Introduction to Mineral Processing
Froth flotation is considered to be the most widely used method for ore beneficiation. In ore beneficiation, flotation is a process in which valuable minerals are separated from worthless material or other valuable minerals by inducing them to gather in and on the surface of a froth layer. Sulfide and non-sulfide minerals as well as native metals are recovered by froth flotation. This process is based on the ability of certain chemicals to modify the surface properties of the mineral(s). Other chemicals are used to generate the froth and still others are used to adjust the pH. Certain chemicals are even capable of depressing the flotation of minerals that are either to be recovered at a later time or are not to be recovered.
froth flotation cells
|The process of froth flotation entails crushing and grinding the ore to a fine size. This fine grinding separates the individual mineral particles from the waste rock and other mineral particles. The grinding is normally done in water with the resultant slurry called the pulp. The pulp is processed in the flotation cells, which agitate the mixture and introduce air as small bubbles.
The ability of a mineral to float depends upon its surface properties. Chemical modification of these properties enables the mineral particles to attach to an air bubble in the flotation cell. The air bubble and mineral particle rise through the pulp to the surface of the froth or foam that is present on the flotation cell. Even though the air bubbles often break at this point, the mineral remains on the surface of the froth. The mineral is physically separated from the remaining pulp material and is removed for further processing.
Frothers - Orfom® F series frothers
Frothers are liquids that produce the froth or foam on which the flotation process depends. The froth resembles soap suds and provides the physical separation between the mineral(s) floated and the pulp containing the waste. The froth must be strong enough to support the weight of the mineral floated and yet not be tenacious and non-flowing. It should have the tendency to break down when removed from the flotation cell. The frother should not enhance the flotation of unwanted material. Many other characteristics are required for a foaming agent to be a good flotation frother. Typical frothers include:
- pine oil
- certain alcohols
- low molecular weight polypropylene glycols
Collectors - Orfom® CO series collectors
A collector is a chemical that attaches to the mineral surface and produces a hydrophobic (water-fearing) surface. While certain minerals are naturally hydrophobic and do not require a collector, recovery is often improved when a collector is used. This water-repellent film facilitates the attachment of the mineral particle to the air bubble. Many different chemicals are used as collectors, such as:
- petroleum sulfonates
- fatty amines
Depressants - OrfomR D Series Depressants
Depressants are chemicals that inhibit the flotation of minerals. They are used to improve the selectivity of a flotation process. They often make it feasible to separate minerals that were initially floated together.
The response of many minerals to the flotation process is often dramatically affected by pH. Flotation circuits are often operated at a pH range of 7.5 to 11.5. The exact range at any given plant is optimized for the ore at that site. Lime is often used to raise the pH of the pulp and also reduce the flotation of iron pyrite.
The particle size to which the ore is ground depends on the nature of the ore. The grind must be fine enough to liberate the mineral grain from associated rock, but producing too small a particle size is both expensive and detrimental to recovery.
Froth flotation is generally limited to size fractions between roughly 65 and 100 mesh. Particles larger than 65 mesh Tyler (210 micron) are difficult for the air bubble to lift while particles smaller than 400 mesh (37 micron) often will not attach to the air bubble.
Reagent Dosage Rate
Many factors influence the amount of reagent required for a particular application. Variations in particular size, mass of particle, quantity of mineral, and the character of the host rock are some of the many factors that influence reagent usage.
Water quality, flotation equipment size and type, temperature, and ore body variation are just a few of the variables affecting the flotation process.
The figure above is a simplified flow sheet showing the basic steps involved in a copper mill producing the byproduct molybdenum. The crushed copper ore (containing, for example, 0.75% Cu and 0.015% Mo) is fed to the grinding circuit where water is added and the ore is ground to the proper size. Lime and an oil for collecting molybdenum are usually added in the grinding circuit. A copper collector may also be added at this point.
The pulp progresses to the flotation cells. The frother and copper collector are usually added to the pulp just before the roughers. Additional copper collector is often added to the scavenger flotation cells and may be added at several other points. The froth, enriched in copper and molybdenum, is removed from the flotation roughers and scavengers. This enriched material is now called the rougher concentrate or cleaner feed. The combined rougher-scavenger concentrate may be 5% Cu and 0.06% Mo (a concentration factor of 3 to 10 times is typical).
The scavenger or combined rougher/scavenger concentrate may go to regrind where it is ground to further liberate the minerals. The rougher and scavenger concentrates ultimately reach the copper molybdenum cleaners. The cleaners, which represent another cycle of froth flotation, provide additional cleaning and upgrading of the rougher scavenger concentrate.
The concentrate, which is now approximately 30% Cu and 0.35% Mo, is pumped to the molybdenum plant where the molybdenum disulfide is separated from the copper sulfide. Chemicals referred to as depressants are added to the pulp to reduce the flotation of the copper minerals while allowing the molybdenum minerals to float. The tailings from the molybdenum plant are the final copper concentrate (typically greater than 30% copper). Following molybdenum rougher flotation and several stages of cleaning, a final molybdenum concentrate is produced. The final molybdenum concentrate is typically greater than 55%.
Lead and zinc often occur in the same ore. Copper is also present in some ores. Froth flotation is again employed to produce marketable concentrates of both. The process is identical in principle to the flotation of other minerals. The figure on the right is a simplified version of a Pb/Zn flotation circuit. The main difference in lead/zinc flotation is that zinc is usually depressed while lead (and copper) is floated. The tailing from the lead flotation circuit is the feed to the zinc circuit. The ability to selectively float the separate minerals depends on the ore, the chemicals, and other factors.
Gold, silver, copper, lead, zinc, molybdenum, iron, potash, phosphate, and even sand for glass are often processed by froth flotation. Without froth flotation, many of the metals and minerals used every day in our modern world would be much more expensive.
Solvent extraction is a hydrometallurgical process. The process entails:
- Dissolution of the metal in an aqueous, typically acid, solution
- Transfer of the dissolved metal to an organic solution
- Transfer of the dissolved metals to a second aqueous solution
- Production of a product
The product is usually electroplated to produce the pure metal or precipitated from the solution to produce a metal salt.
Copper, uranium, vanadium and other metals are produced by solvent extraction.
The figure below is a simplified schematic of a typical copper solvent extraction process.
Oxide copper ore, low-grade copper sulfide overburden, and other copper-containing materials are stacked into a heap. Leach solution (pH approximately 2) is sprinkled or flooded on top of the heap and percolates through. The leach solution containing the dissolved metal eventually flows out of the heap into a collection pond. This material is referred to as pregnant leach solution. Oxide copper minerals are very soluble in the leach solution. Sulfide minerals are rendered soluble by certain bacteria that feed on the sulfide minerals and convert them to soluble sulfates and other soluble forms. This biological action tends to maintain the acidity of the leach solution. Typical operations will leach some areas of the heap, enlarge other sections, and let others rest.
The pregnant leach solution is pumped to the extraction mixer where it is intimately mixed with organic solution. The organic solution is a chelating agent diluted in a hydrocarbon solvent. The chelate reacts with the copper and removes it from the aqueous phase. The mixture flows to the settler where the organic phase and aqueous phase separate. The aqueous phase, now called raffinate, is pumped to the raffinate ponds and eventually back to the heap to leach more copper.
The loaded organic phase goes to the strip circuit where it is mixed with a strong acid solution. The chelate releases the copper to this aqueous phase, called the strip solution. Again the mixture goes to a settler where the organic and aqueous phases separate. The organic solution returns to the extraction circuit to load more copper, thus repeating the cycle.
The strip solution is pumped to the tankhouse where the copper is electrowon (electroplated). This solution then returns to the extraction circuit to repeat the cycle.
Direct electrical current is applied to the electrowinning solution in the tankhouse. A copper starter sheet functions as the cathode, and additional copper deposits on the surface of the sheet. When the copper cathode has reached a certain size it is removed from the electrowinning tank, washed, and prepared for sale. The cathode will be copper of greater than 99% purity.
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