THE ROLE OF TABLE SALT IN THE THERMAL CALCINATION OF CALCIUM LIGNOSULFONATES

V. M. VITYUTIN, V. V. SHVETSOV

(Presented by the scientific seminar of the Department of General Chemical Technology)

Impurities (aluminum, iron, calcium) present in charges for producing green silicon carbide are partially or completely transferred during the furnace process from the reaction zone to the peripheral, colder layers [1]. There are highly contradictory data concerning the mechanism of migration of these impurities. Some investigators believe [2] that table salt, introduced into the charge as an auxiliary raw-material component, forms volatile chlorides with the impurities, thereby facilitating their removal from the carbide-formation zone (the theory of “chlorinating roasting”). Other investigators are of the opinion that table salt plays the role of a heat carrier [3].

In producing silicon carbide from a granulated charge, the thermal properties of the latter change substantially. This, naturally, also leaves a special imprint on the role of table salt in the process. In addition, the granulated charge contains sulfite-alcohol stillage (SAS), which is in close contact with the table salt.

The dry substance of SAS is a lignosulfonate complex bound with the calcium ion. Unlike impurities contained in the main raw-material components, the calcium of SAS is part of an organic compound; therefore its behavior during thermal calcination in the presence of table salt may prove to be specific.

We investigated the role of “chlorinating roasting” in the thermal calcination of SAS samples under reducing conditions. In the investigations, a concentrate of grade LKBT was used, with the following technical characteristics:

moisture content on working mass — 15%;

ash content on dry mass — 10.5%;

volatile yield on dry mass — 59.5%;

content of nonvolatile carbon — 30%.

The ash of SAS contained:

calcium oxides — 26.3%;

sesquioxides — 0.6%;

silicon dioxide — 0.9%;

total sulfur — 18.4%.

A weighed portion 2 of the dry SAS substance mixed with table salt (Fig. 1) was placed in corundum crucible 1 and covered with a layer of anthracite 4. The weighed portion and the anthracite were separated by an interlayer of dense pap-

crucible 3. The crucible was closed with lid 5 and placed under corundum bell 6. The samples were calcined at 800–1000° C in a laboratory muffle furnace, and in the temperature range 1200–1600° C—in a kryptol furnace for 1 hour. In parallel, control samples containing no table salt were calcined.

Fig. 1. Placement of the CLS sample in a crucible during thermal calcination. 1—corundum crucible; 2—sample of a mixture of CLS and table salt; 3—gasket of dense paper; 4—layer of anthracite with particle size 2–0 mm; 5—lid; 6—corundum bell.

After calcination, chlorides were washed out of the sintered beads. In the residues, the calcium oxide content was determined by the gravimetric method. In all cases, the CaO content in the investigated and control samples was the same.

The results of the chemical analysis were checked by means of the radioactive-isotope method.

A mixture of CLS containing the isotope Ca\(^{45}\), and table salt, was calcined at 1640° C for 0.5 hour until the evaporation of the salt ceased. After calcination, the activity of the main mixture, the activity of the anthracite, and also the activity of the walls of the crucibles and of the salt deposit were measured successively. The analysis showed that practically all the calcium remains in the sample, i.e., formation of calcium chloride volatile at 1640° C does not occur.

Thus, as applied to a mixture of CLS and table salt, the theory of “chlorinating roasting” is not applicable.

LITERATURE

1. N. E. Filonenko, V. A. Alferov, L. A. Borovkova. Ogneupory, No. 7, 1954.
2. M. V. Kamentsev, O. A. Bulanin. Bulletin of the Central Scientific-Research Laboratory of Abrasive and Grinding Materials, No. 1, 1936.
3. Mervald Karl-Heinz. Tsvetnaya metallurgiya (Express Information), No. 10, 1966.