Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Patent
1992-07-07
1994-07-19
Bell, Mark L.
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
106707, 106710, 106DIG1, 106792, C04B 204
Patent
active
053305728
DESCRIPTION:
BRIEF SUMMARY
There are two main categories of fly ash from electricity power stations. Silico-alumina ash has pozzolan-like properties and is used in concrete and mortar. Sulfur-lime ash has both pozzolan-like properties and hydraulic properties and could be used more widely as a hydraulic binder in concrete and mortar. This second category occurs, in particular, as a by-product resulting from power stations burning a powder mixture of coal and lignite in varying proportions. This fly ash includes a high proportion of calcium compounds, in part as initially present in the combustion products, and in part as added to the combustion by-product during treatment to remove sulfur from the smoke, hence the term "sulfur-lime".
Quicklime, which is sometimes present in large quantities in the calcium compounds confers an undesirable tendency on mortar or concrete constituted in part by such ash, whereby the mortar or concrete swells, thereby limiting the uses to which they may be put. It is known to be desirable to slake this quicklime without simultaneously spoiling the hydraulic and pozzolan-like potential of the other materials in the ash.
Quicklime swells on hydration because of a phenomenon whereby quicklime gives rise to slaked lime having an apparent volume of more than twice the volume of the quicklime. In addition, since the quicklime in fly ash hydrates very slowly, swelling phenomena may appear several weeks or months after concrete containing untreated raw ash has been cast.
For some types of ash (e.g. from a fluidized bed) in which the quicklime is slaked by water together with setting phenomena, it may also be advantageous to slake the quicklime prior to use in order to avoid, inter alia, giving off the heat which corresponds to slaking during implementation.
It is difficult to slake quicklime contained in ash. The water applied to the raw ash should not hydrate the lime aluminates and silicates that confer pozzolan-like properties to the ash. In addition, the quicklime present in fly ash obtained by burning a fuel hydrates very slowly, apparently because of the presence of the silicates and the aluminates which, by combining with the slaked lime in the first place, slow down hydration to the cores of grains of quicklime. It can thus be understood that slaking ash must satisfy two contradictory conditions: hydrating the mixture for long enough to slake all of the quicklime; while simultaneously interrupting the effect of this hydrarich prior to destroying the hydraulic and pozzolan-like properties of the mixture.
Solutions to this contradiction are disclosed in Document FR-A-2 501 669. It is known that hydration at above 120.degree. C. does not spoil the hydraulic properties of the silicates and aluminates present in ash. Proposals have therefore been made to slake ash at this temperature and under pressure so as to have liquid phase water available, since quicklime does not have affinity for steam when the ash is too hot to allow the steam to condense. However, the method proposed in Document FR-A-2 501 669 suffers from a certain number of drawbacks since it requires the ash to be prior heated and it requires a reaction vessel into which superheated hydration water can be injected at a temperature greater than 130.degree. C., which water must not vaporize before it has reacted with the ash, the method further requiring means for stirring the ash in order to ensure that it mixes intimately with the superheated water.
The present invention seeks to provide a cheaper alternative to this method of treatment, enabling ash to be treated at ordinary temperature in a first stage of the method and then proceeding with a second stage of the treatment at a higher temperature and under pressure to finish off the hydration of the quicklime while inhibiting reactions that could lead to a loss of the hydraulic potential of the mixture.
To this end, the present invention provides a method of slaking fly ash containing quicklime, the method comprising a first stage in which the ash is wetted at ambient temperature and a treatment
REFERENCES:
patent: 5100473 (1992-03-01), Mitsuda et al.
Chemical Abstracts, vol. 100, 1984, (Columbus, Ohio, US) B. A. Shikhov et al: "Two-step treatment of lime with water in the production of highly dispersed calcium hydroxide", voir p. 109, resume 36400c Khim. Prom. st. (Moscou) 1983, (11), 674-6.
Chemisches Zentralblat, No. 13, 1960, E. Pischunger et al: "Das Loschen von kalk mit NaCl-und CaCl.sub.2 -Losungen" voir p. 4319 & Przemysl chem. 38. 97-99. Feb. 1959 Torun, Uniw. M. Kopernika, Kat. Techn. chem.
Baalbaki Oussama
Blondin Jacques
Bell Mark L.
Charbonnages De France (Etablissement Public)
Marcantoni Paul
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