GB1408523A - Process for regenerating spent photographic silver bleaching solutions - Google Patents

Abstract

1408523 Regenerating photographic silver bleaching solutions; automatic control of concentration AGFA-GEVAERT AG 27 April 1973 [27 April 1972] 20157/73 Headings G2C and G3R A photographic silver bleaching solution containing alkali metal ferricyanide, iron (III) complex, or complex-free iron (III) salts or copper (II) salts, and alkali metal halide is regenerated by reoxidation with bromine of the ferrocyanide formed, a quantity of bromine equivalent to the concentration of ferrocyanide at any given time being produced continuously in aqueous solution in a closed vessel controlled by the redox potential of the bleaching solution and being run into the bleaching solution under the surface of the solution. The bromine may be formed by reaction between alkali metal bromate and HBr, alkali metal bromate, alkali metal bromide and HCl, alkali metal bromate, alkali metal bromide and orthophosphoric acid, or hydrogen peroxide and HBr. The bleaching solution may be continuously removed from a processing tank, reoxidized while in through-flow and returned to the tank by a pump circulatory system. While the bleaching solution is being regenerated, other processing baths particularly those in which the concentration of active substances varies in proportion to the quantity of elementary silver bleached in the bleaching bath may be regenerated at the same time by the addition to each bath of the appropriate regenerating solution in a quantity which is proportional to the quantity of bromine required for regenerating the bleaching solution, the quantity being controlled by the redox potential measured in the bleaching solution. The other processing bath may be a developing, fixing, buffering, hardening or stabilizing bath. The apparatus disclosed comprises a control instrument 326 comprising a direct current amplifier 327, electrode leads 324 and 325, and a mV-measuring instrument 328 with lower and upper limit voltage contacts 329 and 330, connected to a dosing unit 331 containing a common drive motor 332 for two dosing pumps 333 and 334 which are connected at the input end to the storage tank for hydrohalic acid 335 and storage tank for oxidizing agent 336 by flexible tubes while the output ends are connected to inlet tubes 301 and 302 of a reoxidation apparatus. The threshold contact 330 is adjusted to the redox potential corresponding to the required ratio of potassium ferricyanide concentration to potassium ferrocyanide concentration in the solution being reoxidized the motor 332 being switched off when the redox potential reaches that to which contact 330 is adjusted. The threshold contact 329 can be used to signal when the potassium ferrocyanide concentration exceeds a previously adjusted maximum level. A relay may be connected to contact 329 so that the dosing unit 331 is switched off as soon as the redox potential drops below the value to which contact 329 is adjusted.