of the waste solution coming from the pcb manufacturing process is usually about 104 u00b0 c no additional advantages can be obtained by increasing the temperature of the aqueous solution to exceed the boiling point under pressurization when the reaction temperature is too low the reaction proceeds at an unduly low velocity as a matter of course it is preferable in order to ensure smooth and rapid proceeding of the reactions that the additive metal is in the form of a powder granules small shots or thin flakes having a relatively large surface area although the substitution reaction expressed by the above given reaction equation i proceeds at a high velocity proceeding of the reaction may be inhibited when the surface of the additive metal is covered by the precipitates of the copper metal in this regard it is important that the reaction mixture is agitated in such an intensity that the precipitates of the metallic copper deposited on the surface of the additive metal can be rapidly removed off from the surface of the additive metal when the additive metal is manganese or zinc in particular the reaction of the additive metal with the free hydrochloric acid according to the reaction equation ii given above is sometimes so violent as to cause splashing of the solution by the evolved.