PRECIPITATION OF CHROMIUM-CONTAINING TANNING SOLUTION OF SUSPENSION MAGNESIUM HYDROXIDE

Abstract

Comparative studies of the deposition of tanning chromium compounds from waste tanning solutions of tannery production containing from 3 to 5 g/l chromium oxide were carried out, using as a chemical reagents a suspension of magnesium hydroxide and sodium carbonate solution. It is shown that the main factors influencing the precipitation process are the pH value and the consumption of the precipitant. The costs of reagents contributing to complete precipitation of chromium compounds in the form of a hydroxide are established. The high efficiency of the use of a suspension of magnesium hydroxide as a precipitator is shown, due to the higher deposition rate and the absence of chromium compounds in the supernatant. The rate of precipitation of chromium using a slurry of magnesium hydroxide is 4.9 cm3/min, and using sodium carbonate 2.5 cm3/min. In the supernatant, after precipitation with sodium carbonate, traces of chromium oxide are found - less than 2.53 g/l. It is shown that when using a suspension of magnesium hydroxide for the precipitation of chromium, periodic mixing of the liquid is necessary to more completely dissolve the precipitator, which in the first stage after dosing drops out of the solution as a precipitate. When sodium carbonate is used, the formation of hydroxyl ions occurs much faster than in the case of a suspension of magnesium hydroxide. However, the formed sediment has a rather loose structure and is characterized by a high moisture content. The precipitate of chromium hydroxide obtained by using a suspension of magnesium hydroxide is characterized by a higher density and better filtration ability. A gradual increase in pH during the precipitation of a tanning solution with a suspension of magnesium hydroxide results in a precipitate with a large particle size and lower humidity.