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The reagent scheme employed in a small flotation concentrator should be as simple as possible consistent with metallurgical goals. The utilization of a multiplicity of reagents having the same function is to be scrupulously avoided. Methods of reagent preparation and associated equipment, or the lack thereof, can have a profound influence upon the success or failure of the facility.
If possible, reagents should be added from a centralised position in the plant. If head tanks are used, the overflow must proceed back to the tank of origin. By this means, solution strengths can be corrected by re-circulation. Screens for the removal of oversized materials should be included at appropriate points in the system. Reagents which are staged to flotation are usually added by means of a cup and disc feeder located at the appropriate position along the flotation bank.
Spillage of reagents will occur and the system must be designed to allow for such spillage. Usually the spill is collected in a tray and directed to a strap from which large quantities of spilled reagents may be reclaimed. Usually, the small amount of material which normally collects in this sump will be intermittantly discarded to tailings.
Brief comments relative to the preparation and feeding of various reagents are as follows:
a. Lime – Calcium hydroxide, usually applied as a water-lime slurry known as milk-of-lime, is the most important flotation pH modifier. In developed areas, pebble lime is usually available. This material is converted to milk-of-lime by means of a lime slaker or small grinding mill operated in closed circuit with a classifier. In underdeveloped areas, the lime sources may be extremely variable. In some cases, the only lime available may be pulverized slaked lime of relatively low calcium oxide availability usually packaged in 50 kilogram bags. The average percent calcium oxide availability and the type of calcined lime which can be acquired locally should be determined prior to the design of the lime preparation system. In some cases it is best to design a dual system which can handle either pebble or bagged lime. The prepared lime product is usually stored in an agitated vessel and delivered to appropriate, addition points in the circuit by means of a circulation loop which commences and terminates at the milk-of-lime storage vessel. The design of a functional milk-of-lime feeding system can be difficult. Usually, milk-of-lime is withdrawn from the loop header by means of air operated pinch valves located in close proximity to the loop line. Valves are opened and closed by an air pulse generated by a solenoid operated valve which in turn is driven by a percent-of-cycle timer.
b. Sulfuric Acid – In certain flotation concentrators, sulfuric acid is used to decrease the flotation pulp pH. In remote areas, the acid may be of doubtful quality. Consequently, fiberglass reinforced plastic or opoxy lined storage tanks should be considered for acid storage. In addition, if precise pH adjustment is required, concentrated acid must be diluted to avoid wide swings in pH as the reagent is added to the pulp. Acid resistant dilution tanks should be supplied and acid resistant piping and valving must be included under these circumstances. The transfer of acid from one location to another should be accomplished by low pressure compress air.
Frothers are usually added undiluted and required only an appropriate cup and disc reagent feeder attached directly to the reagent shipping drum.
Mineral collector reagents are manufactured in several forms including those which can be used as full strength liquids and those which must be dissolved or diluted. Reagents which can be fed directly are usually added in a fashion similar to that employed for frothers. For other reagents, dilution or dissolution tanks coupled with constant head tanks are usually provided. These reagents are added to the flotation circuit in dilute form through percent-of-cycle timers and solenoid valves (similar to the milk-of-lime system) or through rotometers.
Activators and Depressants
Chemicals which are used to activate or depress minerals are usually added as acqueous solutions. Consequently, dilution or dissolution tanks with associated head tanks must be provided. It should be noted that certain depressants tend to be corrosive to mild steel. Therefore, appropriate coatings for reagent tankage must be specified.
Flocculants and Filter Acids
These reagents are only sparingly used in small concentrators and are normally added as aqueous solutions.