Dewatering of slimes from phosphate ores

#This invention relates to a process for dewatering the slime waste product of the phosphate ore mining and beneficiating industry.

#Phosphate ore is mined and is beneficiated in order to recover the phosphate value thereof in a more concentrated form. In a conventional beneficiating process as practiced in the State of Florida, a slurry of phosphate ore is screened on a 150 mesh (Tyler) screen and the minus 150 mesh fraction is discarded. This minus 150 mesh fraction is commonly referred to as "slime". Similar slimes can be produced at other stages of the beneficiating process, for example, when the concentrated phosphate produce obtained by froth flotation is further treated to improve the quality thereof. The slimes are aqueous suspensions or colloidal dispersions containing, typically, on the order of from about one to 5 percent by weight of solids and the balance is water. The solids are clay-like substances. Very large quantities of slimes are produced in the phosphate mining industry. According to conventional practice, the slimes are disposed of by placing same in settling ponds, called slime ponds. The solids do not rapidly settle and it has been reported that some slime ponds have not dried out after 50 years. Slime ponds occupy large areas of land in the phosphate mining region in the State of Florida. The phosphate ore beneficiating process requires large amounts of water, typically about 10,000 gallons of water per ton of phosphate ore. A large percentage of this water is discharged with the slime and it cannot be recycled to the beneficiating process. The water requirements of the phosphate mining industry are very large. Further growth of the phosphate mining industry is hindered by the needs of other water users, such as farmers and the increasing population of Florida, and the need to limit the amount of fresh water that is withdrawn from the aquifer (fresh water source) in Florida in order to prevent serious and possibly irreparable injury to the aquifer.

#It is an object of this invention to provide a process for dewatering the aqueous slime waste product of the phosphate ore mining and beneficiating industry so that the water contained in the slime can be recovered and recycled to the beneficiating process and so that the solids can be recovered in a more or less dry, concentrated, easily handled, agglomerated form suitable for disposal in a land fill or the like.

#The drawing is a schematic flow diagram of the process of the invention.

#As described above, the aqueous slime material treated by the process of the present invention is a pumpable aqueous suspension or colloidal dispersion produced as a waste product of the phosphate ore beneficiation process. It usually contains from about one to about 5 percent by weight, typically about 4 percent by weight, of solids having a particle size of minus 150 mesh and the balance thereof is essentially water. However, the solid content of the slime is not critical and slimes containing greater or lesser concentrations of solids can be used in the invention.

#The aqueous slime is pumped by a pump 1 through a conduit 2. A mixture of treating liquids which preferentially wets the solid particles of the slime is combined with the slime in the conduit 8. During start-up of the process, the mixture of treating liquids is fed from the tank 3 by pump 4. During steady-state operation, most of the mixture of treating liquids is recycled treating liquid supplied from conduit 9 and "make-up" treating liquid is supplied from tank 3. The liquid mixture is comprised of (1) a bridging liquid which is immiscible with water and displaces the water from the surfaces of the suspended solids and (2) one or more conditioners which change the wetting properties of the solids of the slime by being selectively absorbed on the surfaces of the solid particles to alter their normal surface properties, whereby the bridging liquid can displace the water from the surfaces of the solid particles. The bridging liquid is supplied from tank 5 and the conditioner or conditioners are supplied from tanks 6 and 7.

#Useful bridging liquids include hydrophobic organic liquids such as liquid hydrocarbons including fuel oil, toluene and kerosene, nitrobenzene, chlorinated hydrocarbons such as carbon tetrachloride, perchloroethylene, and trichloroethane, and the like. The hydrophobic organic liquids are referred to as bridging liquids in view of their function of creating bridges between the solid particles whereby to form agglomerates. Conditioners are substances which are physically or chemically adsorbed at the solid-liquid interface. The conditioner will usually have a balanced hydrophilic-hydrophobic molecular structure and it is absorbed on the solid with the hydrophobic portion oriented outwardly so that the solid will be wetted by the bridging liquid. Organic acids such as tall oil, xanthates and their soluble salts, alcohols and long chain aliphatic amines, particularly primary amines RNH.sub.2 wherein R is alkyl having 8 to 15 carbon atoms, all having a large hydrocarbon nucleus, are useful as conditioners, as is well known.