APPLICATION
Anionic PAM for Tailings and Coal Washing: Clarifying Slime Water at Scale
Coal preparation and mineral processing move enormous volumes of fine solids in water, and the economics of the whole site ride on how fast that water clarifies and recycles. Anionic polyacrylamide is the workhorse flocculant here - but charge selection and dose point decide whether it works like a chemical or like a costume.
Choosing the charge and molecular weight
Coal slime and most mineral tailings carry negatively charged surfaces, which is why medium-to-high charge anionic PAM with high molecular weight is the default: the charged groups adsorb onto particle edges and the long chains bridge particles into large, fast-settling aggregates. Very high charge can over-flocculate clay-rich slime into tight, sticky masses that raking handles badly; very low charge under-bridges and gives slow settling.
Molecular weight does the bridging work, so high-molecular grades settle faster per milligram - but the same long chains shear-break if you pump the dosed slurry hard. Dose polymer into a gentle mixing point, and let gravity do the rest. When suppliers offer a dozen anionic grades, the practical filter is: run two charges side by side on your slime water and compare settling curves and underflow consistency.
The two-step: PAC first, PAM after
Heavily loaded slime water clarifies better in two steps. A modest PAC dose - industrial 28% grade (GB/T 22627-2022) - collapses the fine, stable colloids into micro-flocs; a following anionic PAM dose at a few tens of grams per tonne of dry solids bridges them into settler-sized aggregates. The PAC also cuts the PAM requirement noticeably, because polymer is not wasted on particles the coagulant could catch more cheaply.
Separation distance matters: give the coagulant a brief vigorous mix, then add PAM where the flow is calm - typically the thickener feedwell - with enough gentle agitation to distribute it without shredding the flocs it just built. If clarification improved but underflow thickened badly and rakes torque up, the PAM dose is high or the charge is wrong, not the idea.
When recycle water keeps drifting
- Clarity falls as production rises - dose is per tonne of solids, and solids throughput has grown; re-scale the dosing pump accordingly.
- Water gets harder to clarify week by week - fine-particle fraction is accumulating in circuit; increase PAC share or bleed more slime water out.
- Flocs form then fall apart in the launder - shear; move the PAM addition point closer to the thickener.
- Thickener overflow clear but underflow thin - over-flocculation with high-charge polymer; step the charge down and re-base the dose.
- Seasonal swing - water temperature and salinity change both PAC and PAM behaviour; re-run the settling tests at the season's actual water.
Frequently asked questions
How much anionic PAM does slime water need?
As a starting corridor, a few tens of grams per tonne of dry solids, set by settling tests on your actual slime water - fine-particle share and water chemistry move the number widely. Clarifying per cubic metre of water is the wrong basis; always scale on solids handled.
Can I run PAM without PAC on coal slime water?
On lightly loaded circuits, yes. On high-fines, high-turbidity slime water, the PAC-plus-PAM two-step usually gives a clearer overflow at lower total chemical cost, because each chemical handles the particle size class it is efficient at. Trial both trains before fixing the flowsheet.
Which PAM for tailings going to paste or filter press?
Thickeners favour high-molecular anionic grades for fast settling; filter presses often run better with a slightly lower charge to keep cake release clean. It is worth holding two grades - one for the thickener, one for dewatering - rather than forcing one product across both duties.