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666.762.5:622.75/77 In the manufacture of ceramics from oxides and highly refractory compounds, the process by which the finely milled raw material is washed from iron chlorides may take 2-4 weeks as a result of the stability of the finely milled suspension. It may require as many as 5-8 decantations [i]. The washing of zircon containing ~5-7% of metallic iron can take 4 weeks under the conditions of acid separation used at the Experimental Plant of the Ukrainian Scientific-Research Institute of Refractories which uses ammonium sulfate as the coagulant.In some production processes, various coagulants combined with synthetic high-molecular, flocculants [2,3] are used for the rapid precipitation of carbonaceous, ferrous, quartz, clayey, and other slurries including magnesium hydroxide.We therefore studied, under laboratory conditions, the rapid precipitation of zircon suspension using several coagulants and flocculants either combined or separately.The coagulants used were A12(SO~)3 and FeCls [2] and for comparison we also studied the formerly used (NH4)2SO4. The flocculants were polyacrylamide (PAA): hydrolyzed polyacrylamide; polyethyleneimine (PEI) with a molecular weight of I00,000; and the VA-212 flocculant based on aminated polystyrene.The effect of these reagents on the precipitation of the suspension was studied in relation to the changes in the concentration of zircon particles at a depth of 30 mm from the upper level of the suspension after 24 h and from the depth of the cleared layer of a column of suspension over this period.The addition of coagulants to suspensions of various concentrations, including the 30% concentration used at the Experimental Plant, did not improve the deposition of the zircon particles; no differences were observed in the effect of the various coagulants used. The amount of unprecipitated particles in a 15% suspension without the addition of coagulants was 34% and with coagulants, 32.5-34.5%; in a 20% suspension these figures were 42.3 and 39.6-46%; in 25%, 55.6 and 52.3-56.4%; and in a 30% suspension, 67.7 and 65.6-68.8%, respectively. All the flocculants we tested completely removed the particles from the experimental layer after 24 h.The effect of the flocculants was evaluated from the kinetics of the clarification process of a 20% zircon suspension. Of those tested the best flocculant was PAA (Fig. i).Ten minutes after the addition of the PAA the thickness of the clarified layer was 40 mm from the initial level of the suspension. The other flocculants clarified to a depth of only i0 mm in the Same time. The significantly more efficient action of the PAA can be explained by the fact that its chains, via its own active groups, bond the smallest particles of the zircon into large aggregates, floccules, which are easily separated and precipitated in a dispersion medium.Next, we studied the effect of the concentration of the suspension, the pH of the medium, and the concentration and durations of the PAA addition [2,4]. The polyacrylamide was in the form of a 0.1% solution.It ...

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Previous work done at the Institute established that refractories based on electrofused baddeleyite can be used in contact with liquid metal [i, 2]. To clarify the grain-size composition requirements of bodies made of such material, and in order to study the influence of additives on the properties of the refractories, we carried out further investigations which are described in this article.Specimens of two standard sizes were used: cylinders of diameter 25, height 35 mm; and cubes with edges of 30 mm, pressed from granular bodies based on baddeleyite, stabilized with 6% CaO, electrofused under a pressure of i00 N/mm 2, and fired at 1580 and 1750~The chemical composition of the starting materials is shown in Table I, and the grainsize and properties in Tables 2 and 3. An increase in the content of AI20 ~ from 1 to 3% improves the sintering and reduces the open porosity of the specimens after firing; it also increases their density.Further increase in the alumina addition from 3 to 5% has no marked effect on sintering, and hardly changes the properties of the articles after firing.There is some improvement in the sintering of the body with increase in the amount of finely dispersed constituent from 30 to 50% (specimens 62-a and 65). In this case in specimen 65 the highest Ocomp value is attained (154.4 N/mm2), and an adequately high thermal-shock resistance to destruction (9-11 heat

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