Volume 234

1974

EFFECT OF POLYACRYLONITRILE ADDITION ON THE FILTRATION PROPERTIES OF ASBESTOS-CEMENT SUSPENSION

S. L. BABENKO, V. M. VITYUGIN, A. P. VEROZHSKII, N. S. DUBOVSKAYA,
E. B. EZHOVA

(Presented by the scientific seminar of the Department of Silicate Technology)

The existing method for forming asbestos-cement products is based on filtering the suspension through the screens of screen cylinders, followed by dewatering of the primary layers on a forming drum.

The productivity of forming machines and the quality of the products depend mainly on the filtration properties of the asbestos-cement suspension. The filtration rate of the suspension can be increased by coagulating it under the action of coagulating additives. A significant coagulating effect is produced by synthetic organic high-molecular substances of the class of higher polymers [1]. Among the polymers tested in the asbestos-cement industry, polyacrylamide is known [2, 3]. The structure-forming effect of a polymer, as is known [4], depends on the size and shape of the macromolecules, the degree of branching, the number and kind of active functional groups, the number of these groups per polymer unit, and their distribution in the polymer. On this basis, it may be assumed that polyacrylonitrile (PAN) will be a more active additive coagulating an asbestos-cement suspension.

In the present work, studies are reported on the influence of a PAN additive on the filtration properties of an asbestos-cement suspension. The purpose of the investigations was to study the activity of PAN as a flocculant for the suspension.

The high activity of the polymer gave the authors grounds for setting up the experiment as follows: the effect of a PAN additive on the water requirement of cement, its setting rate, and its strength in the hardened state; the filtration properties of an asbestos-cement suspension containing PAN. The PAN studies were carried out in comparison with the known coagulant—polyacrylamide (PAA).

The materials used in the work were from the Yashkino cement plant: additive-free Portland cement of grade “500” and asbestos in the form of a mixture of several grades of asbestos with an industrial degree of opening equal to 63%.

The chemical and mineralogical compositions of the cement are given in Table 1. The asbestos mixture contained grade P-5-65 asbestos in an amount of 60%, P-6-45 in an amount of 15%, and M-6-40—25%.

The effect of the additive on the water requirement of the cement was studied by evaluating the limiting shear stress of the cement paste from the depth of immersion of the rod (pestle) of the apparatus for determining normal consistency. At the same time, the setting times and the strength of the hardened cement were measured. The results of determining the physicomechanical properties of cement mixed with water containing PAN in various amounts are shown in Fig. 1.

Table 1

Characteristics of the cement

Cement paste with a water–cement ratio equal to 0.3, with the addition of PAN, becomes more plastic; the setting interval of the paste is somewhat shortened. A slight increase in strength is observed. The optimum addition is 0.015–0.025%.

The coagulating action of PAN was determined from the change in the height of the sediment and of the clarified column of suspension at different additive contents. The concentration of the suspension studied was 5%.

Fig. 1. Structural-mechanical properties of cement.
1, 2 — plasticity of the paste;
3, 4 — setting interval;
5, 6 — hardening strength at 20°C after 12 hours;
7, 8 — hardening strength at 20°C after 28 days.

The filterability of the suspension was established from the rate of filtration through an industrial screen under model conditions. The experimental data are given in Fig. 2 and Fig. 3.

The maximum flocculating action of PAN appears at a content of 0.025%. In this state the suspension is filtered at the highest rate. Under the microscope, flocculation of the suspension and active adhesion of the cement hydration products to the asbestos fibers are clearly visible. It should be especially noted that entrainment of fine particles of the suspension with the filtrate is practically absent.

In view of the high activity of PAN as a coagulant for asbestos-cement suspension, tests of the additive under industrial conditions are planned at the Yapkinskii cement plant. On the basis of all that has been stated above, the following conclusions may be drawn:

1. PAN is an active flocculant for asbestos-cement suspension, superior to the known additive—polyacrylamide.
2. Under the action of PAN, the water demand of the cement decreases and the setting times are shortened.
3. The physicomechanical properties of the hardened cementing component of the suspension change insignificantly.
4. The filtration rate of the suspension increases.
5. Entrainment of fine suspension particles during filtration is absent.

Fig. 2. Flocculating action of the additive.
0—suspension without additive; I, II, III, IV—suspension with additive, respectively, 0.01%; 0.05%; 0.025%; 0.03%.

Fig. 3. Filtration rate of the suspension

REFERENCES

1. E. V. Gerasimova. Use of high polymers for clarification of sludge waters from coal-preparation plants. Research Works in the Coal Industry, No. 1, Ugletekhizdat, Moscow, 1959.

2. I. I. Bernei. Asbestos-Cement Technology. Collection of Scientific Papers, Magnitogorsk, 1967.

3. I. Ya. Leichenko, Z. M. Borunova, V. N. Tarasenko. Investigation of polyacrylamide as an additive to asbestos-cement stock. Problems of Progressive Technology of Building Materials, Krasnoyarsk, 1965.

4. G. L. Maslenkova. Investigation of the structure-forming properties of polymers by infrared spectroscopy, KZh, vol. 23, issue 5, Publishing House of the Academy of Sciences of the USSR, Moscow, 1961.