METHOD FOR IMPROVING THE CONCRETE-TECHNICAL PROPERTIES OF FLY ASH

Abstract: The invention concerns a method for improving the concrete-technical properties of fly ash produced in the burning of solid fuel and separated from the flue gases. According to the invention, the fly ash is subjected to a removal of the coarse fraction, so that the ash contains, at the maximum, about 50 per cent by weight of a fraction that remains on a screen of 45 mu m. The remaining ash is divided into at least two fractions of different grades of coarseness, to which Na- or Ca-lignosulphonate is added at the rate of about 0.1 to 2.5 per cent by weight, whereat the dosage is distributed among the different fractions substantially at a weight ratio proportional to their average particle sizes. The method is particularly suitable for use in connection with electric filtration of flue gas, in which case the additive can be added to the flue gas flow after the removal of the coarse fraction prior to the electric filters.

Description: Method for improving the concrete-technical properties of fly ash The present invention concerns a method for improving the properties of fly ash produced in the burn¬ ing of solid fuels, so as to make the ash better suitable for an additive for concrete. The fly ash is usually re¬ moved from the flue gases of a combustion plant by means of electric filters or other suitable filter and separa¬ tion apparatuses. The ash separated in this way, in par¬ ticular the ash produced on combustion of coal, has been used as an additive for concrete, partly to substitute for the binder material in concrete. It has, however, proved problematic to bring out the favourable properties of the ash, as the fly ash also includes fractions that reduce the effect of the ad¬ vantageous • fractions when used in concrete and may even suppress said effects completely. A particular problem has been the variation of the proportion of the harmful frac¬ tions in relation to the advantageous fractions, which va¬ riation results, e.g., from the operating conditions of the combustion plant. The variations may be considerable even within one combustion plant, but in particular ashes obtained from different combustion plants cause inpredict- able problems when they are used for concrete products to¬ gether. As is well known, attempts have been made to eliminate these problems by means of various treatments of the ash, such as by fractioning the ashes by screening, cyclone separation, flotation, and magnetic methods. Thereat, it has, however, been necessary to re¬ move most of the ashes (from the recovered ashes, the pro¬ portion of powder that passes through a 45 \_m screen is about 99 per cent) to achieve usable concrete-technical properties with the remaining fraction. This results in high costs of the ashes in the form of separation work and large-scale separation equipment, and correspondingly in low proportion of the usable ash of the total quantity of ashes, whereby the profitability and even the quantity of available usable ash have imposed restrictions on rational utilization of the ashes. By means of the operation of the present inven¬ tion, a substantial improvement has been provided for said problems. According to the invention, it has been dis¬ covered that it is sufficient if a coarse fraction of the ash is separated from the ashes so that in the ash to be recovered, at the maximum, approximately 50 % of the weight of the ashes consist of a fraction that is retained on a screen of 45 μm, and the recovered ash is divided into at least two fractions of different grades of coarse¬ ness, to which Na- or Ca-lignosulphonate is added at the rate of about 0.1 to 2.5 % of the weight of the ash so that the total quantity of the addition is distributed among the ash fractions of different grades of coarseness substantially at a weight ratio proportional to their average particle sizes. In particular in respect of the amounts of addi¬ tion of lignosulphonate, surprising results have been ob¬ tained in so far as the results aimed at were obtained with quite a small quantity of additive, which said result is obviously attributable to a "pickling effect" produced by the additive on the ash particles in the method of the invention. In the application of the additive, it is pos¬ sible to use two alternative procedures, depending on the construction of the flue gas cleaning equipment of the combustion plant. In connection with electric filtration, the fol¬ lowing procedure has proved the simplest one. A coarse fraction is removed from the flue gas prior to the electric filtration so that the ash separated at the electric filters yields a screening result, wherein the The figure has been drafted on the basis of. standard concrete tests, wherein the aim has been a 28-day compression strenght of about 60 MN/m^. The method of the invention has proved partic¬ ularly suitable for the treatment of fly ash produced on combustion of coal, but it is also suitable for the treat¬ ment of ash produced on combustion of peat as well as on combustion of bituminous shale, when the ash is processed so as to make it suitable for an additive for concrete.

Claims: WHAT IS CLAIMED IS: 1. Method for improving the concrete-technical properties of fly ash produced in the burning of solid fuel and separated from the flue gases, c h a r ¬ a c t e r i z e d in that a coarse fraction of the ash is separated from the ashes so that, in the ash to be re¬ covered, at the maximum, approximately 50 % of the weight of the ashes consist of a fraction that is retained on a screen of 45 μm, that the recovered ash is divided into at least two fractions of different grades of coarseness, and that Na- or Ca-lignosulphonate is added to the re¬ covered ash fractions at the rate of about 0.1 to 2.5 % of the weight of the ash so that the overall quantity of the additive is distributed among the ash fractions of diffe¬ rent grades of coarseness substantially at a weight ratio proportional to their average particle sizes. 2. Method as claimed in claim 1, c h a r ¬ a c t e r i z e d in that the quantity of additive ad¬ ministered to the ash is 0.1 to 1 %, advantageously 0.1 to 0.5 % of the weight of the ash. 3. Method as claimed in claim 1 or 2, c h a r ¬ a c t e r i z e d in-that the additive is added to frac¬ tions formed in connection with the separation of the ash. 4. Method as claimed in claim 1 or 2, c h a r ¬ a c t e r i z e d in that the additive is added to the flue gas flow after the separation of the coarse ash frac¬ tion, so as to be separated in subsequent separation sta¬ ges along with the ash fractions.

Inventors: HEIKKINEN AINO, HEIKKINEN, AINO
Applicants: FESCON OY
Application year: 1988
Main classification: C04B18
IPC codes: C04B18