Major operations

Engineers said the alum mine's treatment required three transformative "surgeries".

To restore the surface of the hills, the traditional approach was to cover the area with new soil from elsewhere, but the demand for new soil was too great, and it could not solve the pollution problem, said Jia.

Therefore, the project team chose the most advanced "in-situ substrate improvement technology".

"For the entire alum mine treatment, we didn't bring in a single truckload of soil from outside," said Jia.

The technical team added self-developed neutralizers, soil improvement substrates, and microbial agents to the native mine slag and acidic soil, fundamentally changing the soil's "physique" to meet the standards suitable for plant survival.

"This avoids the situation of curing one area while damaging another, truly achieving both root and branch treatment," said Jia.

For the nearly vertical open-pit mine walls, the team used ecological long bags: workers hung safety ropes, anchored long strip-shaped eco-friendly bags filled with improved substrate to the rock walls, and planted grass seeds and shrub seedlings.

With the help of an intelligent drip irrigation system, they achieved a 99 percent vegetation coverage rate on the sheer cliffs.

To block the outflow of acidic water during the treatment period, the team built an 8-kilometer-long drainage ditch at the bottom of the mine to intercept rainwater that had collected on the mountain and further treat it.

They used microbial mineralization anti-seepage technology to seal underground cracks, preventing acidic sewage from continuing to seep underground.