Routes for Dewatering Fine Suspensions and Colloidal Solutions
Abstract Generated abstract
This paper examines methods for dewatering fine suspensions and colloidal solutions, especially materials with particles below 100 micrometers produced by wet grinding. It compares settling, centrifugation, and evaporation, arguing that settling and centrifugation remain too slow or insufficiently effective for very fine particles, while evaporation using direct industrial-frequency electric heating in electrode apparatuses offers a more suitable route. Laboratory work with graphite electrodes on aqueous alumina suspension identified operating field intensities of about 40 to 45 V/cm initially and 80 to 90 V/cm near completion, reducing a 1:2 solid-to-liquid suspension to 5 to 10 percent moisture in about 4 minutes with 90 to 95 percent useful electrical-energy utilization. The study presents direct electric heating as a compact, controllable, and productive approach for concentrating fine wet-ground materials.
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PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE
NAMED AFTER S. M. KIROV, HOLDER OF THE ORDER OF THE OCTOBER REVOLUTION AND THE ORDER OF THE RED BANNER OF LABOR
Vol. 275
1976
METHODS FOR DEWATERING FINE SUSPENSIONS
AND COLLOIDAL SOLUTIONS
V. P. Pishchulin
(Submitted by the scientific-methodological seminar of the Department of Processes, Apparatuses,
and Cybernetics of Chemical Production)
At the present time, the problem of intensifying the process of dewatering fine suspensions and colloidal solutions is becoming increasingly important, since the operation of dewatering fine suspensions obtained after wet grinding of material in ball, vibration, and colloid mills is the longest stage, limiting the entire process of material preparation.
An analysis of investigations and of the practice of carrying out the process of concentrating the solid phase with particle sizes of \(0 \div 100\ \mu\text{m}\) in fine suspensions obtained in mills indicates that dewatering of fine suspensions and colloidal solutions can be carried out by settling, centrifugation, and evaporation.
Dewatering of fine suspensions is most easily carried out by settling. In industry, various designs of settling tanks are used. Naturally, the rate of the settling process is generally very low, especially for particles of small diameter; therefore, to carry out the settling process, large production areas, a small thickness of the suspension layer, and a long process time are required. Some intensification of the settling process is possible by increasing the temperature of the suspension, as a result of which the viscosity of the medium decreases. However, even when the temperature is increased to \(85 \div 97^\circ\text{C}\), the settling rate of particles of size \(0—10.0\ \mu\text{m}\) in an aqueous suspension of various materials is \(0.7 \cdot 10^{-6} \div 1.5 \cdot 10^{-4}\ \text{m/sec}\) [1]. Thus, the settling process for fine suspensions, characterized by low efficiency and long duration, does not meet the modern requirements of industry.
The dewatering of fine suspensions by centrifugation is carried out more effectively. Thus, sedimentation of solid particles approximately \(50\ \mu\text{m}\) in size, at a drum rotation speed of about \(8000\ \text{rpm}\), is accomplished in \(10—15\ \text{min}\), with a total process duration of about 1 hour; reducing the particle size requires a longer process time and a higher drum rotation speed, that is, carrying out the process of dewatering fine suspensions by centrifugation also proves to be insufficiently effective.
It is most expedient to carry out the process of concentrating the solid phase from fine suspensions by removing the dispersion medium—water—by evaporation or boiling. The process of separating the solid phase can be carried out in spray-type dryers, or with
to complete evaporation to dryness. Dryers with suspension spraying greatly intensify the process of liquid removal; however, at the same time, considerable dust carryover is observed in spray dryers.
The characteristics of the process of evaporating fine suspensions and the advantages of direct electric heating favor the choice of industrial-frequency alternating electric current as the heat-transfer medium in electrode-type apparatuses [2].
As a result of studies carried out on a laboratory installation with graphite electrodes, all the data necessary for calculating industrial installations with direct electric heating were obtained, and the optimal conditions for carrying out the dewatering process of a fine aqueous suspension of alumina, obtained after grinding in a vibratory mill, were determined:
the intensity of the alternating electric field at the beginning of the process is 40–45 V/cm for graphite electrodes; the field intensity at the end of the process is 80–90 V/cm.
In this case, the time required to thicken the initial suspension with a solid-to-liquid phase ratio of 1 : 2 to a final moisture content of the solid phase of 5–10% was about 4 min; the coefficient of useful utilization of electrical energy was 90–95%.
Further operation of the installation for concentrating the solid phase of various materials with particle sizes from 0 to 10 µm, obtained by wet grinding in vibratory and colloid mills, showed good operability and reliability of the installation.
Thus, the use of direct electric heating makes it possible to significantly accelerate the evaporation process, increase the productivity and the utilization coefficients of the installation volume, and provide fine control of heat generation during the process, with a high coefficient of useful utilization of electrical energy, in a simple and compact installation of continuous or periodic operation.
REFERENCES
- U.S. Patent No. 3079243, 1963.
- N. I. Gel’perin. Evaporating Apparatuses. Moscow–Leningrad, Goskhimizdat, 1947.