APPLICATION OF PEAT AS A REDUCING AGENT AND BINDER IN THE CHLORINATION OF SOME NATURAL MATERIALS

S. I. Smolyaninov, R. A. Beskrovnaya

(Presented by Prof. I. V. Gebler, Doctor of Technical Sciences)

Chlorination is preceded by the operations of grinding the components, charge preparation, briquetting, and coking of the briquettes. A definite role must also be played by the choice of carbonaceous material and binder.

An orientation toward chlorination in a stationary bed leads to the need to have a lump material possessing certain physical properties—sufficient mechanical strength and porosity. For this purpose, a special binder material is added to the mixture of the reducing agent—some carbonaceous material (soot, charcoal, petroleum coke, etc.)—and the charge is subjected to briquetting. The subsequent coking of the briquettes is intended to strengthen them, increase their porosity, and also remove volatile substances.

As in any heterogeneous process, an increase in the total contact surface increases the rate of interaction of the reacting substances. Therefore the ore components are usually ground to 100 mesh.

It would be no exaggeration to say that all subsequent operations in raw-material preparation—charge preparation, briquetting, and coking of the briquettes—are determined by the choice of reducing agent.

According to literature data, various materials are used as reducing agents: soot, charcoal, petroleum coke, and anthracite.

The choice of reducing agent should apparently be determined both by technological objectives and by the economic aspect of the matter.

From the standpoint of the chemical process, preference should be given to a reactive reducing agent. Such agents are, first of all, soot and charcoal. A second factor from this same point of view is the ash content in the reducing agent. However, this factor may have determining importance, apparently, only when pure starting substances are subjected to chlorination. With respect to natural ore concentrates containing a considerable amount of impurities, the ash content may be limited by a substantially higher value. The least ash-forming reducing agent is soot. Charcoal is also a low-ash reducing agent. The ash content in charcoal lies within the range ...

1–2.5%; it is almost independent of the species of wood and is determined mainly by the bark content.

All the materials indicated are briquetted in mixture with ore only with special binder additives, which, according to the literature, include sulfite-cellulose liquors, coal-tar pitch, resin, dextrin, and a suspension of colloidal titanium dioxide (1).

The amount of binder that ensures good briquette quality is found experimentally. For example, in the practice of chlorinating pure zirconium oxide, a charge is used consisting of 81% zirconium oxide, 14.5% gas black, and 4.5% dextrin (2).

On a number of grounds, peat should be the most suitable material for the stated purpose. The advantages of peat may be:

1. High reactivity of peat coke. In reactivity toward carbon dioxide and water vapor, peat coke approaches charcoal. Excellent activated carbon and carburizer are produced from peat coke. Thus, peat coke is one of the best reducing agents.

2. Low ash content. Raised bog peats have an ash content of 2–4%.

3. Availability.

4. Low cost of extraction.

With modern highly mechanized methods of peat extraction, peat is one of the cheapest types of fuel. Milled peat has an especially low cost. Thus, at leading peat enterprises in Belorussia, the cost of milled peat is 9–13 rubles per ton; moreover, this cost is not the limit [3]. The cost of lump machine-molded peat is twice as high.

By 1965 the cost of milled peat will be 10–13, and that of machine-molded peat 40–45 rubles per ton of standard fuel [4].

1. Simplification of the briquetting technology.

Work by the Department of Chemical Technology of Fuels of the Tomsk Polytechnic Institute [5, 6] has shown the possibility of obtaining so-called fuel-smelting materials by adding a certain amount of ore and fluxes to raw peat at the moment of its molding. These same studies established the influence of molding pressure, the initial and final moisture content of peat, and the amount of added ore and fluxes on the mechanical properties of peat moldings and the coke obtained from them. By this method, a fuel-smelting material suitable for use as blast-furnace fuel in low-shaft furnaces is obtained by molding a mixture of iron ore, fluxes, and processed peat with a working moisture content of not less than 80% under slight pressure (2 kg/cm²), followed by natural drying of the peat moldings. Thus, the operations of briquetting under increased pressure (not less than 50 kg/cm²) are eliminated, and there is no need for a binder additive—the binding of the ore components occurs due to peat colloids. On coking the indicated peat-ore briquettes, a sufficiently porous and strong coke is obtained, suitable for use in small-volume shaft furnaces.

On the basis of the foregoing, we investigated the possibility of obtaining briquettes from ore and a reducing agent based on peat from the Tagan deposit in Tomsk Oblast.

Experimental Part

Peat from the Tagan deposit is characterized by the following data:

\[ W^{p} — 85\%, \qquad A^{c} — 7.3\%, \qquad V^{r} — 70\%. \]

Raw peat with a working moisture content of 85% was processed once in a meat grinder, thoroughly mixed with a known amount of concentrate of the natural degree of comminution, and the mixture was passed through the meat grinder three more times. From the resulting mixture, briquettes weighing 150 g were formed on a lever press under a pressure of 2 kg/cm² with a one-minute holding time under pressure. The briquettes were dried in the open air under laboratory conditions at a temperature of 18–20° and a relative air humidity of 20–25% to a final moisture content of 18–20%. Upon reaching the specified moisture content, the briquettes were subjected to coking in a muffle furnace to temperatures of 400 and 800°. The rate of temperature rise was 3–4° per minute. To prevent oxidation during coking, the briquettes were covered with charcoal powder. Upon reaching the specified temperature, the briquettes were held for 20 minutes, cooled in air for 15 minutes, and finally in a desiccator with calcium chloride. After this, tests for mechanical strength were carried out. The following were determined: crushing resistance, the strength index according to K. I. Syskov [7] (work of fracture per unit increase in surface area), and abrasion resistance. The latter determination was carried out in a cylindrical drum with metal balls, constructed by us after the type of the drum of the All-Union Institute of Mineral Raw Materials [8] (abrasion coefficient—the percentage yield of the fraction smaller than 3 mm). In the method for determining the strength index, we also took into account the 0.5–0 mm fraction, since this correction makes, in our opinion, this index more characteristic.

Briquettes were prepared with the theoretical amount of reducing agent, and with a two- and threefold excess of it.

The amount of reducing agent generally depends on the composition of the ore concentrate and on the chlorination conditions.

According to literature data, up to temperatures of 600°, during chlorination in the presence of carbon, the main portion of the oxygen is bound into carbon dioxide; above 600°, the reaction proceeds with the formation of carbon monoxide. In the latter case, the amount of reducing agent is doubled. Some authors believe [9] that an excess of reducing agent is harmful, since the latter accumulates in the chlorination reactor and clogs it. This formulation of the question seems incorrect to us. On the contrary, with the theoretical amount of reducing agent, chlorination in a fixed bed will inevitably lead to fully chlorinated briquettes crumbling, thereby disrupting the hydraulic regime of the chlorinator. Clogging of the chlorinator when there is an excess of reducing agent can be avoided by design measures, by ensuring continuous discharge of the residue.

The theoretical amount of reducing agent was calculated by us on the basis of the formation of carbon monoxide. Conversion of this amount of ashless coke to working peat with a moisture content of 85% gave the following composition of the briquettes: a) theoretical amount of reducing agent—raw peat 87%, ore concentrate 13%; b) twofold excess of reducing agent—raw peat 93%, concentrate 7%; c) threefold excess of reducing agent—peat 95.5%, concentrate 4.5%.

Table 1 gives the test results for air-dry peat–ore briquettes and thermally treated briquettes. As can be seen, the pressure that destroys the briquette is high for air-dry briquettes (150–210 kg/cm²). With an increase in the final heating temperature, it drops to 30–60 kg/cm² at 800°. Depending on the amount of concentrate added, the crushing resistance of the air-dry briquettes

Table 1

Testing the mechanical strength of peat–ore briquettes

up to a 7% addition it does not change, and then it decreases sharply. At 400°, the splitting resistance is maximal with an 8% addition. At 800°, as the amount of additive increases, it rises.

The abrasion of the briquettes increases with temperature up to 100% at 800° for additions of 13 and 7% concentrate. For briquettes with a 4.5% addition, abrasion is maximal at 600–700°.

The abrasion of air-dry briquettes is very low and practically does not depend on the amount of additive. When the percentage of additive is increased above 7%, at temperatures of 600–800°, abrasion increases strongly with increasing additive and reaches 100% at 800° and a 13% addition of concentrate.

The strength index of the briquettes, with an increase in the final heating temperature, drops rapidly up to a temperature of 400°, and then changes almost not at all. Depending on the percentage of concentrate added, the strength index falls at all temperatures.

Analysis of the results obtained makes it possible to conclude that peat can fully be used as a reducing agent, with the production of peat–ore briquettes by forming a mixture of concentrate and raw peat. Air-dry peat–ore briquettes are extremely strong at all tested amounts of concentrate additions. Their strength is not lower than, and even exceeds, the strength of metallurgical hard-coal coke. Consequently, such briquettes are transportable and can easily withstand transportation and reloading.

The strength of the coke obtained from these briquettes is 3–5 times lower than the strength of the air-dry briquettes. However, their strength is quite suffi-

sufficient for use in a fixed bed in chlorinators with a not very great bed height. In this case, it appears that combining the processes of coking and subsequent chlorination in one apparatus may be especially effective.

The indicated strength of the briquettes and of the coke obtained from them is not the limiting value. With an ore component of finer grinding, the strength of peat–ore briquettes increases sharply. Thus, briquettes with an addition of 7% ore having a particle size of less than 200 mesh have a crushing resistance of 100–130 kg/cm².

As regards the ratio of reducing agent to concentrate, from the standpoint of strength it is possible to use all the mixtures tested, and the final decision will be determined after investigation of the chlorination process itself.

We carried out chlorination using peat and charcoal as the reducing agent. The experiments showed that peat as a reducing agent is not inferior to charcoal.

Conclusions

1. Peat is proposed as a reducing agent and binder for the chlorination of natural minerals.

2. The mechanical strength of coke from peat–ore briquettes permits their use for chlorination in a fixed bed.

3. Peat as a reducing agent in chlorination is not inferior to other previously used carbonaceous materials, but it is favorably distinguished from the latter by the low cost and simplicity of obtaining briquettes.

References

1. Meerson G. A., Zelikman A. N. Metallurgy of Rare Metals. Metallurgizdat, 1956.

2. Zirconium and Zirconium Alloys, Amer. Soc. for Metals, 1953.

3. Mikhnevich P. A. Peat industry and peat transport in Belorussia. Peat Industry, No. 7, pp. 24–28, 1959.

4. Tsuprov S. A. On the state of technology and the directions of its development for 1959–1965 in the peat industry of the USSR. In: Prospects for the Comprehensive Utilization of Peat Resources of the Novosibirsk Economic Region. Publishing House of the Novosibirsk SNKh, 1959.

5. Gebler I. V., Smolyaninov S. I., Martynov A. F., Severin B. M. Effect of pressure and moisture on the properties of peat as a metallurgical fuel. Peat Industry, No. 8, pp. 16–20, 1959.

6. Gebler I. V., Smolyaninov S. I., Potapenko V. E., Kosolapov V. Influence of additions of ore and fluxes on the properties of peat as a metallurgical fuel. Proceedings of TPI, vol. 111.

7. Taits E. M., Letova V. K. Evaluation of coke strength from laboratory specimens. Factory Laboratory, No. 10, pp. 10–15, 1947.

8. Koifman B. K. Micromethod for testing the mechanical stability of coals. Factory Laboratory, No. 6, pp. 741–745, 1947.

9. Bardin I. P., Reznichenko V. A. Investigations in the metallurgy of titanium. In: Metallurgy of the USSR, 1917–1957, vol. 1, Metallurgizdat, pp. 583–623, 1958–1959.