Method of covering waste mounds using a fill

Abstract: The present invention relates to the use of a fill and ground-­ binding agent for covering waste mounds, rubbish tips and other less harmful material consisting of 1-30 % by weight cement and 99-70 % by weight waste materials selected from one or more of the following materials: fly-ash, blast furnace slag, de-­ sulphurization product, various types of other waste, such as pyrite, metal-hydroxide slag, poor ash, etc., whereby the mound first is covered with a first layer of dry materials which can be sulphate-activated by or reacted with a cement solution extract selected from one and more of fly-ash, de-sulphurization product, granulated blast-furnace slag, and which material must not be allowed to react before providing an external second layer containing a basic reserve which can be leached out such as all types of concrete, fill with more than 10 % by weight of cement and a mixture of fly-ash, and/or granulated blast-furnace slag which has been activated with de-sulphurization product and cement.

Description: An object of the invention is to provide a means whereby waste material which is dangerous to the environment can be made harmless. Many such waste materials are dumped on tips, and should be covered with a sealing layer to avoid water from penetrating therethrough. The finer the waste the more susceptible it becomes to water-leaching processes, since the specific surface area of the material increases greatly with fine-grain waste. The object of the invention is further to use and utilize waste and residual products produced in extremely large quantities and which must be dumped. For example, the combustion of coal and other sulphur-containing fuels creates serious environmental problems with respect to acidification, and consequently it is necessary to cleanse the flue gases of fine dust particles and other deleterious substances. The residual products obtained from such cleansing processes are fly-ash, bottom ash, de-sulphurizing products or combinations thereof. Large quantities of such products are dumped, which results in an additional load on the external environment when the products are in an extremely finely divided form and many poisonous substances can be leached-out therefrom. Other products also dumped are metal-hydroxide sludge, fine industrial waste, blast furnace slag etc., and since many of the residual products are chemically unstable, such products when dumped can be a serious hazard. In accordance with the invention it is now possible to mix cement and fly-ash to form a fill exhibiting very good properties. The fly-ash functions as a fine ballast in the hydration of the cement and prevents such cracking as would normally occur when using conventional concrete as a fill. Very good results are obtained with a mixture composition of 1-15 % by weight cement or lime and 99-85 % by weight fly-ash. Water is added to this mixture in an amount to obtain a water binding index WBN, of between 0.20 and 0.60. The expression WBN may include all materials having a binding effect, for example slag, de-sulphurizing products, lime etc.. The consistency of the mixture is controlled by the water binding index, for example in the manner which will enable the mixture to be pumped into cavities which are filled more or less easily, or which will enable the mixture to be shovelled into pits and holes. The mechanical strength of the mixture varies with the amount of cement used and also depends on the water binding index. Our fill is more suited as a covering material for covering mounds of dumped waste, so as to prevent the leaching of poisonous and deleterious substances therefrom. In addition to the fact that the fill will not settle or undergo changes in volume, it can also be used in all weathers and be readily removed even after having been in place for a long period of time. For example, it can be readily "cut away" with the aid of a mechanical digger, or even with a spade. The mechanical strength of the fill after one year or more is at maximum 10-15 MPa, depending on the E-modulus of the material. The fill quickly hardens after being laid, and after 18 hours the penetration resistance is so high that a force of 0.3-1.0 MPa results in a maximum deformation of 2-20 mm. The fill constituents can be mixed in a conventional cement mixer, either industrially for transportation as a pumpable ready-mix fill, or directly on the working site in smaller cement mixers. Another alternative is to place the constituents in a transit or truck mixer, and mix the constituents during transit to the working site. Such a fill is produced by mixing 5-15 % by weight cement with 95-85 % by weight fly-ash. The fly-ash may be admixed with various other wastes, for example blast furnace slag, while different environmentally harmful waste materials may also be incorporated in the mix, for example such waste materials as metal-hydroxide sludge, poor ash and other waste, such as pyrite etc. which is difficult to dispose of. Fly-ash having a high residual carbon content can also be used, despite the fact that such ash has previously been considered unusable because of its low pozzolanic activity. The quantities in which the environmentally harmful wastes are incorporated in the fill are determined by the environmental authorities, so that the leaching water does not contaminate the ground water, since a large cement and fly-ash content results in greater denseness or higher degree of impermeability. The aforementioned de-sulphurizing product comprises mainly a plurality of different calcium salts and minor quantities of different compounds of trace substances. The composition is directly dependent on the coal combusted. The de-sulphurizing product can be produced in various ways, for example by a wet-dry de-sulphurizing process, in which a lime solution is sprayed into the flue gas, to produce a dry residual product. In this respect, the major part of the fly-ash can either be separated prior to the de-sulphurizing process, or may accompany the process and be mixed with the de-sulphurizing product. A characteristic composition of de-sulphurizing products is given in Table 1 below. Chemical Compound Percent by Weight CaSO₃ x 1/2 H₂O 50-80 CaSO₄ x 2 H₂O 5-20 CaCl₂ x 2 H₂O 1-10 Ca(OH)₂ 5-20 CaCO₃ 5-15. The de-sulphurizing product can be flocculated around fly-ash particles in fine-powder form having a particle size of 2-5 µm. In the absence of fly-ash, the de-sulphurizing products are agglomerated to composite agglomerates of very high porosity. The problem encountered with the de-sulphurizing product is associated with the stability of the calcium sulphite and the question of whether a chemically unstable residual product can be used directly, whether oxidation can be prevented or whether the material must be oxidized prior to being used. Even though oxidation of the material has given good results, an oxidizing plant is highly expensive and consequently the direct use of the product of de-sulphurization to be preferred. This can be effected in the fill and has been found to give a very good result, particularly when the fill is used as a covering material. The de-sulphurizing material is able, in itself, to activate the glass phase in granulated blast-furnace slag and in fly-ash. The growth in the strength in the material is controlled by adding Portland-cement even though the mechanical strength of the material is not in a chemically stable phase. Even though an excessive content of Portland-cement results in an unacceptable swelling reaction, the cement is a suitable activator since the material obtains thereby an internal basic environment which greatly impedes oxidation of the unstable sulphite. The strength values of the mixture are also sufficiently high to noticeably impede the diffusion of oxygen into the material. The readily dissolved salts contained in the product of de-sulphurization promote the uptake of water and reduce the resistance to frost activity, and one condition for the successful use of the material is consequently that the matrix is sealed with another, stronger binder or that there is created a system of air pores which provided the requisite degree of swelling. A mixture of cement, de-sulphurizing product, fly-ash and slag is particularly suited for encasing dangerous operational waste. Impervious covers for less dangerous tips can also be formed from this mixture. Layers of clay and sand-bentonite-mixtures are at present used for this purpose, although concrete is avoided due to the risk of cracks forming therein. Particularly good properties have been obtained, by adapting the quantities in which the various constituents are present in the mixture in accordance with the invention. If the product of de-sulphurization is used in excessive quantities, the material becomes sensitive to water and there is a risk of a swelling reaction if the sulphite converts to sulphate. The use of an excessive quantity of cement also results in a swelling problem, due to the formation of ettringite, and the maximum permissible sulphur trioxide content of cement is 4 %. The harmful effects of our mixture, however, are restricted by the fact that the cement content is so low that the ettringite formed will not swell to an extent which causes ruptures in the material. A suitable composition of our fill for ground binding purposes is a cement content of 2-30 % by weight, a quantity of 10-40 % by weight de-sulphurization product, and the remainder comprising a puzzolanic material which reacts upon contact with a given substance, for example a high percentage of OH-ions, and which can be sulphate activated. The puzzolanic material may be fly-ash or granulated blast-furnace slag. Thus, a sealing mass according to the invention can be produced by mixing 60-90 % by weight fly-ash with 10-40 % by weight de-sulphurization product, and 80-98 % by weight of this mixture admixed with 2-20 % by weight Portland-cement. A mound of garbage, domestic waste, coal ash or similar less harmful material can be covered with a layer of some material which can be sulphate activated or reacted with cement-solution extract, i.e. a high OH-ion content, such as a material having a pH of 12-14. Such materials include fly-ash and granulated blast-furnace slag which can be laid out wet, and various mixtures which must be laid out in a dry state, such as a mixture of fly-ash and de-sulphurization product, a mixture of de-sulphurization product and granulated blast-furnace slag or fly-ash mixed with both slag and de-sulphurization product. This material must not be allowed to react before providing an externally lying layer. This outer, hard layer contains cement in quantities such as to provide a basic reserve which can be leached out, consisting of 1-30% by weight cement and 99-70% by weight waste materials selected from one or more of the following materials: fly ash, blast furnace slag, de-sulphurisation product, pyrite metal hydroxide sludge or poor ash. The mound is covered with a top layer of soil or the like, which serves as a substrate for plant life, grass, shrubs and eventuelly trees. When the mound of waste settles no caving will take place, but the hard external shell will crack and remain on the inner powderous material in larger or smaller pieces. The hard material forming the shell is of a nature which enables it to crack relatively easily and to fall together as soon as there is a tendency to caving. The underlying powder layer must also be of a thickness such that when the mound settles there is continously formed a coherent layer without interruption, this thickness in practice being at least about 1 m. When cracks have formed, rain water will progressively enter through the cracks and activate the underlying powder material, causing the same to harden. During passage of the water over the hard layer and through the cracks, OH-ions are also leached out, therewith to raise the pH of the rain water so as to additionally activate hardening of the powder material. Normally, all cracks are closed after 2-3 months. The invention offers an important service by proposing valuable usage of waste products which previously have only presented a dumping problem.

Claims: Method for covering waste mounds, rubbish tips and other less harmful material characterized in that the mound is covered with a first layer of dry materials which can be sulphate-activated by or reacted with a cement solution extract selected from one or more of fly-ash, de-sulphurization product, granulated blast-furnace slag, and which material must not be allowed to react before providing an external second layer containing a basic reserve which can be leached out consisting of 1-30% by weight cement and 99-70% by weight waste materials selected from one or more of the following materials: fly ash, blast furnace slag, de-sulphurisation product, pyrite metal hydroxide sludge or poor ash

Inventors: TUUTTI KYOESTI, TUUTTI, KYOESTI, Tuutti, Kyösti
Applicants: CEMENTA AB
Application year: 1985
Main classification: B09B1
IPC codes: B09B1, C04B28, E01C11, C04B40