PROCESS FOR RECOVERING PRECIOUS METALS
This invention relates to a process of recovering metals from dilute aqueous solutions of metal salts by contacting a dilute aqueous solution containing dissolved metals with a fibrous proteinaceous material of animal origin. In one of its more specific aspects，this invention relates to a process for recovering precious metals and base metals from an aqueous liquid containing one or more of such metals in solution by contacting said aqueous liquid with a proteinaceous substance selected from the group consisting of feathers，hair，hoof meal and horn meal.
Industrial waste waters often contain various metals which for ecological or economical reasons it would be desirable to recover or remove from the water. It is known, for example，that large quantities of gold and other valuable metals are contained in sea water, but, up until the present time at least, there has been no economic method for recovering it. Many industrial waste waters contain dissolved salts of such economically important metals as platinum, rhodium, palladium ，ruthenium，iridium，gold and silver，as well as base metals, such as zinc, aluminum, iron, copper, tin and nickel. Such dissolved metals are present as anions as well as in the form of anionic complexes and are contained in wastes，such as spent plating liquors and refinery waste solutions.
Precipitation by chemical methods and electrode position ( or electrowinning) have been used for the removal or recovery of metal values from dilute aqueous solutions. Frequently, the concentrations of the metals are so low that the cost of recovering them from solutions by these methods exceeds their value. This invention now provides a process by which it is not only possible but economically feasible to recover noble metals and other valuable metals from dilute aqueous solutions by reaction with an animal protein, especially fibrous proteinaceous materials of relatively little economic value, i. e. hair, horns, hoofs and feathers.
It has been proposed heretofore in U. S. Pat No. 2，371, 119 to recover metals from dilute aqueous solutions in which such metals are present by contacting the aqueous solutions in which such metals are present by contacting the aqueous solution with wool. The interaction of wool with metal cations is reported by Masri et al，Textile Research Journal, April, 1974，page 298，which shows that wool is capable of taking up many metals，including platinum，palladium and silver，from solutions of the chlorides and nitrates of these metals. It has now been discovered，quite unexpectedly，that certain waste or low value by - products of the meat and poultry industries are as effective or more effective than wool for the recovery of precious metals from aqueous solutions.
In the process of the present invention, common animal wastes or by 一 products of the meat industry, e, g. , hair, feathers，hoofs, and horns，are utilized for the recovery of precious metals from aqueous solutions. The process is carried out by contacting the aqueous solution with the proteinaceous material for a period of time sufficient to remove the dissolved metals from their solutions. The time of contact between the solution and the proteinaceous material may vary from about 10 minutes to 60 hours, preferably 1 to 12 hours. It has been found that some proteinaceous materials are more selective to certain of the precious metals than others. The optimum contact time required for substantial removal of any given metal from solution will depend to some extent upon the particular proteinaceous material and the particular metal undergoing recovery. Optimum contact times may be determined for any given combination of aqueous solution and proteinaceous substance. The effectiveness of the proteinaceous material for removal of a metal in accordance with this invention depends to some extent also upon the extent to which the material has been already loaded with recovered metals.
The pH of the solution undergoing treatment preferably is within the range of 2 to 3. While extraction of metals from aqueous solutions occurs over a wide pH range, test results indicate the pH values in the range of 5 to 11 are less favorabe than pH values within the range of 1.2 to 5, while maximum recoveries of precious metals occur within the preferred pH range of 2 to 3. The process suitably is carried out at a temperature in the range of from just above freezing to as high as 90°C.，preferably, in the range of 5 °C to 50°C.
The extraction of metal values from dilute solutions by the process of this invention is preferably carried out at atmospheric pressure and temperature although higher or lower pressures and temperatures may be used.
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