Patent GB-830674-A (GB)
Title: Improvements in or relating to a process for recovering noble metals
IPC: C22B11/02
Filed: 1957-07-05
Abstract: Noble metals entrained in a gas are recovered by passing the gas in succession through any two of the following layers or layer (c) alone:- (a) an apertured member of metallic material stable at red heat which rests on the next following layer; (b) a gas-permeable layer of acid-insoluble refractory solid; (c) a gas-permeable layer of granular solids which are heat-stable at the prevailing temperature and consist, at least at their surface, of one or more oxides of calcium, strontium, magnesium or barium, except that when passed through layers (a) and (b) the gas is first passed through a separate layer of acid-insoluble refractory material; said gas when passing through layer (b) or (c) being at a sufficiently elevated temperature to enable the noble metals to be recovered on the surface of the solids; thereafter, if layers (a) and (b) have been used, treating layer (b) with a solvent for the noble metals in which the refractory solid is not appreciably soluble and, if layer (c) has been used alone or with another layer, treating layer (c) with an acid in which the noble metals are not appreciably soluble.  The process is of particular value in gaseous catalytic processes using a noble metal catalyst which is partially volatilized and entrained in the gaseous reactants, e.g. in the catalytic oxidation of ammonia over a platinum or platinum-rhodium catalyst.  The elevated temperature in the layer or layers will be produced by the reaction heat if the reaction is exothermic.  The oxide of layer (c) may be formed in situ by decomposition of the hydroxide or carbonate, and may be a naturally occurring form such as marble, magnesite or dolomite; it should be substantially free of fluxes and have a silica content below 0.2%.  If desired, it may consist of heat-resisting insoluble solids coated with the collecting material; in such case, the consumption of acid will be reduced and the insoluble centres can be reused.  When the acid-soluble material is dissolved the metallic sludge remaining is separated, dissolved and recovered normally.  The collecting activity of the layer is increased by covering with a layer (a) which may be a metal sheet with holes or slots or a grid or wire net formed of non-sealing material stable at red heat, such as steel alloys or a noble metal such as platinum, e.g. a spent catalyst net.  The layer itself may be sub-divided into several sub-layers separated by such metal nets in which case the layers after the first need not be treated as often as the first layer for recovery of metal.  Such "activating" effect can also be achieved by covering the layer (c) with a shallow layer (b) consisting of baffle bodies of ceramics, quartz, vitreous silica or other natural or synthetic refractory material.  The layer (b) does not collect the metal to any appreciable extent and therefore need not be treated for the recovery of the metal unless this is desired, and can be used again.  In the case where layers (a) and (b) are used in conjunction the layer (a) appears to have an "activating" effect.  The collecting layer is preferably situated some distance from the catalyst in a region of reduced temperature or in a separate chamber.  The collecting bodies do not need to be treated frequently but can be left for several reaction periods without loss of collecting activity even though a metal coating is formed on the collecting bodies.  The layer may be sub-divided as described for the combination of layers (a) and (c) above.  The collected metal is dissolved, e.g. in aqua regia or chlorine water and air, and recovered normally.