately 7 and at a temperature of approximately 35.degree.-40.degree. C.

It should be noted that a pH of at least 5, and preferably 7, is fairly essential in the present process. The fact is that at a lower pH of, for example, 1 to 3 a silicate employed as a material containing silicon oxide will be converted into a silicic acid polymer, such as is disclosed in U.S. Pat. No. 4,404,105, from which it is no longer possible to form a material having a stable ordered structure with the characteristics of clay.

The application of a temperature of 30.degree.-45.degree. C. is favourable for obtaining a fairly rapid conversion. Although this range is not very critical, it will be obvious that at a lower temperature the reaction velocity will also be lower, while at a temperature above 45.degree. C. the reaction velocity will indeed have increased, but that this can be such that it will become impossible to obtain an ordered structure that is present throughout the whole final product due to the formation of impermeable films on the various reagents.

Furthermore, an activator is employed preferably for the formation of clay in order to accelerate the conversion process for the formation of a material with a stable ordered structure. Transition metal compounds such as magnesium sulphate, ferrous sulphate and manganese oxide have proven to be very suitable for employment as an activator.

A clay modifier is further added to the mixture of starting substances to be converted. Preferably a calcium compound is employed as a clay modifier.

An embodiment of the process according to the invention that particularly deserves preference includes the employment of a hearth ash containing a glass fraction, obtained through the incineration of household or industrial waste, for the formation of a clay-like material with a stable ordered structure.

It should be noted that the composition of such a hearth ash depends on the waste material supplied and the process conditions implemented, and consequently is inconstant. At present this hearth ash is mainly employed in fills, in road foundations and as an aggregate in concrete. There is no application for the remainder (approximately 150,000 tons a year) as a result of which it has to be dumped. The problem with hearth ash is that it contains heavy metals, which upon leaching pollute the environment in an extremely undesirable way.

Surprisingly, it has now been found that hearth ash can be employed as a raw material for the formation of a layered, clay-like material by utilizing the glass fraction present in hearth ash. The metallic elements of hearth ash prove to be incorporated or absorbed into the clay matrix. Thus a fixation of the hazardous metallic elements is obtained. By subsequently incorporating the formed, layered, clay-like material into a cement matrix, a building material is obtained in which the various metallic elements are no longer perceptible in the leaching tests carried out.

Furthermore, the invention relates to a layered, clay-like material obtained by having waste, which is polluted with inorganic compounds and subjected to incineration, in a pulverized state at a mainly neutral pH, a temperature of 30.degree.-45.degree. C. and in the presence of water is reacted with a pulverized, mainly amorphous, silicon oxide source.

The amorphous silicon oxide source present in this may be a glass fraction that is already present in the material, which has been subjected to incineration. In that case, a very valuable product is obtained from waste material.

If, however, the waste material which has been subjected to incineration has an insufficient quantity of a silicon oxide source, boron glass or cullet, for example, may be added so that the formation of the clay matrix can still take place.

Finally, the invention relates to a process for immobilizing waste material polluted with inorganic material, whether subjected to incineration or not, by incorporating the inorganic material into a layered, clay-like material obtained by applying the process according to the invention.

So-called AVI (waste processing industry) silicate slags that were finely ground to a particle size of mainly about 100 .mu.m were used as a starting material. AVI silicate slags are generated by burning household waste and sewage sludge and contain heavy metals as pollutants. Furthermore, these slags contain glass beads in which a part of the heavy metals has already been fixed.

A quantity of aluminum sulphate was added to the finely ground slag material and the pH of the mixture was brought up to a value of approximately 7. Furthermore, as an activator for the formation of clay, magnesium sulphate was added. The mixture thus obtained was brought up to a temperature of approximately 40.degree. C. and brought into contact with water, in the course of which the mixture was continuously turned over.

After approximately 1 month the starting material had converted into a layered, clay-like material.

In this example, cullet that had been finely ground to a particle size of approximately 80 .mu.m was used as a starting material. Analysis of the cullet employed showed that the material contained approximately 3% Al.sub.2 O.sub.3.

An equivalent quantity of manganese oxide was added to the finely ground cullet as well as just enough water so that the particles almost began to adhere to each other. The pH of the mixture formed was brought up to approximately 6.

By continuously turning the mixture over at a temperature of approximately 32.degree. C. a layered, clay-like material was formed, in which the conversion was complete after approximately 5 weeks.

Adding ferrous sulphate to the mixture to be turned over resulted in completion of the conversion in approximately 3 weeks.

The clay-like material obtained had the capacity to absorb various metal salts present in an aqueous solution.

The procedure described in Example I was repeated. The clay-like material formed was incorporated into a cement matrix of blast furnace slag cement according to the manner known per se. From the product obtained after 28 days of hardening, not a single heavy metallic element could be detected in leaching tests.