Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Alkaline earth metal
Patent
1997-05-28
2000-08-08
Hruskoci, Peter A.
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Alkaline earth metal
210710, 210712, 210718, 210724, 210726, 423169, 42324308, 423554, C01F 540
Patent
active
060998148
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a humid method for the treatment of effluents containing sulfric acid, free SO.sub.2, SO.sub.3.sup.-, or SO.sub.4.sup.=.
These effluents come for example from the washing steps of fumes or flue gases by means of water.
These liquid effluents contain sulfuiric acid and consequently may not be discharged in rivers. Until now, these effluents have been treated by means of CaCO.sub.3 or lime, producing gypsum CaSO.sub.4.xH.sub.2 O as by-product, in other words producing mountains of waste material that have to be stocked or spread.
This invention aims to avoid this disadvantage, but also enables the recovery of a valorizable by-product namely magnesium sulfate.
BRIEF DESCRIPTION OF THE INVENTION
The method according to the invention is a humid method for treating effluents containing H.sub.2 SO.sub.4, free SO.sub.2, SO.sub.3, or SO.sub.4.sup.= and having a pH lower than 5, in which these effluents are reacted with a basic compound. In the method according to the invention, particles of CaCO.sub.3.MgCO.sub.3, MgCO.sub.3 or a mixture of these, possibly mixed with CaCO.sub.3 are used for treating the said effluents, the said particles having a particle size such that at least 95%, preferably about 100% of the particles of CaCO.sub.3.MgCO.sub.3 and/or MgCO.sub.3 have a particle size smaller than 75 .mu.m. After treatment, magnesium sulfate in the form of a solution or brine is recovered.
Preferably, in order to obtain a very efficient treatment, the said particles of CaCO.sub.3.MgCO.sub.3 and/or MgCO.sub.3 have a particle size such that the median diameter d.sub.50 (50% in weight of the particles having a particle size greater than the said median diameter) is between 3 and 30 .mu.m, in particular between 5 and 20 .mu.m, and is more specifically about 10 .mu.m.
Tests have shown that, by using particles of CaCO.sub.3 and of dolomite of particle size greater than 100 .mu.m, a slow and therefore non-industrial neutralization of acid solutions was obtained. Such particles are therefore not suitable for valorizing acid liquid effluents by producing magnesium sulfate brine or MgSO.sub.4.xH.sub.2 O.
Other tests have shown that the use of particles of CaCO.sub.3 and of dolomite with a too fine particle size (less than 3 .mu.m) for the industrial neutralization of acid liquid effluents was to be proscribed due to the costs of the crushing operation, the small gain in neutralization time, the dosage problems and therefore the risks of using quantities of dolomite in excess, in other words risks of discharging unreacted particles of dolomite when separating the liquid phase containing magnesium sulfate.
Furthermore, the mixing of too fine solid particles of dolomite with liquid effluents is difficult and creates problems.
It has now been observed that particles of MgCO.sub.3 or of dolomite with a well-determined particle size are to be used for ensuring an optimum industrial treatment.
Because the particles used in the method according to the invention have an optimum particle size, they can easily be mixed and dispersed in the acid solution (substantially homogeneous dispersion), so that the substantially "homogeneous" formation of a gaseous phase in the solution (formation of a gaseous phase as well in the upper layers as in the lower layers of the solutions to be neutralized) will improve the mixing of the particles, and lead to a better solidtliquid contact and thus a better reaction.
The particles of MgCO.sub.3 and/or CaCO.sub.3.MgCO.sub.3 used in the method according to the invention advantageously have a wide particle size distribution. Their distribution factor at 80%, and even at 90% is preferably greater than 1.5, in particular greater than 1.9 (for example about 2).
The particle size factors at 90% and 80% are calculated as follows: ##EQU1## in which .O slashed..sub.x% is the upper diameter of the particles of the weight fraction passing at x%
According to one embodiment, the effluents are neutralized in two steps, for example by using in each of the said steps dolomite with a r
REFERENCES:
patent: 1765424 (1930-06-01), Hageman et al.
patent: 3617560 (1971-11-01), Deul et al.
patent: 3637347 (1972-01-01), Jonakin et al.
patent: 3904742 (1975-09-01), Akimoto
patent: 4388283 (1983-06-01), Abrams et al.
patent: 5282977 (1994-02-01), Schinkitz
Colombier Simon
Langelin Henri Rene
Gunter Jr. Charles D.
Hruskoci Peter A.
Lloist Recherche et Developpement S.A.
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