Published on December 12, 2009
SILVER EXTRACTION FROM HYPO FIXER SOLUTION
Silver is a very precious and important metal. So recycling of silver from waste solutions of X-ray clinics, photographers is a profitable business. Silver is a major cost component of photographic films. Every film either X-ray or photographic, has a thin coating of silver over it. During image formation, silver transfers from image portion of films and comes to the fixing solution called 'hypo fixer'. Due to repetition of fixing process of several films in same solution, more and more silver keeps on dissolving in hypo fixer solution. When the amount of dissolved silver reaches to saturation level, no more fixing of films can be done in such solution. This exhausted waste solution is the basic raw material for business of extracting pure silver.
RAW MATERIAL ( HYPO)
The concentration of silver is hypo/fixer solution varies from process to process and time to time. During photographic film processing, silver leaches into the fixer solution (sodium thiosulphate) from the photographic film and forms silver thiosulphate complex ions. This is metal displacement process. Generally hypo solution is rich in silver (6-10gm/lit.). Before processing the solution, the silver concentration can be checked using 'SILVER ESTIMATION PAPER'.
Silver testing process is very simple. A small silver estimation paper is dipped in the solution and then the change is colour is compared with the colour chart accompanying it. By this method silver concentration can be determined in gms/liter within 5 seconds. Purchase price of fixer can be estimated accordingly. Moreover it helps to determine the profitability and reduce the risk factor.
EXTRACTION OF SILVER FROM HYPO
The methods mainly used are; A) Chemical method and B) Electrolytic method.
A) CHEMICAL METHOD:
1) Determine gm/lit of fixer solution using silver estimation paper.
2) Take 1 lit. solution and to it add approximate amount sodium sulphide ( Na2S) and caustic soda ( NaOH). Using common ion effect we can remove silver from it.
3) Using the proper proportion as per the upper estimation add proper amount of sodium sulphide ( Na2S) and caustic soda ( NaOH) to all the solution.
4) We get black residue at bottom due to reaction and the solution at the top becomes yellow in colour. Filter the solution and retain the black residue. Dry the residue using filter paper. Get it melted and molded into bricks by Goldsmith. While melting we can use Borax / Na2CO3 / CaCO3 etc. In this way shiny silver metal is recovered from black silver residue.
B) Electrolytic method:
silver can be extracted from hypo using semi automatic or fully automatic machines. The most appropriate model of such machine can be selected according to our working and financial capacity.
Anode used in such machine is made up of carbon or graphite and Cathode is mostly made up of large stainless steel or other alloy plates. By filling the waste solution into the machine and connecting to the electricity mains, machine starts working. Silver present in the solution starts depositing over silver collecting plates. The machine should be operated till all the silver present in the solution gets deposited on cathode plates as black flakes. The process time can be estimated by checking silver availability using silver estimation paper time to time during operation.
It is not essential to attend the machine continuously while operating. Machine can be operated with full tank / half tank /quarter tank filled. No special training, industrial process , power, load line or license is required for this operation.
This information is compiled by Deepak V. Doddamani, Mumbai. Email: firstname.lastname@example.org
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