Most of the cleaning ramps (tolva) used by miners of Madre de Dios are wood structures, placed at the extraction tube outlet. The ramp is used to receive and sort the auriferous material in order to retain the gold flakes.
The tolva is generally used in different ways:
- A steel grid (zaranda) is placed at the extraction tube outlet. This grid helps sorting rocks from auriferous sand. A wooden framework is arranged around the grid, in order to prevent rocks from bouncing and falling on the ramp.
- Once the grid has been passed, the mixture of sand and water strucks into a wooden board in order to spread the flow over the entire width of the ramp. This obstacle is also useful to reduce the speed of the flow so that water can flow slowly, maximizing gold flakes collecting on the ramp carpets.
- Finally, the mixture of sand and water flows on the ramp, covered by carpets which retain fine gold flakes.
Further to this process, carpets are washed in an annexed pond in order to recover a concentrate of gold and sand.
Then, miners put this concentrated mixture in a tank and add mercury. The mercury amalgams/retains gold flakes and separate it from sand. At the end of this step, miners obtain a sort of compact bullet of mercury, containing gold flakes.
The analyse of miner activities in several miner communities helped us to identify many issues related to this gold extraction process:
- The ramp installation takes too much time for the miners.
- Each time the ramp is moved to another mine site, the vertical wood profiles are buried under the stones and cannot be removed. That is why the miners have to rebuild the vertical wood profiles, cutting down tree branches.
- The system is quite inefficient: flow and ramp slope are too important to maximize gold flakes retaining on the carpets.
- The grid system does not provide an efficient refining of auriferous material.
- The wood structure rotten quickly.
- Rejected water are often troubled and polluted.
- The ratio Time of work VS number of operations is not profitable and the final result is not really optimal.
In this perspective, the idea of the Wanamei team was to propose an improved device to recover polluted water and to improve gold collecting.
The co-design process with local miners helped us to design a new concept of ramp: ”the portable tolva”, which fulfill legal requirements. This concept is more efficient in recovering a concentrate of auriferous sand and is more respectful towards the environment (no wood used, recovering of stones of washing process, water used in a closed circuit system).
The prototype has been imagined to be modular and portable. It can adapt to different types of lands of the region.