This progress report discusses the development of a non-hazardous process for leaching metals from electronic scrap using sodium thiosulfate. The report outlines problems with an initial experimental procedure that attempted to recover gold from gold pins using nitric acid, sulfuric acid, and hydrogen peroxide solutions. These caused gold to separate prematurely from the solution. The report proposes using either an acid/peroxide or aqua regia process instead, and provides details on making a "poor man's aqua regia" solution using sodium nitrate, hydrochloric acid, and water. Maintaining the sodium thiosulfate solution at 65°C or above and using a hydrogen peroxide concentration of 3-5% are also recommended

1. COMSATS Institute of Information and Technology Lahore
Department of Chemical Engineering
Progress Report
Development of non-hazardous process for leaching of metals
from electronic scrap and their characterization

2. Problem Statement
Recovery of gold and silver from electronic scrap using non-hazardous leaching process.
Introduction
Thiosulphate leaching
Two kinds of thiosulphate is used in leaching of gold, sodium thiosulphate and ammonium
thiosulphate. Here we will discuss sodium thiosulphate [Na2S2O3] which is commonly available
as table salt. Gold and thiosulfate will form a stable complex with the presence of oxygen[1].
Being an alkaline process it is operated in the pH range of 8 to 10, therefore, there are no risks
of corrosion of materials. Other advantage of this process is that it is nontoxic and thiosulphate
is less expensive.
Thiosulfate when combines with the gold, a complex formation occurs i.e. auro-thiosulfate [Au
(S2O3)2
3-
] which is accountable for the gold recovery as shown in equation.
4Au + 8S2O3
2-
+ O2 + 2H2O 4Au (S2O3)2
3-
+ 4OH-
The kinetics of sodium thiosulfate leaching gold was studied by Moore [2], taking into account
that thiosulfate will be prone to decomposition at high temperature, the suitable temperature
is between 65 to 750C.
Heath [3] studied the reaction of thiosulfate leaching in ammonia medium where he showed
the higher the concentration of reagents, the faster the leaching reaction goes.
Yen [4] found that the dissolution of copper minerals is the main reason for the increase of
thiosulfate consumption.

3. Experimental Procedure:
This process is done to recover and refine gold from gold pins. The process is as follows:
1- Gold plated pins are separated from the processor by using flame gun.
2- We make leaching solution which contains 50% nitric acid, 25% Sulfuric acid and 25%
water.
3- We take 3g gold pins and 15 ml of leaching solution in a beaker and heat it to 35 ◦C
with magnetic stirring. As this reaction is corrosive, it is done in a fume hood.
4- The solution turns green and we filtered it.
5- We add 12ml HCl and 6 ml H2O2 in the pins filtered earlier. The solution starts to
bubble and black color is obtained.
6- We again filtered the solution and add 2g sodium thio-sulphate. The solution was then
filtered out.
Problems:
The problems faced are:
1. Gold was already separated in 3rd
step which should not happen.
2. Gold was separated out in the 5th
step of procedure which was hard to separate from
filter.
3. Our solution had no gold in it because gold was already separated in earlier steps.
4. On adding sodium meta bi-sulphate, a white sludge is produced in the beaker which is
CuCl (I) due to excess Cl.

4. Solution:
There are two types of processes used[5] :
1. Acid/Peroxide process (AP)
2. Aqua Regia process (AR or Poorman AR)
AR ( HCl/Nitric) and Poor Man's AR ( HCl/Sodium Nitrate, Potassium Nitrate or Ammonia
Nitrate) are pretty much the same processes they both have pretty much the same
outcome and effect on the material you are working with. They both have the same
advantages and pit falls when working with them.
 They are both using HCL and a nitrate whether it be processed nitric or a nitrate bearing
substance such as sodium nitrate, potassium nitrate or ammonia nitrate.
 They will both create silver chloride when elemental silver is involved in what you are
processing.
 They can both be used in recovery and or refinement of PM's.
 They both will put pretty much all metals and or PM's into solution when properly used
in their correct ratios.
 They both will work in ambient temperature solutions but work better in warmed
solutions.
 They will both can cause a runaway reaction due to exothermic heat produced from
their chemical reaction from processing different types of material.
 They both can cause a runaway reaction when excessive heat is applied to their
reaction.

5. AP is a recovery process where as AR is a refining process of gold.
1. A/P can be considered a recovery process if you are using it in the following
examples:
 Removing base metals from pin's or CPU to concentrate your gold
 Processing MLCC's to recover Pd from the material without disturbing the
state of silver in the process.
2. You can also use AP as a refinement process to reprocess recovered gold but it is not
recommended.
3. AR would be considered a recovery process when it is used for pins, CPU's or other
material to remove base metals and other contaminants to concentrate your PM's.
4. AR would be considered as a refining process when it is used to reprocess your
concentrated PM's.
H2O2 is a catalyst and should not be used in greater concentration (≤5%). If we use it in high
concentration as we do earlier then it will make reaction very fast and gold will start dissolving
in it. Its concentration used should be 3-5% so that reaction becomes stable[6].
The temperature for sodium thiosulphate solution should be 65 ◦C minimum which was not
maintained.
Poor Man's AR can be made using 8 oz Sodium Nitrate (added in small increments), with 480 ml
water and 960 ml Muriatic Acid. Heat slowly.
This can be used to dissolve high karat gold, gold powder, gold foils, dissolves Platinum when
hot. When using (poormans Aqua Regia).
The excess Nitric Acid (HNO3) has to be removed as in regular Aqua regia. You can do this by
the evaporating method or sulfamic Acid. If using sulfamic Acid, add the crystals until the
solution stops fizzing when stirred.
The reaction is as follows:
2Au + 3H2O2 + 6HCl 2AuCl3 + 6H2O
Au dissolved in solution depend on two factors:
1. Amount of chlorine present in solutions.
2. Peroxide concentration.

6. Reference
[1] Yanhua Zhanga, Shili Liua, Henghua Xiea, Xianlai Zengb, Jinhui Lib, "Current status on leaching
precious metals from waste printed circuit boards
" 2012.
[2] Z. X. Moore DM, Li CX, et al, "Using thiosulfate as a leach reagents instead of cyanide," Metallic
Ore Dressing Abroad, 2005.
[3] Z. X. Heath JA, Lin S, et al, "Anaerobic thiosulfate leaching - Research on in situ gold leaching
technology," Metallic Ore Dressing Abroad, 2008.
[4] X. C. Yen WT, " Effects of copper minerals on ammionical thiosulfate leaching of gold.,"
Proceeding of X X IV International Mineral Processing Congress, 2008.
[5] Ken_S. http://refinementofpreciousmetals.com/viewtopic.php?f=74&t=456 citation 27/11/2014
[Online].
[6] Lazersteve. Peroxide Acid Method https://www.scribd.com/doc/29035575/Peroxide-Acid-
Method citation 27/11/2014 [Online].