EP0730884A1 - Method of decreasing the content of heavy metals in incineration and pyrolysis residues - Google Patents

Abstract

In the redn. of the heavy metal content of combustion and pyrolysis residues using aq. solvent, the residues are washed with an aq. soln. contg. a ligand (I) with at least 2 metal-co-ordinating gps. Pref. (I) contains at least one COOH gp. and at least one COOH or OH gp. Washing with aq. (I) soln. is carried out in several stages. This is followed by washing with water, without (I), to reduce the organic C content. The washed residues are then subjected to controlled ageing.

Description

The invention relates to a method for reducing the heavy metal contents of combustion and pyrolysis residues containing heavy metals.

Household waste, commercial waste similar to household waste and bulky waste represent an apparently small share of the total waste volume with 25 - 30 million tons or 10% annually. However, due to their heterogeneous composition of pollutants, they deserve special attention when disposed of. In 1991, 9.5 million tons of this was thermally treated in 47 waste incineration plants and in a pyrolysis plant in Germany.

Waste incineration has meanwhile developed into a proven and reliable waste recovery process. Nevertheless, the environment is polluted by emissions and residues, as with other disposal methods.

One of the main tasks in the thermal treatment of waste is the generation of largely inert, ecotoxicologically harmless and usable residues. As the number of landfills decreases drastically due to backfilling and due to political decisions, this applies in particular to the slag, which is the main residue fraction in terms of quantity. With conventional waste incineration, approx. 250 to 350 kg of slag and 40 to 60 kg of filter dust are generated per ton of waste.

In waste treatment in waste incineration plants, the majority of the environmentally relevant elements (heavy metals) contained in the waste are transferred to the slag with regard to the pollutant loads. For this reason, the properties of the combustion product "slag" and the handling of it largely determine the environmental compatibility of the waste incineration process.

Several processes have been developed for washing out heavy metals and salts from residues from waste incineration.
Most of the processes are carried out in an acidic environment, since for most of the heavy metal hydroxides the minimum solubility is in the pH range from 9 to 12. At the neutral point, the solubilities are several powers of ten higher. At pH 4, the heavy metals are largely soluble. Some of these methods are from Gleis, M .; Hoffmann, G .: Treatment methods to improve the environmental compatibility of residues from the incineration of household waste, further training center for health and environmental protection Berlin (ed.), Seminar residues from waste incineration plants 31 (1992) 43-61 known.
With a basic wash of the slag with water as z. B. is also carried out in the waste incineration plant in Bamberg, easily soluble salts (z. B. 50% of the chlorides) can be removed from the slag. Such a process is known from Reimann, DO: Combined quenching and washing process of MV slag using the example of the MHKW Bamberg, manual for the VDI seminar Slag Processing, Recycling and Disposal, Munich (1992).

Studies show that the heavy metals, even with acids, can only be partially washed out of the residue matrix of residues from waste incineration, and the latter is thereby severely decomposed. Heavy metals firmly integrated in the matrix of residues are thus dissolved and do not remain inert. The acidic treatment of the waste materials from waste incineration with acids also greatly reduces the alkalinity, which is therefore no longer available for buffering the remaining residues that still contain pollutants. Heavy metal hydroxides have their lowest solubility in the pH range between 9 and 12.

For the acidic washing of slag, there is not enough waste gas scrubbing water during waste incineration (approx. 0.2 m ³ waste gas scrubbing water per ton of slag). To make one for the sour To be able to elute laundry with a sufficiently low pH value of at least 4, very large amounts of acid must be used because of the high basic buffer capacity of the residues.

In the basic washing of residues (without the addition of complexing agents) from waste incineration, only a few easily soluble salts, e.g. Chlorides are washed out of the residues. Residual chlorides and especially heavy metals remain in the residues because heavy metal hydroxides have their lowest solubility in the pH range from 9 to 12.

The object of the invention is to significantly reduce the heavy metal content and thus the toxicity of combustion and pyrolysis residues containing heavy metal.

This object is achieved by the characterizing feature of patent claim 1. The subclaims describe advantageous embodiments of the invention.

The novelty of the invention is that heavy metals are washed out of residues from the incineration with complexing agent solutions. These complexing agents work particularly well in the basic area. (Waste incineration residues are basic). The complexing agents can be used both as individual substances and as mixtures. Basically, all carbohydrate and / or protein-containing substances and residues are suitable as sources for complexing agents. The mixtures can be released inexpensively from residues from the food industry. This can be done chemically, electrochemically or enzymatically. Another advantage of the complexing agent mixtures are synergistic effects that increase leaching efficiency. In contrast to the "acid wash" that has been used most of the time, the residue matrix is less attacked. Heavy metals that are firmly embedded in the matrix of residues are hardly dissolved and remain inert.
Another advantage of our invention over the "acid wash" is that the basic buffer capacities of the Waste incineration residues can only be reduced insignificantly. Since heavy metal hydroxides have their minimum solubility in the pH range from 9 to 12, heavy metals are partially immobilized due to the basicity of the residues from waste incineration.

The advantage of the washing process with complexing agents compared to "basic washing" is that not only easily soluble metal chlorides but also heavy metals bound to the surface can be washed out of the residues of waste incineration. This significantly improves the leaching behavior of the residues, since the basic buffer capacity of the residues is largely preserved, as in basic washing without complexing agents.

Washing residual waste from waste incineration with complexing agent solutions thus combines the advantages of "acid" and "basic" laundry without having their disadvantages. The disadvantage of contaminating the residues with organic material can be eliminated by washing them again.

Through this type of processing of residues of waste incineration and waste pyrolysis, z. B. in road construction, expensive landfill costs can be saved.

The invention is explained in more detail below on the basis of exemplary embodiments.

The slag or other residues (boiler ash, filter dust) from waste incineration and pyrolysis plants are washed with complexing agents dissolved in water. The added complexing agent solutions elute the heavy metals contained in the residues of the waste incineration except for an immobile, largely ecotoxicologically harmless residue. The organic complexing agents used in the invention, both as pure substances and as complexing agent mixtures are used, are of biogenic origin and therefore easily biodegradable. Residues, mainly from the food industry, are used as a source for the complexing agent mixtures. As a rule, these residues of the food industry have to be treated hydrolytically or converted oxidatively. In principle, all carbohydrate and protein-containing substances or residues are suitable as a source for complexing agents. The chelating agents used consist of compounds with two or more metal-coordinating groups of molecules, e.g. B. carboxyl (-COOH) -, hydroxyl (-OH) - and amino (-NH ₂ ) groups.

The low molecular weight organic compounds such as aliphatic (hydroxy) carboxylic acids, ketocarboxylic acids, sugar acids, keto sugar acids, aminosugars and amino acids are essentially responsible for the complex formation capacity of the mixtures and thus the release of the heavy metals.

The eluates containing heavy metals could be concentrated and the heavy metals contained therein could be reprocessed.

The residues from waste incineration plants treated in this way undercut the limit values for landfill prescribed in the TA waste and are also suitable for safe reuse e.g. B. as a base in road construction.

The residues from waste incineration are crushed, if necessary, and then washed with the complexing agent solutions. The washing can be carried out directly at the discharge of the slags (e.g. in the wet deslagger), boiler ash and filter dusts (e.g. combined with a cooling of the residues), or after some time of storage. The composition of the complexing agent solutions and the ratio of residue / complexing agent solution differ in different applications.
They can be optimized for every residue. The wash can be carried out in several stages with varying concentrations and compositions. Several washing stages with each lower chelating agent concentration show a better result than a one-step washing process with the same amount of chelating agent.

The table shows the concentration of some heavy metal ions in the elution of boiler ash from a waste incineration plant with complexing agent solution (gluconic acid 0% to 5%; molasses 1.8%). The molasses was oxidatively split with 30% nitric acid analogous to sucrose oxidation (solid / liquid ratio) 1/10.

Particularly good elution rates are obtained for iron, chromium, copper, lead and zinc.

Protein-containing residues can be hydrolytically split into amino acids by hydrochloric acid.

Since the residues of waste incineration generally have a high pH, those complexing agents are used which have particularly high complexing capacities compared to heavy metals in the basic (e.g. through the formation of alcoholate groups from OH groups).

After elution of the heavy metals with the complexing agent solution, the treated residue is washed with water in order to minimize any organic complexing agent solution remaining in the residue. The TOC limit values (TOC = total organic carbon) of TA waste can thus be complied with. At the same time, additional heavy metals that were dissolved by the first washing process are eluted in this washing process.

Controlled aging of the washed residues can further improve the immobilization of the heavy metals remaining in the residues. This is done by storing with ventilation and humidification (a few days to weeks).

The complexing agent solution used and the wash water can, for. B. be processed by evaporation using the waste heat of the waste incineration plant. The wash water can thus be reused; the eluted heavy metals can U. can be recovered. table unit 0% glucons 0.01% glucon 0.1% glucon 0.5% glucon 1% glucons 2% glucons 5% glucons 1.8% molasses Sb ppb 9.8 <9.8 12.4 194 244 291 531 625 Fe ppb 18th <4.7 178 2020 3360 3690 7670 1780 Co ppb 32.8 120 24.9 36.1 43.7 45.8 52.6 56.5 Ba ppb 112 117 <4.2 <4.2 <4.2 <4.2 411 1260 As ppb <13 <13 <13 <13 <13 144 325 458 Ni ppb 72.3 253 60.9 95 119 111 109 116 Cr ppb 8.9 27.8 85.8 338 648 780 1040 705 CD ppb 12700 38400 13500 1840 2530 5060 13700 1230 Cu ppb 17.9 128 470 4230 10200 15,000 26000 21900 Ti ppb 6.3 6.3 6.3 183.4 521.2 856.5 1563.4 710.7 Mn ppb 6.76359 16.6 8.7 5.4 6.7 8th 10.3 8.6 6 Pb ppb 1100 3140 5800 6000 14600 67500 412000 60500 Zn ppb 27700 954000 77000 95000 174000 309000 782000 355000

Claims (5)

Process for reducing the heavy metal contents of combustion and pyrolysis residues containing heavy metals with the aid of aqueous solvents, characterized by washing the residues containing heavy metals with an aqueous solution which contains chelating agents with at least two metal-coordinating molecule groups. A method according to claim 1, characterized in that the metal-coordinating molecular groups are at least one carboxyl and at least one carboxyl or hydroxyl group. A method according to claim 1 or 2, characterized in that after washing out the content of organic carbon in the residues is reduced by a further washing process with water without a chelating agent. Method according to one of claims 1 to 3, characterized in that the washing out with the chelating agent-containing aqueous solution is carried out in several stages. Method according to one of claims 1 to 4, characterized in that the washed combustion and pyrolysis residues are subjected to controlled aging.