NL9400684A - Method for treating contaminated dredgings and/or soil to obtain building materials - Google Patents

Abstract

It is an object of the method according to the present invention to produce useful products from contaminated dredgings and soil by means of a combination of separation and purification techniques which focuses on melting and crystallization of the residues. In the process, the remaining clay and organic material are used to produce a synthetic basalt which is comparable with natural basalt in terms of possible uses and qualities. After separation and cleaning, the bulk of the dredgings and/or soil can be reused as raw material in the construction industry. The organic pollutants, concentrated in the residues, are destroyed in the thermal process, and the heavy metals are immobilized. <IMAGE>

Description

Method for the treatment of contaminated dredged material and / or soil for obtaining building materials

The invention relates to a method for the treatment of contaminated dredged material and / or soil for obtaining building materials.

Large amounts of contaminated soil and dredging sludge are released annually during earthworks and maintenance work on waterways. In addition, highly contaminated soil and dredging sludge are released from locations that are cleaned up for environmental reasons. Current techniques allow only part of these materials to be cleaned at an acceptable cost.

Most cleaning methods for contaminated (water) soils are often unsuitable, or become expensive at a high clay and / or organic matter content due to the fact that the contamination is both inorganic and organic in nature, and consists of a large number of different components. exists. The alternative - dumping of these materials - also comes up against a number of drawbacks, in particular the following: 1. Increasingly stringent requirements are imposed on the environmental hygiene properties of waste materials, such as toxicity and leaching behavior, before permission is granted to dispose of them. apply or deposit in a controlled manner; 2. The construction and management costs of depots in which contaminated dredging sludge, soil materials and other highly contaminated waste streams may also increase sharply; 3. The lack of space in the Netherlands and in many areas in Europe, in combination with the high resistance of local residents to landfills, makes the choice of location for permanent disposal of highly contaminated waste problematic.

All this has led to a sharp increase in the landfill costs of the aforementioned bulk waste. Furthermore, the Dutch government has set a policy objective that a significant part of the dredging sludge and soil must be recovered, whether or not after cleaning. Landfilling of organic waste will also be prohibited in the future.

In summary, the invention to be described herein emphatically responds to the following trends in modern environmental policy: 1. The wish that more waste materials should be put to good use in order to limit the use of scarce raw materials; 2. The increase in the landfill costs of contaminated materials; 3. The strict environmental hygiene requirements for reuse and landfill of waste; 4. The possibilities offered in current environmental legislation for the recovery of waste after immobilization of the heavy metals present therein.

The object of the invention is now to provide such a method that after treatment of the contaminated dredging sludge and / or soil, as much of the material obtained as possible can find useful application in construction, and thus meets the physical and environmental hygiene requirements imposed on it.

The method according to the invention is characterized in that the dredging sludge and / or soil is / are separated, by techniques known per se, into relatively clean partial flows of sand and loam and a relatively small residual stream of highly contaminated sludge, which residual stream is subjected to a controlled oxidation, melting / crystallization process to produce crystalline basaltic material.

A large number of building materials can be produced using the method according to the invention. As a result, the amount of material to be dumped can be greatly reduced, and the use of natural raw materials such as sand, gravel and basalt can be reduced.

In the method according to the invention, the first step may consist of sieving the coarse materials from the dredged material and the soil. This coarse material usually consists of construction rubble and waste. After any separation into metals and debris, these materials can be reused or landfilled. By adding chemicals to the dredged material, de-agglomeration and thus the effectiveness of its separation and cleaning can be promoted. De-agglomeration can be enhanced by high-pressure slurry techniques.

By means of separation and cleaning techniques it is possible to concentrate contaminants in a highly contaminated residual stream. Such techniques include hydrocyclonage, gravitation separation, and flotation, optionally in combination with extraction techniques. The sandy and loamy, relatively clean residual flows that are released when these techniques are used, can possibly be used directly as fill material or in concrete and asphalt production. Partial flows of cleaned loamy material may also be used as raw material in the coarse ceramic industry.

The application of separation and flotation techniques to dredge spoil and contaminated soil is described, inter alia, in European patent no. 9001210. The described cleaning technique produces a highly contaminated residue consisting of organic material from the dredge spoil or soil, the very fine sludge and clay from the dredged material or soil and the impurities that have been washed out from coarser fractions. From an environmental point of view, it concerns a very highly contaminated residue, which must be dumped or incinerated at a high cost.

The core of the method consists of a new application of the melt / crystallization technology, which addresses the problem of the residual flow of highly contaminated clay and organic matter, which is released in the said separation and cleaning techniques, from a new vision. It has been found that by not only attuning the said separation techniques to the desire to obtain clean fractions, but also attuning to the desired composition of the contaminated residue, a coupling can be made using a melt / crystallization technique. The organic matter and lutum-rich, highly contaminated residues of the various separation and cleaning techniques are converted into a basalt-like product by means of a special form of melt / crystallization technique which has been further developed for this application.

Melting and crystallization of residues is a known technique, as is apparent, inter alia, from European patent no. 0033133. In this invention, a mixture is composed of different waste materials based on the content of SiO 2, Al 2 O 3, Fe, Fe 2 O 3, alkali metals and alkaline earth metals, after which this mixture is degassed, melted and optionally crystallized. When melting according to this process, part of the iron is separated by carbothermal reduction. Issues such as heavy metal immobilization and environmental hygiene aspects do not play an important role in said patent.

It has been found that dredged material and soil are not simply suitable for melting in the most common furnaces. The use of special furnaces and extreme process parameters would be necessary for this. Due in particular to the high SiO2 content (including quartz sand) of dredging spoil or soil, problems arise with the too high viscosity and adverse chemical properties of the melt. The required process temperature becomes too high and the handling of the melt becomes extremely difficult, with a high risk of damage to the furnaces.

An important part of the invention is a method which in particular focuses the pretreatment of the original dredging sludge and / or ground streams on a reduction of the silica content of the contaminated residual stream. This makes it considerably more suitable for processing in a melting process. This result can be achieved in particular by using hydrocycloning techniques to remove coarse and fine sand. The separated sand can be used in construction, whether or not after further cleaning steps (flotation and extraction).

It has further been found that in conventional thermal treatment under reducing conditions, a large proportion of the inorganic impurities volatilize and the impurities remaining in the melt cannot be properly immobilized in the product. In order to process the aforementioned residue of soil and dredging sludge, which contains high contents of heavy metals and organic impurities, it is necessary that by choosing the correct process conditions an optimal immobilization of the metals is achieved and the organic components are completely oxidized.

The invention also consists in that the best attainable immobilization can be achieved by carrying out the processing under oxidizing conditions, during which the metals are incorporated in oxidized form into stable crystal structures during the crystallization. When processed under oxidizing conditions, all toxic organic compounds such as PAHs and PCBs are destroyed simultaneously. The crystallization of the formed products also leads to greatly improved physical / mechanical properties, which makes it possible to use them as a building material. Compared to the glass-like end products of most melting processes, crystallized materials have markedly improved strength properties, and the fracture surfaces are significantly stiffer than those of glass-like slag.

The energy requirement of the melting / crystallization process places a financial burden on the process. Significant cost savings can be achieved by delivering as many clean partial flows as possible in the process through less costly separation and cleaning technologies. With the proposed integrated processing of the soil and dredging sludge, the cost per ton of dredging sludge or soil can be considerably reduced.

Flotation techniques developed in the mining industry can be applied to further purify contaminated soil and dredging sludge. The relatively clean bulk waste flows that are released hereby contain a high content of sand. The highly contaminated residual stream thus formed is the raw material for the production of the basalt by means of melting and crystallization techniques. This material is relatively low in silica and rich in organic matter.

According to a further elaboration of the invention, provision can be made for the residual flow of the highly contaminated sludge by means of the admixture of waste materials, or combinations of (waste) materials, which contain a considerable amount of calcium and magnesium, at the desired basicity. is being brought. Experimental research shows that the average ratio between Si, Al, Ca and Mg as found in natural basalt species should be sought. One way of expressing this composition in the production of slag and rock from smelting is basicity.

The basicity of a material is the ratio in molar units between the so-called network formers (especially silicon) and the network interrupters (alkali and alkaline earth metals). It can be defined as:

Research has shown that increasing the basicity reduces the viscosity and melting temperature of the product. Preferably, the basicity is in the range from 0.3 to 1.

This ratio of main elements gives the most optimal rheological properties of the melt as well as the best crystallized products. If the composition of the dredging sludge and / or soil residue does not fully meet the required requirement due to the aforementioned pretreatment techniques, the chemical composition may be adjusted by adding additives. Substances, or combinations of (waste) substances, which contain a considerable amount of calcium and magnesium, are suitable for achieving this ratio in the contaminated residual stream. Materials that can serve this purpose are, for example, asbestos, asbestos cement, lime-rich sewage sludges and waste slag.

Certain additives have a positive effect on the degree and speed of crystallization, such as substances with a high melting point that crystallize at an early stage in the melt (crystal germs). These substances can optionally be added at the time of pouring the melt so that they are well mixed.

A further coordination of the aforementioned separation techniques and the melt / crystallization process is aimed at increasing the content of flammable components in the residue. In the thermal processing, as much use as possible is made of the energy content of dredged material and soil in the form of the organic matter content; significant energy savings are hereby achieved.

In some cases it makes sense to further increase the organic matter content of the residue. This is the case, for example, if the dredged material or soil used in the present process contains a low content of organic substances.

Organic waste sludges, such as municipal sewage sludge and industrial waste sludge (paper sludge) and residues from digestion plants from household waste, can provide the desired organic increase in the organic matter content. The addition of contaminated peat soils could also serve this purpose.

There are also conceivable additives that do not have any special effects on the quality of the end product, but which, due to their physical properties, their chemical composition and the nature of their impurities, are extremely suitable for similar processing by means of the melt / crystallization process. Examples of these waste materials are: drinking water sludges, fly ash, combustion ash from organic sludge, blasting grit, industrial catalysts and residues from soil cleaning installations that are not linked to the present processing installation.

By including these waste materials as additives in the process, the economic basis and the environmental efficiency of the melting / crystallization process can also be strengthened.

Some of these wastes, however, must undergo pre-treatment similar to that required for the contaminated dredging sludge and soil prior to addition. This is the case, for example, for sewage, sewage and ground sludge and the so-called 'peat soils' from the peat areas. Other additives, such as organic sludges, are only administered after this pretreatment.

The final input material for the thermal immobilization step consists of the residual flows of the aforementioned separation and cleaning techniques, whether or not mixed with various waste sludges. According to the invention, the residual wet stream formed is dewatered and then dried.

During the separation and cleaning steps, water is usually added to the dredging sludge or soil, so that dewatering must be carried out prior to further treatment. This can take place by gravitational dewatering in basins, but also by means of sludge thickeners and mechanical dewatering installations such as sieve belt, chamber fliter, balloon and vacuum filter presses. With this, a dry matter content of at least 50% can be achieved. Drying can then take place by making use of the energy content of the organic substances present in the residual stream. The content can be increased by adding organic waste materials, so that no external energy is required for drying. Many different systems can be used for drying, such as drum dryers, fluid bed dryers, belt dryers, grinder dryers, etc.

The evaporation of the physically bound water is an endothermic reaction that takes place to a temperature of approximately 250 ° C. An important volume and mass reduction occurs when sludge is dried.

In the temperature stage up to 800 to 900 "C, the organic components, including toxic hydrocarbons, will burn. In order for this process to run optimally, an excess of oxygen must be present during this phase. In the presence of sufficient oxygen, no (new) formation instead of the dibenzofurans and dioxins that often occur during large-scale combustion of organic substances.This (new) formation can take place at temperatures between 400 ° C and 700 ° C. These substances are considered to be extremely environmentally hazardous, and the eventual disposal of both products. this should be kept as waste (flue gases).

In the melting technique, this temperature step will be a separate step in the entire process. In this temperature trajectory, energy is released which can be used elsewhere in the process. Part of the emissions (including heavy metals, Cl, S0X, CO, CxHy) will take place in this phase. Depending on the type of melting furnace used, these flue gases can be afterburned in the melting zone of the furnace, after which, in particular, the heavy metals and SO2 from the flue gases must be collected with a flue-gas cleaning installation. The energy released during the oxidation step can be used in the drying process.

Pyrolysis is a possible alternative to the oxidation of sludge streams rich in organic material. This makes it possible to utilize the energy content of the organic substances in a more controlled manner. Due to their composition, the products formed here should, however, be burned at high temperature under oxygen-rich conditions in the melting furnace or flue gas cleaning installation.

During the drying and oxidation of the sludge, there is a strong reduction in the mass and volume of the residue. It makes sense to only mix those additives with the cleaning / separation residue at the drying stage, which are supplied dry and no longer contain any organic substances, such as fly ash and asbestos-containing materials. However, it is also possible to mix these dry additives with the mechanically dewatered sludge, so that the dry matter content of this sludge is increased. This facilitates handling and any necessary shaping by pelletization.

The material stream that has been dried, oxidized and optionally mixed with additives determines the subsequently required capacity of the melting furnace.

The invention is based on producing a melt under oxidative conditions. The metals are bound by an excess of oxygen in the oven atmosphere. When melting under reducing conditions, situations arise similar to those in blast furnaces where large chunks of free iron are formed and other metals evaporate. The metallic iron causes a physical degradation of the basalt produced. The heavy metals are then also present at a reduced level. The volatilization is high and what remains in the melt is not properly immobilized. Because of the environmental hygiene objectives and preconditions imposed on the method, metals should be incorporated in stable compounds as much as possible.

The melt should be heated at least above the pour point so that a sufficiently low viscosity of the melt is achieved. The required low viscosity is necessary not only to be able to tap and shape the melt, but also to enable dust transport in the melt for crystallization and for the elimination of gases (mainly air present in the material, but also formed gases as CO2 and SO2). For most compositions, a temperature of 1200-1500 ° C is high enough to achieve the desired result.

The cooling range and composition of a melt determine the structure and mineralogy of the crystalline material produced. This also determines the physical properties. The temperature maintained during crystallization should be high enough to maintain a viscosity in the given composition to promote crystal formation. For the crystallization phase, the melt must be completely degassed. Bad degassing causes large voids in the solidified slag to lead to unwanted breaks in the products. In order to achieve optimum degassing of the melt, it may be useful to apply an underpressure above the melt or to add leaching agents (degassing agents).

The 'handling' of the melt determines, without adding energy, the temperature gradient. The degree of cooling can be controlled, for example, by varying the surface / volume ratio, or by using insulating materials. If necessary, forced cooling can be used, using the heat released elsewhere in the process. However, too rapid a cooling causes a glassy slag to form. Optionally, by adding energy to the material in a crystallization oven, the desired temperature profile can be accurately monitored and (partial) remelting and recrystallization can improve the quality of the final product. The crystallization time can be shortened by ensuring that germs or crystallization surfaces are present.

Depending on the cooling range used and the composition, the following minerals, or associations of the following minerals, will mainly arise: oxides (spinels), silicates, aluminum silicates such as pyroxenes, feldspars, olivine and optionally quartz or feldspar substitutes.

The crystallization of the melt is carried out with two objectives: on the one hand, a material must be produced that meets physical / mechanical requirements as required by the buyers and processors of the rock-like material. On the other hand, heavy metals must be immobilized in the crystallization. There are two possibilities of metal immobilization in the rocks produced: namely, the fixation of heavy metals in resistant crystal lattices and the encapsulation in a glassy matrix. The most stable method of fixing, however, is the installation of the metals in a crystal grid, which itself is very resistant. Most aluminum silicate grids meet this requirement, as do a number of non-silicates, such as — for example — spindle group minerals. Due to good process control during the cooling phase and by properly carrying out the pre-treatment of dredging sludge and soil, heavy metals present can largely be stored in resistant crystals.

Due to this crystallization, the mechanical properties of a crystallized slag are also superior to the properties of a glass slag. Tests have shown that the crystalline synthetic basalt produced has properties comparable to that of natural basalt. The crystalline material produced can be used as shaped building material, cast as columns (starches), tiles and slabs, or large (sawn) blocks and - if broken - as unformed quarry stone in asphalt and concrete. In principle, all applications for which natural basalt-like rocks are used are eligible.

Necessary in this processing is the use of a water purification and flue gas cleaning installation. As much of the process water released during dewatering should be reused in the separation and flotation steps used. The surplus process water from the various cleaning steps and flotation, as well as water from the flue gas cleaning, must be cleaned before discharge to the surface water. The exhausted flue gases must comply with the applicable emission guidelines.

Example I

The processing of dredged material in combination with soil cleaning residues by means of the melt / crystallization technique.

The method according to the invention was applied to contaminated dredging sludge from the Malburgerhaven in Arnhem. The following steps were used for this: 1. Hydrocycloning of the dredged material, resulting in: relatively clean or cleanable sand: underflow; and contaminated sludge: headwaters.

2. Cleaning the underflow by means of (acid) extraction.

3. Mechanical dewatering of the underflow which can then be usefully used as an embankment material.

4. Mix in the cleaning residue from the bottom channel to the top channel.

5. Supplementation of the organic matter content with other waste sludges, such as flotation residues, up to approx. 40%.

6. The composition of the inorganic part is influenced by means of waste sludges: + 2% K20; + 5% MgO; + 3% CaO.

7. Dewatering of the mixture with mechanical thickeners and presses.

8. Drying in the drying oven at a temperature up to ± 250 ° C.

9. Oxidation in the oxidation oven at a temperature up to ± 900 ° C. The oxidation step can optionally take place in the same oven as the drying step.

10. Melting the oxidized material. The melting temperature is 1300-1400 ° C to form a sufficiently low-viscosity melt.

11. Degassing of the melt.

12. Pouring the still liquid melt into molds in the crystallization oven.

13. Crystallization of the melt.

In order to obtain the desired crystal structure after cooling, the temperature range between 1000 ° C and 900 ° C must be slowly followed during cooling (0.5-1 ° C / min). A new crystal formation cycle can be initiated, if necessary, by raising the temperature again to the melting temperature after cooling to 800 ° C and thus increasing the degree of crystallization. The minerals formed are mainly aluminum silicates and oxides (pyroxenes, spinel group minerals, feldspars, gehlenite). The heavy metals are effectively immobilized herein.

14. The product is a microcrystalline material. The material properties of the material in broken form meet the requirements for use as quarry stone or gravel substitute. The cast products can serve as a starch instead of basalt or as a replacement for other high-quality designed building materials.

Example II

Residue of dredged material after hydrocyclonage and flotation separation.

The method according to the invention was applied to contaminated dredging sludge from one of the inland ports of Rotterdam. The following steps were used for this purpose: 1. Hydrocyclonage of the dredging sludge, whereby the underflow can be used immediately after dewatering.

2. Flotation separation performed on the upper course. The organic matter content of the contaminated flotation residue is ± 50% (on a dry matter basis).

3. Re-use after dewatering of the part of the dredged material cleaned by flotation as fill material.

4. Mechanical dewatering of the flotation residue to a dry matter percentage of 50-55%.

5. Drying the material in a drying oven to a temperature of ± 250 ° C.

6. The composition of the inorganic part is influenced by means of dry additives: + 10% CaO; + 7.1% MgO.

7. The continuation is the same as that of Example I.

Claims (10)

Method for the treatment of contaminated dredged material and / or soil to obtain building materials, characterized in that the dredged material and / or soil is separated, by techniques known per se, into relatively clean partial flows of sand and loam and a relatively small residual stream of highly contaminated sludge, which residual stream is subjected to a controlled oxidation, melting / crystallization process for the production of crystalline basalt-like material. Method according to claim 1, characterized in that the residual flow of the highly contaminated sludge is adjusted to the desired basicity by means of the admixture of waste materials, or combinations of (waste) materials, which contain a considerable amount of calcium and magnesium. brought. Method according to claim 2, characterized in that the (waste) substances can be formed by: asbestos, asbestos cement, cement residues, lime-rich sewage sludges and waste slag. Process according to claim 3, characterized in that heavy metals and chemically related elements, such as arsenic, molybdenum and vanadium, are immobilized and all organic impurities are destroyed. A method according to any one of the preceding claims, characterized in that the degree and speed of the crystallization are enhanced by adding certain (waste) substances. A method according to claim 5, characterized in that the substances are added at the moment of pouring out the melt. A method according to any one of the preceding claims, characterized in that the degassing of the melt is promoted by applying negative pressure above the melt and / or adding purifying agents. A method according to any one of the preceding claims, characterized in that the temperature during the cooling phase of the melt is controlled such that it is between 1200 ° C and 1500 ° C, so that the melt retains a sufficiently low viscosity to form crystals. A method according to any one of the preceding claims, characterized in that the liquid basalt is poured into molds which have the desired shape of the product to be manufactured. A method according to any one of the preceding claims, characterized in that organic substances such as sewage sludge, peat, industrial sludge and the like are added to dry the residual streams involved in the melting process.