FLOCCULATION AND FILTRATION OF COAL SUSPENSIONS

S. A. BABENKO, D. D. BABYCHEV

(Presented by the scientific seminar of the Department of OChT)

The filtration rate, other conditions being equal, depends on the permeability of the cake [1]. Therefore, the filtration process can be intensified by improving the structure of the cake through aggregation of fine and very fine particles into larger aggregates by adding structure-forming reagents. The reagents used for this purpose were selected by us on the basis of an evaluation of their flocculating properties on coal suspensions with a density of 100 g/l and higher. The following substances were tested: lime, slaked in the form of lime milk of 5% concentration; sodium chloride; ferric chloride; calcium chloride—all at 1%; hydrolyzed and nonhydrolyzed PAA, 0.05–0.1%; K-4, 0.1–5%.

The procedure for carrying out the flocculation experiments was as follows: a coal suspension of a specified concentration, placed in a cylinder of 250 ml capacity, was treated with the calculated amount of flocculant, and the entire contents were thoroughly mixed. At specified time intervals, the percentage of clarification was recorded. Settling was taken as 100% when the suspension, after standing for 2 days, had settled and occupied the corresponding volume, depending on the concentration of the suspension. The flocculated suspensions were filtered on a laboratory vacuum filter. All experiments were carried out on coals from a series of seams of the Chertinskaya 1 and Baibanakovskaya mines, having the following screen composition: size class −0.5 + 0.25 mm—12%; −0.25 + 0.1 mm—37%; −0.1 mm—51%. During the experiment the slurry temperature was maintained at 18–21°C. Filtration time was 2 min; vacuum, 0.5 atm.

As a result of the investigations carried out, it was found that, among the substances considered, only lime, K-4, and ferric chloride have the ability to flocculate coal suspensions noticeably. The optimum consumption of K-4 is 100 g/l, and that of lime and ferric chloride is 1 kg/t. At these consumptions, for suspensions containing 100 g/l of solids, a high percentage of settling of coal particles is ensured. (Clarification in 10 min—80%.) With an increase in solids in the suspension, the visible flocculation effect decreases, especially for coal from the seams of the Chertinskaya 1 mine. Thus, at 300 g/l solids, K-4 does not flocculate; ferric chloride clarifies by 15–22% in 10 min; lime clarifies by 30% over the same interval. K-4, lime, and ferric chloride, along with their flocculating action, improve the filterability of coal suspensions.

The specific resistance of the cake, calculated by equation [2], decreases 13-fold with the addition of K-4 (100 g/t), 18-fold with the addition of slaked lime (1 kg/t), and 4-fold with ferric chloride (1 kg/t). PAA at dosages of 50, 100, and 200 g/t does not improve the filterability of coal suspensions.

Considering the high effect achieved in filtering coal suspensions with K-4 and lime, it may be expected that they will find industrial application at the coal-preparation plants of Kuzbass.

K-4, which we recommend for industrial introduction in order to intensify the process of filtering coal suspensions, according to data in [3], is obtained by incomplete saponification of polyacrylonitrile in aqueous caustic-alkali solutions.

K-4 dissolves well in water; a K-4 solution does not change its aggregating properties over time. These qualities favorably distinguish it from PAA, a widely advertised flocculant for accelerating the processes of thickening and filtering coal slurries. K-4 is three times cheaper than PAA.

Thus, as a result of the studies carried out using coal suspensions as an example, a direct relationship has been established between the flocculating properties of a number of reagents and their ability to reduce the specific resistance of the cake.

REFERENCES

1. A. E. Scheidegger. The Physics of Flow Through Porous Media. Gostoptekhizdat, 1960.

2. A. G. Kasatkin. Basic Processes and Apparatuses of Chemical Technology. Goskhimizdat, 1960.

3. S. Zainutdinov, K. S. Akhmedov. Preparation of an artificial structure-former for soils of Central Asia. In: Humic and Polymer Preparations in Agriculture. Publishing House of the Academy of Sciences of the Uzbek SSR, 1961.