Abstract: The present invention is a Ca(OH)2 particle composition and a method of making the composition, the composition consisting essentially of dried calcium hydroxide particles having a moisture content of less than 2 percent by weight of the total composition, a specific surface area greater than 30 m2/g, a total nitrogen desorption pore volume of at least 0.1 cm3/g, and a CO2 content of less than 2 percent by weight of the total composition, the aforementioned particles being in the form of a mixture comprising first fraction of particles having a size of less than 32 micrometers and a second fraction of particles of a size greater than 32 micrometers, the weight percent of the second fraction ranging from 20 to 50 percent, the aforementioned particles having a nitrogen desorption pore volume consisting of pores with a diameter ranging from 100 to 400 Angstroms.

Abstract: In the method for the humid treatment of effluents containing a compound selected from the group consisting of H.sub.2 SO.sub.4, free SO.sub.2, SO.sub.3.sup.-, or SO.sub.4.sup.= and having a pH lower than 5, the effluents are reacted with a basic compound. Particles selected from the group consisting of CaCO.sub.3.MgCO.sub.3, MgCO.sub.3 and a mixture of these, are used for treating the said effluents, the said particles having a particle size such that at least 95% of the particles selected from the group consisting of CaCO.sub.3.MgCO.sub.3 and 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.