Magnesium Production Wastewater Treatment: Chloride, Fluoride and Process Routes

Magnesium is produced by two routes that could not be more different in their water signature. The silicothermic route — the Pidgeon process — makes almost no liquid effluent but generates fluoride-bearing dust and spent residue; the electrolytic route dissolves magnesium chloride and makes a chlorine- and chloride-rich brine that has to be managed carefully. Treating magnesium plant water means knowing which route you are on, because the two produce entirely different problems and pretending one treatment fits both is a mistake.

Two Routes, Two Problems

The Pidgeon process reduces dolomite with ferrosilicon in closed retorts under vacuum. It is dry by design, so the liquid effluent is small — mostly wash water and scrubber bleed — but the solid and gaseous residues carry fluoride and silicone.

The electrolytic route dissolves anhydrous magnesium chloride and electrolyses it to metal and chlorine. The chloride-rich bleed from this circuit is the dominant water issue, and chlorine handling runs through the whole water balance.

A site running both has to keep the streams separate. Mixing a fluoride-bearing scrubber bleed into a chloride brine dilutes neither problem and complicates both treatments.

Know the feed. Dolomite composition varies and so does the residue chemistry; a plant assuming a fixed analysis underperforms when the ore changes.

Chloride From Electrolysis

Electrolytic magnesium generates a magnesium chloride brine that circulates and bleeds. Chloride concentration is high and stays high; there is no biological route that likes it. A related discussion worth reading here is Coal Gasification and Syngas Plant effluent.

The bleed is usually evaporated and the salt returned or disposed of, because discharging high-chloride brine is rarely permitted and corrodes anything it touches. Evaporation is the control, not treatment. Plants dealing with comparable loads face many of the same trade-offs covered in Waste Oil Re-refining Plant Wastewater Treatment.

Chlorine recovery and reuse keeps the circuit closed. Where chlorine is vented or scrubbed, the scrubber water becomes another chloride stream that has to be accounted for. The same selection criteria reappear in polyolefin production wastewater treatment, which is worth opening alongside this one.

Corrosion is the practical limit. High-chloride water wrecks ordinary steel quickly; materials selection on this duty decides whether the plant survives its own water.

Removal Rates Across Our Installed Base

Across 42 commissioned plants where we hold complete COD records, influent COD ranged from 200 to 172,000 mg/L and treated effluent from 50 to 5,000 mg/L. Average removal across that set is 87.1% — not a marketing figure, but the measured mean.

That average is worth pausing on. It sits well below the 95%-plus numbers most suppliers quote, because the set includes genuinely difficult streams. On some electroplating and municipal duties the installed configuration only reaches about half the influent COD, and saying that up front is more use to you than a number the plant will never hold.

Fluoride From the Residue

Both routes produce fluoride-bearing residue or dust. The Pidgeon process dolime and the electrolytic cell liner both carry fluorine, and any water contacting them picks it up.

Calcium precipitation as fluorite is the established removal route, and it works well when the fluoride is in solution rather than locked in a solid that first has to be leached.

The residual limit is low and calcium fluoride solubility sets a floor, so a polishing step — adsorption or selective resin — is needed where the limit is tight.

Keep fluoride-bearing solids out of the main water. Contacting dry residue with wash water creates the problem; handling the residue dry avoids it.

Operating Notes

Separate the two routes’ water. One chloride brine and one fluoride scrubber bleed are far easier to handle apart than mixed.

Close the chloride loop by evaporation. Discharge of high-chloride brine is rarely an option and corrodes the plant.

Finally, treat fluoride on contact water only. Keeping residue dry is cheaper than treating the water it would make.

Why Choose Baihuipu as Your Manufacturer

Two decades in this industry teaches you that most treatment problems are procurement problems in disguise. Someone bought the cheapest skid, nobody checked the equalisation volume, and eighteen months on the plant cannot hold its consent limit. We build differently.

Manufacturing Base in Guangdong

Owned production lines mean we control tolerances, materials and delivery. Standard modular units and bespoke skids are both built by our own people, so schedule commitments are ours to keep.

Documentation You Can Actually Audit

Each system ships with as-built drawings, material certificates, welding records and a functional specification. If a client's own consultant wants to review the process calculation basis, we hand it over rather than treating it as proprietary.

Two Decades of Field Data

Food and beverage, chemicals, electroplating, textile dyeing, mining, municipal work — we have commissioned across all of them. The useful part is not the project count, it is having seen how plants fail in year seven and designing those failures out.

We Engineer the Whole Train

Screening, equalisation, biological or chemical treatment, membranes, evaporation, brine handling — the complete line comes from one organisation. Process design, mechanical, electrical and PLC programming sit under the same roof, so there is no gap for finger-pointing at handover.

Certification and Export Practice

CE marking is standard and we work to ISO 9001 procedures. Where a project calls for specific material grades, pressure vessel certification or ATEX-rated equipment, we build to that standard and ship the full documentation pack.

Spares, Service and Commissioning

Membranes, dosing pumps, diffusers, instrumentation and blowers are stocked for the systems we sell. Our control systems support remote diagnostics, and we can put an engineer on site for commissioning, training or an unplanned shutdown.

Twenty-Plus Export Markets

We have shipped to the United States, Canada, Russia, Indonesia, India, Nigeria and Vietnam among others. Crating, export documentation and certificates are handled in-house, which keeps customs surprises to a minimum.

Let's Look at Your Numbers

Share your water analysis and site constraints and we will give you a straight assessment of the process route and the realistic operating cost before you commit to anything. Contact our engineers at WhatsApp: +86 13631765076, or find us at hkbhp.com.

WhatsApp: +86 13631765076

Frequently Asked Questions

What does it cost to run, per cubic metre?

Energy dominates, then chemicals, then sludge disposal. We give you a per-cubic-metre breakdown during proposal so you can compare it against your sewer charge — treating too aggressively is a common and expensive mistake.

What happens if the incoming magnesium production load exceeds the design figure?

Load swings are the most common reason plants underperform. We set the design envelope from your worst-case samples, not your average ones, and specify enough buffer in the biological stage to ride out shocks. If your process genuinely shifts up permanently, we can usually add capacity in stages.

How much operator attention does the system need day to day?

Automatic control handles dosing, backwash and level control. Realistically your team spends time on chemical top-up, sample testing and sludge handling — maybe one to two hours per shift on magnesium production duty, less if you opt for remote monitoring.

What treatment capacity do you normally design for magnesium production?

Skid-mounted units cover roughly 27 to 1169 m³/day depending on the duty, and parallel trains extend that further. We size against your actual peak hour flow rather than the daily average, because equalisation rarely absorbs the whole spike.

What information do you need to quote?

Water analysis for your worst-case stream, peak and average flows, discharge limits, available footprint and utilities. If you do not have a full analysis yet, we can start from a partial one and flag the gaps.