Description

The present invention relates to a pellet comprising a consolidated dry mixture that comprises granular material and bentonite. The present invention further relates to a method for manufacturing a pellet of this kind, as well as to the application of a pellet of this kind.

British publication GB 1 ,439,734 relates to a soil sealant composition that is obtained by the dry mixing of bentonite, a water-soluble polymer, and a water- soluble dispersant, after which a dry, granular, or powder-like composition is formed. This dry composition can easily be introduced into the soil and then mixed with the soil. The mixture of soil and soil sealant composition obtained is mixed intimately and after contact with water, hydration of the bentonite occurs, resulting in swelling. The voids in the soil mixed with soil sealant composition will become filled, so that a layer impermeable to water will be formed.

Mixtures of sand and bentonite in a loose granular form or in the form of a slurry are known per se. International application WO94/18284 relates to a claycontaining mixture that comprises a pulverulent or ground smectite and/or a smectite-containing natural rock and 1-10 wt% of a water-soluble polymer, for forming a moisture-resistant gel.

Bentonite is a strongly swelling type of clay. This clay formed about 70 million years ago during the Cretaceous and the Tertiary, from volcanic ash, which in a damp climate was deposited in depressions without outlet. Under the effect of water, the ash was hydrolysed to Si and Al (hydr)oxides with colloidal properties. These deposits occur in various places throughout the world, but not in the same composition everywhere. Bentonite consists in principle of the clay mineral montmorillonite.

Wyoming bentonite largely contains monovalent (Na ⁺ ) ions, so that it has high swelling capacity. European or Indian bentonite has a much higher proportion of divalent (Ca ²⁺ ) ions, so that it has a smaller swelling capacity. Activated (calcium) bentonite that has undergone Na treatment, wherein some of the (Ca ²⁺ ) ions have been replaced with Na+, has a swelling capacity that is comparable to that of Wyoming bentonite. For forming sealing layers, bentonite is added to moist soil and mixed. The relatively small bentonite particles are then located between the sand grains in the pores of the sand skeleton. When this system takes up water, the bentonite will swell, so that the pores of the sand skeleton become filled, and in this way the permeability of the soil is reduced.

In the East of the Netherlands, parts of the Twentekanalen (Twente canals) are located above the groundwater level and it is known that the natural subsoil consists of (water-) permeable material. In these parts there is therefore a risk that the canal will leak, both to the subsoil and the environment (seepage), as a result of dredging of an impermeable layer at the bottom of the canal.

In order to manage this risk, Rijkswaterstaat (the Directorate-General for Public Works and Water Management) has conducted experiments with a so- called "Sand-Bentonite mixture" (SBM), said mixture to be regarded as a slurry, and thus a liquid form. SBM is applied by means of a hose, with which the mixture is spread on the bottom of the canal.

A possible drawback of this method is the loss of fine material owing to the flow of water and segregation of the material during application. Furthermore, the SBM must first be reduced to a liquid form before it can be applied on the bottom of the canal. A layer made with this liquid form has low mechanical stability. There is therefore a risk of sagging of the constituents in the SBM (especially possible on slopes), with possible cracking as a consequence. Moreover, application itself is difficult with pumping using a hose system, which may lead to blockage problems.

An object of the present invention is to prevent or greatly reduce the aforementioned problems that occur with mixtures in the form of a slurry.

Another object of the present invention is to provide a mixture that can be used for sealing, said mixture being easy to produce and store before the actual application thereof takes place, and is thus ready to be used immediately.

The present invention thus relates to a pellet of a consolidated dry mixture that comprises granular material and bentonite, and possibly one or more additives.

"Consolidated" is to be understood to mean that the mixture has a solid consistency, and thus does not have a liquid or slurry form, but a solid form, wherein the dry mixture of solid constituents has the form of a pellet. "Consolidated" is further to be understood to mean that the mixture is already consolidated beforehand, namely before the application thereof as sealing material takes place. These pellets are easy to transport and store, they retain their shape and stability and are ready to be used immediately. The present pellets are obtained by exerting pressure on the starting materials, for example by pressing, extruding, stamping, and rolling, or a combination thereof. An extruder or pelletizer may be mentioned as an example of a device for making pellets. It is also possible to obtain a consolidated mixture by means of a device in which rolls rotate against each other, and this is then comminuted further to the desired size.

By exerting pressure on the aforementioned starting materials, a particular form is obtained, namely the pellet. In a pellet, the aforementioned starting materials, namely granular material, bentonite and possibly one or more additives, have a mutual cohesion or consistency. Owing to their solid form, because consolidation has already taken place, the pellets can be transported and packed in bags and the like, for example such as in so-called big bags, more easily than a loose mixture, and are stored and transported to the location where the pellets are to be applied. The solid, consolidated form then ensures that they sink more easily in water and also that the layer thus formed does not need to be compacted.

The term "pellet" used here is to be understood as not only pellets, but also bars and granules, and blocks, for example in the form of a brick. Pellets are thus used to endow certain materials, particularly the aforementioned starting materials, namely granular material, bentonite and possibly one or more additives, with a form that can be handled and metered.

According to an example, the pellet is obtained by exerting pressure on granular material, bentonite and possibly one or more additives so as to obtain the pellet, wherein the aforementioned step of consolidation is selected from the group of pressing, extruding, stamping, and rolling, or a combination thereof.

In an example of the present pellet, the moisture content of the pellet is at least 0.75 wt%, preferably at least 2 wt%, and in particular preferably at least 3 wt% and at most 25 wt%, preferably at most 15 wt% and especially preferably at most 10 wt%, calculated on the basis of the total weight of the pellet. If the moisture content is too high, the pellets will adhere to one another. A moisture content that is too high will also cause problems during storage of pellets, particularly with respect to premature and undesirable swelling of the pellets. A moisture content of the pellets that is too low will lead to an unstable pellet, with the result that the pellet will disintegrate prematurely, so that the processability of the pellet is adversely affected. In an example of a method for manufacturing a pellet by extrusion, a moisture content in the range 2-5 wt% is desirable. In an example of a method for manufacturing a pellet by rolling, a moisture content in the range 4-10 wt% is desirable. The difference in moisture content of the final pellet is probably caused by a temperature rise in the extruder, so that the moisture content of the extruded material will decrease. A way of measuring moisture content that is known by a person skilled in the art employs a so-called KERN DBS 60-3 Moisture Analyzer. It is also possible to determine the moisture content by drying the sample for 24 hours at a temperature of 105°C. In the present description, a moisture content for example of 5 wt% signifies a dry weight of 95 g, and 5 g of moisture.

Sand, for example, may be mentioned as granular material. Other examples of granular material are gravel, stone chippings, crushed rock, secondary building materials, such as broken rubble, concrete and glass, cleaned soil and bottom ash, or a combination thereof. It should be noted that granular material has a certain moisture content naturally, for example 1-20 wt%. Bentonite also possesses a certain moisture content naturally, for example 5-25 wt%. The term "dry" should thus be understood to mean that the starting materials are used as such, so that there will be some moisture content in the consolidated mixture naturally. In an example of a mixture of starting materials yet to be compacted, namely granular material, bentonite, and possibly one or more additives, the moisture content of the loose mixture was 7-9 wt%, calculated on the basis of the total weight of the loose mixture.

According to an example of the consolidated mixture, the aforementioned one or more additives are selected from the group of adhesives, activating agents and polymers, or a combination thereof.

According to an example of the consolidated dry mixture, the polymers are selected from the group of water-soluble polymers with a molecular weight of more than 900 000, in particular a molecular weight from 2 000 000 to 12 000 000. These polymers provide strengthening of the network that arises after swelling of the bentonite.

An acrylate polymer in primary form may be mentioned as an example of a water-soluble polymer.

To obtain a pellet in which the different constituents display improved mutual adhesion, in a certain embodiment the use of one or more adhesives is desirable, particularly adhesives selected from the group of lignin, fats and oils, or a combination thereof. The aforementioned adhesives are environmentally friendly and have no adverse impact on the environment in which the pellet is used as a consolidated dry mixture.

Alkali metal carbonate and/or an alkali metal phosphate may be mentioned as examples of activating agents. These activating agents ensure improved swelling and network formation.

According to an example of the consolidated mixture, granular material (for example sand) of type D50 in the range 0.05-2.0 mm is used. A granular material of this type is particularly desirable from the viewpoint of the strength of the final network.

According to an example, a pellet comprises an amount of granular material in the range 50-99 wt%, an amount of bentonite in the range 1-50 wt% and an amount of one or more additives in the range 0-5 wt%, wherein all percentages by weight are based on the total weight of the pellet, and the sum total of the separate constituents is always 100 wt%.

According to an example of the consolidated dry mixture, the pellet density is 1200-2500 kg/m ³ , in particular 1600-2200 kg/m ³ . Such a value is especially favourable from the viewpoint of the settling rate. With such a value, the likelihood of washout as a result of water flow is also greatly reduced. According to an example, the density of dried pellets is in the range 1800-2200 kg/m ³ .

According to an example of the consolidated dry mixture, a particle of the consolidated mixture is to be regarded as a particle with an arbitrary shape, for example an elongated or spherical general appearance whose length L is in the range 0.2-25 cm and the diameter D or width B in the range 0.2-25 cm. Examples of this are for example pellets, bars and granules, but also blocks, for example in the shape of a brick.

The present invention also relates to a method for manufacturing a pellet as discussed in the foregoing, wherein the method comprises the following steps: a) providing granular material and bentonite in dry form, b) mixing granular material and bentonite from step a) to form a mixture, and c) compacting the mixture from step b) so that pellets of the thus consolidated mixture are obtained. According to an example of the method for manufacturing a pellet as discussed above, step c) is carried out according to one or more of pressing, extruding, stamping, and rolling, or any other means of exerting pressure or by means of a pelletizer. According to a certain embodiment step c) is repeated a number of times, meaning that the consolidated mixture is compacted again. It is also possible, by means of a device in which rolls are rotated against each other, to obtain a consolidated mixture that is then comminuted further to the desired size. Using such a method, the inventors assume that the moisture content of the starting material will decrease less, wherein pellets are obtained with a moisture content for example in the range 5-7 wt%, calculated on the basis of the total weight of the pellet.

It should be noted that granular material has a certain moisture content naturally, for example 1-20 wt%. Bentonite also has a certain moisture content naturally, for example 5-25 wt%. The term "providing granular material and bentonite in dry form" is thus to be understood as meaning that the starting materials are used as such, so that there will naturally be some moisture content, so that mutual bonding between the starting materials in step c) may occur. A way of measuring moisture content that is known by a person skilled in the art is by means of a so- called KERN DBS 60-3 Moisture Analyzer. It is also possible to determine the moisture content by drying the sample for 24 hours at a temperature of 105°C.

In an embodiment of step b), it is also possible that a minimum amount of water is added to bring about mutual bonding between granular material and bentonite. Thus, it is possible that in step b), optionally addition of extra water is carried out if the starting materials contain insufficient moisture for bonding of the dry constituents during consolidation to the pellet form. It should be noted that in step b), extra addition of water does not take place in such a way that a slurry is obtained. Now, in such a situation, step c) could not be carried out.

According to an example, step c) is carried out as an extrusion step, but step c) may also be carried out batchwise in a mould. In an extrusion step, for example the diameter of the consolidated dry mixture finally obtained can be influenced by the choice of the die. After the extrusion step, an additional step of comminution may take place, if it is desirable.

According to an example, in step b) one or more additives, which are also to be understood as meaning water, are incorporated, as discussed above. The present invention also relates to the application of a consolidated dry mixture as discussed above, or a consolidated dry mixture obtained by a method as discussed above, as a layer impermeable (difficult for water to permeable) to water in an aqueous environment.

An example of such an application is sealing the bottom of a river, canal, or stream. A quantity of pellets is introduced into the water and spread on the existing riverbed. It is possible to provide the layer thus applied with a layer of ballast so that the layer of pellets applied is embedded between the existing bed and the layer of ballast. In certain embodiments, said layer of ballast may be omitted.

It is also possible to use the consolidated dry mixture as discussed above, or a consolidated dry mixture obtained by a method as discussed above, as a vertical seal, for example by milling in the bottom by means of a modified chain digger or by producing a trench first, in for example sheet piling, by filling the resultant trench with the consolidated mixture that comprises granular material and bentonite, and possibly one or more additives.

The present invention also relates to a civil engineering structure, comprising a sublayer, and upon that, an intermediate layer of a consolidated mixture as discussed above, or a consolidated dry mixture obtained by a method as discussed above, and optionally a layer of ballast, wherein the aforementioned structure forms a layer impermeable to water in an aqueous environment. According to an example, said intermediate layer has a thickness in the range 0.04-1 m, preferably 0.1-0.5 m.

In order to exert an influence on the degree of water permeability of the intermediate layer, according to an example it is possible that the aforementioned intermediate layer additionally comprises one or more granular inert materials, selected from the group of sand, gravel, stone chippings, crushed rock, secondary building materials, such as broken rubble, concrete and glass, cleaned soil and bottom ash, or a combination thereof.

The present invention will be explained in more detail hereunder on the basis of a number of examples, wherein it should be noted that the invention is in no case limited to said examples. Moreover, the figures merely serve for explanation of the present invention.

Fig. 1 shows an example of a consolidated dry mixture according to the present invention, obtained after extrusion. Fig. 2 shows an example of a layer packet, wherein the intermediate layer comprises the consolidated dry mixture according to Fig. 1.

Fig. 3 shows an example of the layer packet according to Fig. 2 after some time has passed.

Example 1

850 kg of granular material (type D50= 0.05-2.0 mm) and 150 kg of bentonite were mixed and fed to an extruder. The resultant mixture was extruded through a die and was produced as pellets with a solid form, in particular a length between 0.5-3 cm and a diameter of about 0.5 cm. The moisture content of the pellets finally obtained was 2.5 wt%.

Example 2

The same starting materials as in Example 1 were used, except that 849 kg of granular material, 150 kg of bentonite and 1 kg of acrylate polymer in primary form were used. The resultant mixture was extruded through a die and was produced as pellets with a solid form, in particular a length between 0.5-3 cm and a diameter of about 0.5 cm. The moisture content of the pellets finally obtained was 2.3 wt%.

Example 3

The same starting materials as in Example 1 were used, except that 840 kg of granular material, 150 kg of bentonite and 10 kg of hydrogenated palm oil were used. The resultant mixture was extruded through a die and was produced as pellets with a solid form, in particular a length between 0.5-3 cm and a diameter of about 0.5 cm. The moisture content of the pellets finally obtained was 2.1 wt%.

Example 4

The same starting materials as in Example 2 were used, except that 840 kg of granular material, 150 kg of bentonite, 1 kg of acrylate polymer in primary form and 9 kg of sodium phosphate were used. The resultant mixture was extruded through a die and was produced as pellets with a solid form, in particular a length between 0.5-3 cm and a diameter of about 0.5 cm. The moisture content of the pellets finally obtained was 2.2 wt%.

The pellets in dry solid form according to Examples 1-4 were applied on the bottom of a canal in a thickness of 10 cm. On top of the layer thus applied, a ballast layer consisting of gravel was applied, with a thickness of 15 cm. After a week, the layers thus applied were inspected visually and a bonded network was clearly visible.

Fig. 1 relates to the consolidated dry mixture that is obtained after extrusion of the mixture according to example 2. The consolidated dry mixture is to be regarded as separate pellets with a length of about 1-2 cm and a diameter of about 0.5 cm.

Fig. 2 shows an example of a layer packet wherein the intermediate layer comprises the consolidated dry mixture according to Fig. 1. The intermediate layer is thus assembled from pellets wherein it can clearly be seen that as a result of the size of the pellets there are considerable voids.

Fig. 3 shows the layer packet according to Fig. 2 after some time has passed, wherein it can clearly be seen that the pellets have swollen so that the voids have been filled. Owing to the swelling of the pellets, the intermediate layer forms a layer that is impermeable to water.