Printed circuit boards (PCBs) are one of the richest sources of gold found in electronic waste. A single ton of mobile phone circuit boards can hold anywhere from 142 to 700 grams of gold, a concentration that far exceeds what most gold mines produce from the same weight of ore. Yet a large share of this value is still lost, because many recyclers keep relying on outdated methods or ship the boards out untreated. For anyone running a circuit board recycling operation, knowing how to pull this gold out efficiently and safely is the difference between a profitable plant and a wasted resource.
Why thiosulfate instead of cyanide
For decades, cyanide has been the standard leaching agent for gold. It works well and the process is simple, but cyanide is highly toxic. Handling, storing and treating cyanide effluent demands strict control, and in many countries the regulations around it have become a serious barrier for small and medium recycling plants. Thiosulfate has emerged as the most promising alternative. It is inexpensive, far less toxic, and it can dissolve gold from material that cyanidation struggles to treat. The gold dissolves into a soluble and stable complex, Au(S2O3)23-, which stays in solution under neutral and alkaline conditions.
The chemistry behind the process
The leaching reaction itself is slow, so it needs a catalyst. Copper (II) ions play that role, while ammonia keeps the copper in its active form and stabilizes the solution. In practice, this means the leaching solution is a mixture of sodium or ammonium thiosulfate, ammonia and a small amount of copper sulfate, kept alkaline at a pH around 10. Oxygen from the air also takes part in the reaction, which is why leaching is carried out in open, well-agitated tanks rather than sealed reactors.
Step-by-step gold extraction with thiosulfate
The whole route can be broken down into four clear stages. Following them in order keeps reagent consumption down and recovery up.
Step 1: Prepare the boards
Start by removing components that can be recovered separately, such as batteries, capacitors and large connectors. The boards can then be leached whole or lightly crushed. Studies have shown that heavy grinding actually loses gold, because the metal sits in thin surface layers and fine particles are easily carried away. Most of the gold is concentrated in the outer layers of the board, so aggressive milling is not only unnecessary but counterproductive.
Step 2: Make up the leaching solution
A typical starting point is a solution containing 0.1 to 0.5 mol/L thiosulfate, 0.2 to 1.2 mol/L ammonia and 0.01 to 0.04 mol/L copper sulfate. Adjust the pH to around 10 with sodium hydroxide or sulfuric acid. The exact recipe depends on the type of boards you are treating, so it is worth running a few small trials before committing a full batch.
Step 3: Leach under controlled conditions
Keep the solution at 25 to 30°C with continuous stirring. Reaction time ranges from 4 to 24 hours depending on the reagent concentration: higher concentrations shorten the time, while lower concentrations need longer. Under optimized conditions, leaching ratios above 95% have been reported, which puts thiosulfate on par with cyanide for most PCB feeds.
Step 4: Recover the gold from solution
The gold is recovered from the pregnant solution by electrowinning or cementation. Electrowinning deposits the gold onto a cathode and typically recovers around 94% of the gold held in solution. The copper left behind can be recovered in a second electrowinning step, which also cleans the solution for reuse.
Key parameters at a glance
Parameter
Typical range
Thiosulfate concentration
0.1 - 0.5 mol/L
Ammonia concentration
0.2 - 1.2 mol/L
Copper sulfate
0.01 - 0.04 mol/L
pH
10 - 10.5
Temperature
25 - 30 °C
Leaching time
4 - 24 hours
Stirring speed
Around 300 rpm
Limitations to plan for
Thiosulfate leaching is slower than cyanide, so throughput depends on tank capacity and batch planning. Copper in the boards competes with gold for reagent, consuming thiosulfate and pushing up cost. The solution is also metastable and can decompose if the pH or temperature drifts. None of these are deal-breakers, but they are exactly the kind of variables that a well-designed process and proper equipment keep under control.
From the lab bench to a working plant
At laboratory scale, thiosulfate leaching is straightforward. Scaling it to a commercial operation is another matter. The boards must be shredded, separated and fed consistently; the solution must be circulated, heated and monitored; and the pregnant liquor must be handled safely. This is where purpose-built circuit board recycling equipment comes in. A complete circuit board recycling plant combines crushing, separation and dust collection so that the metal-rich fraction is concentrated before it ever reaches the leaching tank, improving reagent efficiency and overall recovery.
Thiosulfate leaching offers a practical, low-toxicity route to recover gold from circuit boards, and it is especially attractive for recyclers who want to avoid the regulatory burden of cyanide. The chemistry is well understood, the reagents are cheap, and recovery rates above 90% are achievable when the process is controlled properly. Paired with the right recycling equipment, it turns a waste stream into a reliable source of precious metal.
