Most phosphate rock contains a trace but toxic element – arsenic, the same substance that makes "white arsenic" (arsenic trioxide) notorious. During yellow phosphorus production, this impurity follows a complex journey, and understanding it is key to producing high‑quality phosphorus products.
How does arsenic hide in phosphate rock?
Arsenic doesn't usually exist as a separate mineral. About 80–95% of it sneaks into the crystal lattice of apatite by substituting phosphate ions (PO₄³⁻) with arsenate ions (AsO₄³⁻) – a classic case of "chemical impersonation." The remaining 5–15% occurs as sulfide minerals, mainly arsenopyrite (FeAsS).
Arsenic levels vary by region
Yunnan's Anning and Jinning areas have relatively low arsenic: 12–16 ppm (parts per million) on average.
Hubei's Yichang and Xiangyang regions, where black‑shale‑type phosphate rock prevails, show higher levels: 20–35 ppm.
What happens to arsenic during smelting?
In an electric furnace (1400–1600°C), arsenic is reduced and vaporised under strong reducing conditions, with 75–90% entering the gas phase as As₄ vapour. As the furnace gas rises and cools, arsenic undergoes a series of transformations: reduction‑volatilisation → re‑capture by CaO → secondary reduction → condensation along the gas pipe → final partitioning.
A crucial physical property helps here: arsenic vapour has a dew point of about 560°C, while phosphorus vapour condenses at a much lower ~280°C. This means arsenic starts to solidify earlier during cooling, allowing for pre‑separation before phosphorus condenses.
Where does arsenic finally end up?
Most (75–90%) remains in the furnace gas (to be treated later).
Some partitions into slag (5–10%) or ferrophosphorus (3–5%).
Only a tiny fraction (0.1–3%) actually makes it into the final yellow phosphorus product – but even that is enough to downgrade its quality. Unrefined commercial yellow phosphorus typically contains 50–200 ppm arsenic.
How to control arsenic?
The most cost‑effective measure is pre‑roasting the phosphate rock at 800–1000°C, which can remove 40–70% of arsenic before smelting. Without this step, high‑arsenic ore (>50 ppm) can easily yield yellow phosphorus with over 300 ppm arsenic – exceeding the acceptable limit for even the lowest product grade.
In a nutshell: Arsenic is a stubborn companion of phosphate rock, but once we know its hiding places and condensation behaviour, we can "intercept" most of it before it contaminates the final product.
