Patent Number: 045333955
Section: summary

FIELD OF THE INVENTION The invention relates to a method of making a fixation product of harmful water-containing waste materials and cement with or without additives (fixation matrix). BACKGROUND OF THE INVENTION Solid end products of harmful water-containing waste materials and cement with or without additives such as metal oxides, metal hydroxides, silicates of the alkaline or alkaline-earth metals or aluminosilicates, have been known for some time (DE-AS No. 2,426,641). Such solid products, however, contain only a relatively small amount of harmful wastes, generally less than 10% by weight based on the dry weight of the waste in the end product or the end product may have a water content of substantially more than 45% by weight on a water-to-cement basis or both. Harmful water-containing wastes are aqueous solutions or slurries such as slurries having suspended therein compounds which are harmful or dangerous for the biocycle, for example, poisonous or radioactive compounds. The solid end products of this type known so far have certain limits given by either the water-to-cement ratio, that is, the water-to-fixation compound ratio or the charging of the fixation compounds with waste. The smaller the volume of aqueous waste in relation to the fixation compounds, the smaller is the portion of harmful waste in the end product and also, the smaller is the relative amount of water-to-fixation compounds (weight percent to weight percent) in the end product. If the volume of the aqueous waste to be solidified is increased while the amount of fixation compounds remains unchanged, then the weight percent of dry waste is increased in the end product but the weight ratio of water-to-fixation compounds is also increased which, however, is only possible to a point that depends on the type of fixation matrix. Any amount of liquid to be solidified beyond that point cannot be contained by the fixation matrix. As a consequence of an addition of an excessive amount of liquid, the fixation matrix will not solidify or, if the fixation matrix can solidify, it will bleed liquid, that is, water or watery solutions will be left over after solidification of the matrix. In this connection, it is not foreseeable whether this bleed liquid contains harmful waste materials. Certainly, it is not reasonable to undergo a solidification process and, after solidification, have harmful liquid waste left over. The aqueous waste may be concentrated, for example, aqueous salt solutions may be subjected to evaporation so that the concentration of harmful waste in the end product is increased by a certain amount but this may cause problems with regard to the quality of solidification. In order to obtain a sufficiently large concentration of waste material, the aqueous solutions would have to be concentrated to such a degree that mechanical problems would occur which may render the process impossible to practice; for example, crystallization, transport problems, clogging by dried compounds might occur. The salt content of the end products which are made in accordance with presently known methods, particularly by encapsulating in barrels with cement, is limited by the salt concentration in the waste solution. The solidification of aqueous waste with a content of harmful salts of about 40% by weight leads to solid end products with a salt content of not more than 16% by weight in spite of the fact that the waste-to-cement ratio is about 0.4. The higher the water-cement value, that is, the weight ratio of water-to-solidification compound, in the end product, the smaller is the resistance of the solid end product to leaching by water or salt solutions. In addition to high resistance against leaching, it is necessary that the solid end product has also a relatively high mechanical strength. Of solid end products containing radioactive waste, a compression strength of 10 N/mm.sup.2 or more is required. Also, for harmful materials there should be: on the average, a leaching rate of better than 10.sup.-3 g/cm.sup.2 per day (when measured over the period of a year), PA1 a differential leaching rate of better than 10.sup.-4 g/cm.sup.2 per day (as measured at the end of the first year), and PA1 a diffusion parameter of better than 10.sup.-4 cm.sup.2 .times.d.sup.-1. PA1 (a) contains 33 to 50% by weight of harmful waste materials based on the dry weight of the waste in the end product (solid fixation products), and PA1 (b) has a water content corresponding to a water-to-cement ratio of 0.20 to 0.40. Only if these conditions are fulfilled, the end product may be considered leach resistant. It is the object of the present invention to provide a solid fixation product of harmful aqueous waste materials and a fixation matrix on the basis of an inorganic solidification compound, which fixation product is resistant to leaching by water and water solutions and is capable of containing a relatively large amount of waste in combination with a low water-to-solidification compound ratio. The fixation products shall have good mechanical properties, a good leach resistance and at least the retaining qualities of the known fixation products (with much lower content of harmful waste in the solid end product). The novel solid fixation product shall be producible in a continuous as well as in a batch-type process and, in any case, shall have especially crushing strength and leach resistance. It is, furthermore, an object of the present invention to provide a method of making such solid fixation products. SUMMARY OF THE INVENTION A method of making a solid fixation product of harmful water-containing waste materials and cement with or without additives (fixation matrix) which is resistant to leaching by water or salt solutions and which is composed of 20 to 50% by weight of harmful waste based on the dry weight of the waste in the solid end product and a water content in the end product corresponding to a water-to-cement ratio of 0.2 to 0.45 (water-cement number). If the waste materials are ion exchanger resins charged with poisonous waste, the amount is 26 to 50% by weight. Preferably, the solid fixation product: In the method according to the invention, in which the water-containing waste materials are concentrated by evaporation and in which the concentrate is mixed with the fixation matrix and solidified, the evaporation process and the mixing process are performed concurrently at temperatures of 100.degree. C. to 180.degree. C. The evaporation and mixing time is adjusted so as to obtain a water content in the end product corresponding to a water-cement number of 0.2 to 0.45. Evaporation and mixing may be performed on a continuous basis in a continuous mixer; but it may also be performed in a batch-type operation in a container. If necessary, evaporation and mixing is performed in the presence of a setting retardant. Also, evaporation and mixing may be performed in a vacuum environment of 20 to 50 m bar. It has been impossible, so far, to make leach-resistant solid end products of harmful aqueous waste materials which have a fixation matrix of mainly inorganic fixation materials which had such a high content of harmful waste materials in the end product. The product according to the invention has a number of important advantages: For example, the waste volume in the end product to be disposed of is substantially reduced when compared with the volume before fixation, that is, the volume of the solid product is only slightly larger than the volume of the dry portion of the waste because of the addition of only a relatively small amount of fixation materials. The solid end products are easy to produce in spite of a relatively large waste content and they may be produced with relatively low costs, little space, and small expenditure in time. It has already been proposed in connection with the fixation of radioactive waste water to concentrate the waste water by evaporation even to such a degree that a dry residue is obtained and thereafter admix the dry residue to an encapsulation material for solidification but such a process encounters problems since the relatively high viscosity of the pasty encapsulation material results in a relatively low limit for the content of waste material in the encapsulation material. If such dry residue is admixed to the cement paste, the high waste content in the end product, as it is possible with the process in accordance with the invention, cannot be obtained and the relatively low water-cement number is exceeded. In order to obtain a high waste loading of the end product, a certain amount of bentonite has been added to the encapsulating material, particularly water-absorbing bentonite, but this resulted in an increase of the viscosity of the cement paste so that it could not be properly mixed unless additional water was added to compensate for that disadvantage. A continuous encapsulation process in which a water-based encapsulating compound is the main component of the encapsulation matrix is known from DE-OS 32 02 518. As described therein, radioactive and/or poisonous materials containing water and/or crushed radioactive and/or poisonous solid materials are embedded in an encapsulating material mixture which solidifies and the mixture is filled into barrels or containers. In the process according to this German Laying-Open-Print, the principal idea is to mix the inactive materials such as cement and sand and the radioactive liquid materials such as evaporator concentrates or slurries so as to produce a volume which is as small as possible and thereafter fill it into containers. The arrangement is to be as small as possible and as much as possible of such materials are to be solidified in the smallest possible space resulting also in a saving in encapsulating compounds. Especially equipment and machine parts which are contaminated by radioactive materials should require the least possible volume after their embedment into a solidifying encapsulating material mixture. This is achieved in accordance with the German OS in that a dry encapsulating material mixture is introduced into a radiation protective chamber or similar containment and is mixed therein with the materials to be encapsulated and the mixture is then permitted to set. It is said that it is particularly advantageous if the encapsulating material is mixed with radioactive or poisonous water so as to harden thereafter. With this method, not only radioactive particulate materials but also the radioactive water are solidified. The pre-mixed encapsulating materials (mortar mixture), the particulate radioactive solid materials and finally radioactive ion exchange resins are introduced into a continuous mixer by means of augers and are mixed in a mixing zone with radioactive water or slurries by means of a mixing paddle wheel. Process conditions such as the amount and consistency of the encapsulating materials, the ratio of amounts of encapsulating materials to radioactive and/or poisonous water or similar, the ratio of amounts of encapsulating materials to radioactive or poisonous crushed solid materials, the ratio of amounts of encapsulating materials to radioactive or poisonous ion exchanger resins, etc., are not addressed in the above German OS. It must therefore be assumed that this process operates under the known conditions as usually employed. Increased waste loading and, at the same time, a low water-cement number cannot be achieved with this method. Suitable fixation matrix materials for the method according to the invention are, for example, cements on the basis of Portland cement such as the Portland cements themselves, blast furnace cements, trass cements, pozzuolona cements, etc., or water-setting ceramic cements such as deep drill cements. The cements may be utilized each alone or in any suitable mixture of two or more of the cements or in the form of mixtures with additives of, for example, alkali- or alkaline-earth silicate, etc. Under normal conditions, as they are prevalent, for example, in the building industry, the setting and hardening of cement is accelerated at higher temperatures. This effect is utilized, for example, by hardening concrete and cement building blocks at higher temperatures in autoclaves in order to obtain their final strength already after a relatively short time (one day as compared to one week). Under such conditions, the setting takes place already within a few minutes. It has now been found during research concerning the fixation with cement of NaNO.sub.3 containing radioactive waste concentrates that those mixtures with cement (Portland cement or blast furnace cement) can, surprisingly, be treated over extended periods of time (45-60 minutes) at temperatures of up to 180.degree. C. without resulting in a premature setting. The end products are hardened after several hours and, after 28 days, have achieved a crush resistance comparable to that of the products made in accordance with the known methods. It becomes, therefore, possible to produce, in a single process step (combined mixing and evaporating), cement products with a low water-cement number, that is, with a higher salt content, than it is possible with the known "in-barrel cementing" methods. In comparison with the known two-stage process, in which the waste water is first evaporated and the dry residue is then mixed with cement paste in separate process steps, the process according to the present invention has a number of advantages. It is a one-step process which may be operated either in a continuous or a batch-type fashion. Material dosage is facilitated since only cement and the (for example) waste or waste concentrates have to be mixed in a reaction vessel, that is, the only solid material which needs to be admetered is the cement; there is no evaporation residue to be admetered which is difficult since it generally includes lumps and tends to stick together. An important advantage of the process according to the present invention resides further in the fact that it may be performed in a continuous operation and that the waste content of the end product is easily controllable in a wide range by adjustment of the composition of the initial mixture and the degree of evaporation in the mixer. Another advantage is found in the fact that the higher concentration of radioactive materials in the final storage containers, although providing for increased radioactivity in the containers, results in a substantially reduced expenditure in shielding materials: Taking, for example, 400 l barrels with cast iron shielding walls in connection with the encapsulation therein of radionuclide Cobalt-60: If the shielding wall thickness of such barrels is doubled, the volume of the shielding material is increased by a factor of 2.5. However, the Cobalt-60 radioactivity admissible within the container may be increased by a factor of 40. This relationship becomes even more favorable with further increased radioactivity. For the encapsulation of concentrates of medium-(radio)active wastes (MAW-concentrates), the respective ratios are even more favorable since the nuclide spectrum in those encapsulation products contains low energy gamma radiation which will not penetrate the shielding walls of increased thickness and, therefore, do no longer need to be taken into consideration. Hereafter, the invention is illustrated on the basis of a simplified flow scheme given in the single FIGURE. Encapsulation examples of aqueous wastes are given as follows: