Chemistry of inorganic compounds – Sulfur or compound thereof – Oxygen containing
Patent
1986-02-26
1987-11-17
Straub, Gary P.
Chemistry of inorganic compounds
Sulfur or compound thereof
Oxygen containing
423166, 423146, 423338, C01F 1146, C01B 3312
Patent
active
047073489
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method for neutralizing waste sulfuric acid by adding a salt of a weaker acid than sulfuric acid.
As appears from "Ullmann's Enzyklopadie der chemischen Technologie" 18, 579 (1982), it was common practice up to now to drain waste acid directly into the sea or into rivers. Said draining causes an unacceptable environmental effect on the surface waters. If from the waste sulfuric acid gypsum is formed by reacting it with calcium carbonate, the waste acid problem is only transformed, since in that case one obtains considerable quantities of gypsum for which an insufficient number of applications is available, particularly in view of the impurities present in that type of gypsum. (Consequently, according to the law, said gypsum is still considered a chemical waste).
It was found now, that the waste acid problem may be solved in an environmentally acceptable way by adding magnesium silicate to said waste sulfuric acid. Upon reaction with sulfuric acid two separate or easily separable phases are formed, the first of which consists of an aqueous solution of the sulphate ions and the magnesium metal ions derived from the salt and the second of which contains an SiO.sub.2 -containing precipitate, apart from the magnesium silicate that optionally is used in excess.
From a practical point of view magnesium silicate will be used, that reacts sufficiently quickly with sulfuric acid.
Magnesium silicates are e.g.: forsterite, olivine, enstatite and chrysolite.
Subsequently, the solution of sulfate ions and metal ions may be drained into surface water, or it can be used for an industrial application. This holds also for the precipitate obtained.
Magnesium is the second most important cation in sea-water and consequently there are no serious objections to the draining thereof. A similar argument holds for the sulfate ion, which is the second most important anion. Since further in case of using magnesium silicate a silica gel will be formed below the magnesium sulfate solution, and silica gel may be used for many purposes in the building arts, the use of magnesium silicate is very useful.
The reaction that occurs is illustrated by the following equation: SiO.sub.4.aq+2SO.sub.4.sup.2- +2Mg.sup.2+
Said reaction occurs completely even at room temperature (18.degree.-30.degree. C.). The reaction rate depends on the surface area available and on the temperature. In a series of experiments it appears that at a temperature of 50.degree. C. the reaction time is about 24 hours and at 85.degree. C. it is just several hours. Literature data show that magnesium-rich olivine is the most advantageous of all magnesium silicates. It may be expected that if other magnesium silicates would be used, the reaction time would be at least ten times higher.
Olivine occurs in high concentrations in nature in stones that can be obtained easily. In a preferred embodiment of the method according to the invention particularly as regards using it on an commercial scale, olivine is added to the sulfuric acid. Indeed olivine contains apart from magnesium also small quantities of other metals, like nickel, but said metals may be extracted easily by means of known methods. Further iron is present in the solution. Said iron may be advantageous, since upon precipitation of iron hydroxide heavy metals may be precipitated simultaneously and subsequently they can be separated easily.
EXAMPLES
After having observed in a series of experiments that in order to achieve a fast reaction of the liquid olivine blend it has to be stirred continuously, and that a smaller grain size (higher specific surface) and a higher temperature increase the reaction rate, some experiments with waste sulfuric acid were carried out.
SERIES I
350 g. olivine, having the following a composition: SiO.sub.2 40.7 weight percent, MgO 50 weight percent, FeO 8.9 weight percent, NiO 0.4 weight percent, were added to 300 ml waste sulfuric acid (s.g. 1.29) having the following composition: H.sub.2 SO.sub.4 29 weight percent, Fe 1.8 weight percent, Mg 0.5 weight perc
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Rijksuniversiteit Utrecht
Straub Gary P.
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