Английский для Тх / adsorptive Bubble Separation and Dispersed Air Flotation

Fig. 10. De-inking flotation process system for waste paper purification and recycle ( 123 ).

adjust the pH. Then the slurry is conditioned with a fatty acid soap, which selectively coats the calcium carbonate particles with an insoluble soap making them hydrophobic and collectable. In the flotation machine, which mixes as well as disperses fine air bubbles into the slurry, the coated particles attach to the bubbles so that they can float to the surface and be removed from the machine. The clay and silt, which are still water-wetted, remain in the slurry ( 124).

12.2.2. Mechanism of Froth Flotation

Many theories have been advanced concerning the mechanisms involved in surfacing the mineral particles so as to create a hydrophobic hydrocarbon film on the mineral surface, and many investigations have been carried out to define these mechanisms. When froth flotation is used in an aqueous medium that carries the solids to be separated (together with dispersed air bubbles and possibly an organic liquid) a threeor possibly a four-phase system must be considered. In most froth flotation processes, the solid particles are initially completely water-wetted, and the solid–liquid interface must be replaced by

a solid–gas interface by using suitable reagents. Studies of the changes in thermodynamic properties such as free energy and chemical potentials have done much to advance the understanding of flotation ( 124).

Whether the adsorption of the reagent at the surface of the solid is by physical adsorption, chemisorption, or chemical reaction, there is a definite correlation between the flotability of most minerals and the solubility of the compound formed by the collector agent and the compound to be floated. Theory postulates the formation of an insoluble metal organic compound at the particle surface. For example, in the use of xanthates for the collection of sulfides, it has been shown that the insoluble metal xanthate formed with lead and copper results in a floatable particle. With the more soluble zinc xanthate, zinc sulfide is not floatable unless it is activated by the addition of copper sulfate. Although the actual reactions may be more complex, the following simplified reactions of an organic compound with a polar–nonpolar configuration such as a xanthate are indicative:

The organic portion, R, in the xanthate is generally obtained from alcohols ranging

from ethyl to amyl (C2H5OH to C5 H 11OH).

The mechanism for surfacing (collecting) the carbonate and oxide compounds by the use of fatty acid or fatty acid soaps can also be described by the formation of insoluble organic metallic compounds at the surface of the particles. The use of an oleic acid soap (NaO2H33C18 ) for the flotation of limestone (CaCO3) demonstrates the basic chemistry:

13. ANALYTICAL METHODS AVAILABLE FOR PROCESS MONITORING

Influent concentrations and residual concentrations of cationic surfactants, anionic surfactants, cationic polyelectrolyte, anionic polyelectrolyte, proteins, colloids, oxygen, ozone, detergents, suspended solids, and so on, in the adsorptive bubble separation systems can be determined by the analytical methods reported in the literature (82,127–149).

14. GLOSSARY

Important flotation process terminologies are briefly introduced in this section (1,75,124).

Activators: Activators selectively react with particles to cause the collector to surface. The classic example, as mentioned above, is the use of copper sulfate for the activation of zinc sulfide so that it can be collected by standard sulfide mineral collectors. Another example is the surfacing of lead carbonate, copper carbonate, and copper oxide with the use of sodium sulfide so that collection is also possible by the sulfide collectors.

Air Dissolving Tube or Retention Tank: A metal tank in which the water flow and compressed air are held under high pressure for several minutes to allow time for the air to dissolve into water.

Air to Solid (A/S) Ratio: A/S is the ratio of the pounds (or kilograms) of air available for flotation and the pounds (or kilograms) of suspended solids to be floated. The A/S ratio is independent of flotation surface area.

Clarified Effluent: The liquid being discharged from the flotation unit.

Collectors: Collector reagents are used to provide a water-repellent surface on the particles to be floated in order to obtain adherence to the air bubbles. The collectors are classified according to their cationic or anionic reaction and type of minerals to be floated. The anionic types of collectors react with the metal portion (cation) in the compound to be floated, whereas the cationic types react with anion portions.

Degree of Treatment: The desired or required degree of treatment depends on objectives. It may meet the effluent discharge standards, or the requirements for water reuse, or the quantity/quality of recovered material.

Depressants: Depressants act to prevent the surfacing of the collector on a particle. An example is the use of zinc sulfate in preventing zinc sulfide from floating while allowing lead sulfide to be collected. Sodium silicate not only aids in the dispersion of slimes or colloidal material, but depresses silica and silicates. The cyanide ion aids in selectively assisting in the activation of lead sulfide but depressing pyrite (iron sulfide) and zinc sulfide. Lime concentrations at a relatively high pH 10–12 depress pyrite, allowing the copper or zinc sulfide mineral to float. Organic colloids such as starch, glue, and tannins act as dispersants, and an excess can prevent any collection. In controlled amounts, they are used to depress carbonaceous material, clays, talc, and calcium carbonate.

Design Flow: Feed or influent to be applied to the flotation unit for design purpose. It can be obtained by examining existing or expected flow data.

Float or Floated Sludge: The concentrated material scooped or skimmed from the top of the flotation unit. The concentration is measured in percent solids, or mg/L.

Flocculating Agent or Flocculant or Coagulant: Any chemical that can convert soluble or colloidal substances to insoluble flocs.

Flotation Aid: Any chemical that produces coagulation, breaks an emulsion, and/or aids in the adsorption of air bubbles by the liquid or particles to be removed.

Flotation Chamber or Flotation Tank: A tank where the water enters and the air comes out of solution in minute bubbles throughout the entire volume of liquid for flotation of impurities.

Froths: A frothing agent is used to form a stable yet brittle froth at the surface of the flotation machine so that the froth can be removed from the slurry along with the attached particles, thus accomplishing a separation of the hydrophobic and wetted particles. The froth generated should be able to support the particles, but should also readily break down when removed from the flotation machine so as not to interfere with subsequent processing. Frothing agents function by reducing the surface tension of the water. Compounds used are generally heteropolar. They contain an organic nonpolar radical that repels water and a polar portion that attracts water. Used in limited quantities, the heteropolar molecules are aligned at the gas–liquid interface with the polar end toward the water and the nonpolar end toward the air. This monomolecular film tends to retain the size of the bubble formed by the flotation machine and prevents the bubbles from breaking as they

feed concentration Li

Ethe surface excess of the solute, g-mole/cm2, or mg/cm2

Kan equilibrium constant for surface adsorption, cm

Kan equilibrium constant for surface adsorption, g-mole/cm2

Lthe bulk surface active solute concentration, g-mole/cm3 , or mg/cm3

Lthe surface active solute or solid concentration in the net overhead, g-

n

Qthe volumetric rate of liquid upflow in a foam separation reactor, cm3/s.

Qthe volumetric rate of air, cm3/s

Qthe volumetric liquid flow rate of net overhead, cm3/s

rthe surface tension of the solution, dyne/cm

Rthe universal gas constant, dyne-cm/(g-mole K)

Tthe absolute temperature, K

Ttemperature in °C

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