Coagulation of Finely Dispersed Precipitates of Metatitanic Acid

I. V. Gebler, V. M. Vityugin, O. A. Fuks

(Presented by the scientific-methodological seminar of the Chemical-Technological Faculty)

The isolation of titanium from solutions of its salts can be carried out by hydrolysis. In the process of titanium hydrolysis, upon prolonged boiling of the solution, a precipitate of metatitanic acid is formed, distinguished by its exceptional fineness of dispersion, which makes it difficult to separate it from the mother liquor.

In the case of titanium hydrolysis from sulfuric-acid solutions, particles of finely dispersed metatitanic acid combine into micelles. The micelles form aggregates, which determine the particle size of titanium dioxide. The binding of micelles is intensified under the action of the sulfate ion. Such a precipitate with enlarged particles is filtered fairly easily.

During titanium hydrolysis from hydrochloric-acid solutions, the finely dispersed suspension of metatitanic acid does not coarsen, and separation of the precipitate is a very difficult process. The precipitate passes through all ordinary filters, is not centrifuged, and does not settle over a long period of time. In such cases, to isolate the precipitate, a coagulant is used, for which polyvinyl alcohol is employed. However, the use of this coagulant in large quantities entails considerable cost.

We have found that, for acidic suspensions, water-soluble humates can be good coagulants.

Sodium or ammonium salts of humic acids are obtained from peat or brown coal by treating them with a solution of sodium hydroxide or ammonia. Humic acids, which are present in considerable quantities (up to 50%, and sometimes more) in peats and humic brown coals, have a condensed aromatic structure and are oxycarboxylic acids of high molecular weight. In addition to carboxyl and phenolic groups, they also contain other peripheral groups. The most mobile is the hydrogen of the carboxyl group, through which salts of humic acids are formed. Humic acids and their salts have a permutite character, i.e., they are capable of cation exchange. Cation-exchange reactions take place in dilute solutions. Strong acids in solutions of high concentration decompose humates with liberation of free humic acid.

The action of a humate coagulant in hydrochloric-acid solutions of titanium proceeds as follows. In an acidic suspension, the humate decomposes and free humic acid is liberated in the form of a dispersed precipitate possessing a large total particle surface and prone to flocculation. The larger aggregates formed in this process carry along with them the co-

fine particles of metatitanic acid carrying a positive charge. The negatively charged particles of humic acid neutralize the positive charge of the metatitanic-acid particles, and this leads to coagulation of the precipitate. The precipitate becomes coarse-grained and readily filterable. The yield of metatitanic acid in hydrochloric-acid hydrolysis is 96–98%, and the yield on filtration is up to 100%. The filtrate remains completely clear.

A necessary condition when using a humate coagulant is maintenance of an acidic medium of the solution (pH < 1) before and after addition of the humate. The consumption of humate is 1–3% of the weight of solids in the suspension, and after its addition the reaction of the medium changes little. Sodium salts are removed from the precipitate when it is washed, and the organic matter of the humic acids burns off when the precipitate is calcined.

Preparation of a Humate Coagulant from Peat

Peat is dried to a moisture content of 20–25%, ground to 3–0 mm, and placed in bags made of strong cotton fabric, filling them halfway, taking into account the swelling of the peat. The bags of peat are placed in a vertical position on the second perforated bottom of a boiler heated by an open flame. For each kilogram of humic acid contained in the peat, 20 liters of a 1% solution of caustic soda are poured into the boiler. Boiling is continued for 6–8 hours, with periodic addition of evaporating water so that the bags of peat are constantly below the liquid level. At the end of boiling, the solution is discharged through the tap between the bottoms of the boiler into a settling reservoir, and into the boiler, to wash the humate out of the sludge, as much water is added as there had been alkali solution. After one hour of boiling, the liquid is discharged into the same settling tank and combined with the first solution. After settling for 24 hours, the liquids are decanted, leaving in the settling tank the sludge, which is discarded.

The concentration of the resulting solution, which depends on the moisture content of the peat taken and on its humic-acid content, is determined colorimetrically against a standard solution. By evaporating the solution, the desired concentration of humate can be obtained, or it can be isolated in dry form.

Ammonium humate has the advantage that it is completely removed when the precipitate is calcined, without the need to wash out the cation.

To prepare a working solution of ammonium humate, bags of peat are prepared and placed on the grate of a vessel with a double bottom (in a desiccator). Onto the bottom of the vessel is poured a 10% aqueous solution of ammonia, calculated as 1 liter per 10 kg of peat. The peat remains in the vessel for 3–5 days to absorb the ammonia, and after this it is transferred for boiling into a boiler with water. The boiling process and separation of the solution are carried out under the same conditions as in obtaining sodium humate. The concentration of the solution is established colorimetrically against a standard solution of ammonium humate.

The ammonium humate coagulant gives complete separation of the precipitate in the filtration of acidic suspensions.

LITERATURE

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2. G. A. Stadnikov. Chemistry of Coal. Moscow–Leningrad, Scientific-Technical Publishing House, 131, 1932.
3. G. A. Stadnikov. Analysis and Investigation of Coals. Moscow, Publishing House of the Academy of Sciences of the USSR, 124, 1936.
4. V. S. Veselovskii. The Chemical Nature of Combustible Minerals. Moscow, Publishing House of the Academy of Sciences of the USSR, 1955.