BULLETIN
OF THE TOMSK POLYTECHNIC INSTITUTE NAMED AFTER S. M. KIROV,
ORDER OF THE OCTOBER REVOLUTION AND ORDER OF THE RED BANNER OF LABOR

Volume 257　　　　　　　　　　　　　　　　　　　　　　　　　1973

SOME QUESTIONS OF THE DESIGN IMPLEMENTATION OF THE EXTRACTION PROCESS IN TWO- AND THREE-PHASE SYSTEMS

V. I. Poltavtsev, L. A. Popova, Yu. G. Chistokhin

The intensive development of industry in recent years has posed the task of creating high-performance equipment for the process of extracting various substances from the solid phase. The development of equipment design for the process shows that the most difficult tasks in creating new designs of extractors are:

a) transportation of the solid phase;
b) ensuring complete contact of solid particles with the solvent (extractant);
c) elimination of back-mixing of the extractant;
d) reduction of mechanical friction and abrasive wear of the moving parts of the apparatus;
e) attainment of the same residence time of particles of the solid material in the apparatus.

In world extraction practice, the simplest and most widespread apparatuses in the period up to 1940–1950 were percolators, which have not lost their importance even at the present time [1, 2]. As a rule, a percolator is a vessel, most often cylindrical, with a conical bottom and a hinged upper hatch, into which the solid phase is loaded. The principal disadvantages of the apparatus, despite its exceptional simplicity and low cost, are:

a) open discharge of the solid phase, often with the use of manual labor, which leads to intensive contamination of the production atmosphere by volatile and harmful solvents;
b) difficulty of mechanizing the process because of the periodicity of loading and unloading the solid phase;
c) a rapid decrease in the rate of the process and poor conditions of contact of solid particles with the liquid.

Elimination of the latter shortcoming led to the appearance of rotating horizontal extractors with continuous feeding of the extractant, but again with periodic loading of the solid phase [2]. Improvement of the conditions of contact between the particles and the liquid gives rise to the effect of back-mixing of the extractant, which substantially reduces the driving force of the process. This drawback, together with the complexity of the design (the presence of sliding trunnions supplying liquid and steam, the drive, etc.), prevented this design from becoming widespread.

Further development of the extraction process is proceeding along the line of creating apparatuses with discrete motion of the solid phase and continuous motion of the liquid.

One of the most widespread apparatuses of this type is the screw extractor, for example the Hildebrandt extractor.

In it, the motion of the liquid and of the solid material is carried out in countercurrent; moreover, if the liquid moves as a continuous phase, then the solid material is distributed in portions located between the flights of the screw. The advantages of apparatuses of this type are:

a) a high coefficient of filling of the useful volume of the apparatus with solid material;

b) continuity of the countercurrent process, making it possible to include the apparatus in continuous technological lines;

c) simplicity of design.

At the same time, this type of apparatus has certain disadvantages, which depend on the kind of raw material being processed. In the case of plant raw material, plugs form in the extractor at the points where the screws are coupled, which impede the motion of the extractant and create stagnant zones of unprocessed material. Elimination of plug formation led to the appearance of extractors with a rotating screw that simultaneously performs a reciprocating motion (for example, the extractors of Yu. N. Slavyanov [3]), in which the mass of raw material is displaced upward without the formation of stagnant zones.

When granulated materials with a rigid porous skeleton are used, the friction between the screw flights and the material increases sharply; moreover, the degree of crushing of the material entering the clearances between the screw and the housing increases, which raises energy consumption. Contact between the liquid and the material is incomplete.

Various designs of screw apparatuses have been proposed [1, 4, 5], in which the most favorable conditions for contact between the particles and the liquid are created. Among the successful designs of this type should be counted the apparatus of Yu. F. Artamonov et al. [6].

However, the shortcomings of screw-type apparatuses are aggravated in the Artamonov design by the complexity of the drive and stuffing-box seals, as well as by the low filling coefficient of the extractor.

Another line in the development of apparatuses with discrete motion of the solid phase and continuous motion of the liquid led to the emergence of a group of conveyor-type extractors [7, I, 8], with the conveyor arrangement being either vertical or horizontal: the extractors of De Smet, Olier, and others [8, 9].

The high productivity of the indicated apparatuses makes it possible to obtain a low-cost product, which is their undoubted advantage. But practice has shown that these apparatuses are applicable only to plant materials (sugar beet, cane, oilseeds, etc.). The processing of materials with abrasive properties (ores, slags, sinters) in apparatuses with a large number of moving parts is uneconomical owing to rapid wear of structural components. To some extent such wear is reduced in the rotor extractor [I], where the material is loaded into baskets forming a closed rotor. For materials with abrasive action, apparatuses with minimal mechanical friction, of the Kennedy [10] and Bonotto [I] types, are also applicable.

The principal shortcoming of the latter two designs is that at each treatment stage the material moves as a dense mass in front of the scraper, as a result of which access of the extractant to the particle surface is hindered. In addition, in the Kennedy extractor the occurrence of back-mixing of the extractant is possible, while in the Bonotto extractor the flow of fresh extractant mixes only weakly with the liquid in the volume of the stage. The use of a rotating rotor with simultaneous pulsating liquid feed to improve phase interaction is likewise not free from abrasive wear of the moving parts of the apparatus [16].

Thus, the presence of a large number of moving parts operating in a corrosive-abrasive medium, and the difficulty of obtaining good mixing of the liquid and particles by mechanical means, led to the creation of apparatuses with a three-phase system: solid—liquid—gas. Designs with such a system may be made both with pneumomechanical and with pneumatic mixing.

Apparatuses with pneumomechanical mixing may have a central airlift or peripheral airlifts. Such apparatuses, despite their complexity and bulkiness, mix the pulp excellently, approaching in their type ideal-mixing reactors. However, when they are connected into a technological line, it is impossible to obtain a cascade of extractors with countercurrent motion of the liquid and solid phases, which is most

more desirable in leaching. The same drawback is possessed by a cascade of Pachuca tanks [11, 12] with pneumatic mixing, which are used in large-tonnage ore processing.

The energy consumption for mixing in Pachuca tanks is usually greater than in apparatuses with mechanical mixing, but “Pachucas” have no moving parts and therefore are more economical (lower repair and maintenance costs), and also provide better quality of mixing and aeration of the pulp [13]. The impossibility of creating countercurrent contact between the liquid and the solid material in a cascade of Pachuca tanks led to the creation of another method for carrying out countercurrent mass-transfer processes in a liquid medium [14, 15].

The method is implemented in a vertical cylindrical column, each section of which is equipped with a gas lift for mixing the reacting phases. For countercurrent movement of the solid material from bottom to top, an additional gas lift is installed in each section. However, such a design has the following substantial disadvantages:

a) installation of an additional gas lift reduces the economy of the process (the consumption of high-pressure air increases);

b) the additional gas lift creates intensive back-mixing of the liquid, as a result of which the driving force of the leaching process decreases;

c) the series connection of sections requires the use of compressors to supply high-pressure air.

It follows from this that parallel gas feed to each of the sections, arranged horizontally, not only makes it possible to mix the liquid and solid phases, but also to transport the latter from them in countercurrent to the liquid [17].

Conclusions

Analysis of existing designs of extractors and of the principles of their operation shows that:

1. The countercurrent method of carrying out the extraction process is the most preferable.

2. The phenomena of back-mixing of liquid in extractors should be reduced to a minimum.

3. Pneumatic mixing and mechanical pulsations in the T–L system are an effective means of intensifying heterogeneous processes.

4. Apparatuses with a T–L–T system provide countercurrent movement of the liquid and solid material and operate on low-pressure gas.

References

1. R. N. Richles. Chem. Engng. 49, 15, 157 (1965).

2. E. Schechter. Extraction Apparatus. Published by the Scientific and Technical Administration of the Supreme Council of the National Economy of the USSR, Moscow, 1929; translated from German.

3. Yu. N. Slavyanov, N. A. Filippin. Inventor’s certificates No. 128962 and No. 131748.

4. V. A. Vvedenskii. Inventor’s certificates No. 115410 and No. 152872.

5. I. I. Pelishenko. Abstract of a Candidate dissertation. Kharkov, 1965.

6. B. M. Azizov et al. Inventor’s certificate No. 190332.

7. S. S. Il’in. Inventor’s certificate No. 822292.

8. Schütze H., Wetzel K. Chem. Techn. 19, 3, 175 (1967).

9. Ind. Alim. Agr., 1961, p. 799.

10. P. G. Romankov. Proceedings of the Leningrad Technological Institute named after Lensovet, issue 15, 1947.

11. Lamont A. Canad. J. Chem. Engng. 36, 4, 153 (1958).

12. I. Little. U.S. Patent No. 2773749; 1956.

13. N. P. Galkin, V. B. Tikhomirov. Basic Processes and Apparatus of Uranium Technology. Gosatomizdat, Moscow, 1961.

14. I. A. Yakubovich, O. B. Nevskii. Inventor’s certificate No. 144465.

15. I. A. Yakubovich, O. B. Nevskii. “Non-Ferrous Metals” No. 10, 33.

16. V. I. Poltavtsev, M. I. Kurochkina, V. P. Vysotskii, M. M. Dolzhanov; Inventor’s certificate No. 328923.

17. V. I. Poltavtsev. Abstract of a Candidate dissertation. L., 1970.