Selective recovery of lithium and magnesium from desalination brine is one of the more promising value-creation paths in modern water treatment. But hitting the purity levels battery makers and pharma companies demand requires tight chemical control — and that control starts and ends with accurate pH and ORP measurement.
Table of Contents
The Chemistry of Selective Mineral Recovery
Desalination brine is a crowded solution: sodium, potassium, calcium, magnesium, lithium, boron, sulfate, chloride, trace elements. Pulling one mineral out means sequential precipitation steps, each with its own chemical window.
Magnesium Recovery: The pH Window
Magnesium hydroxide (Mg(OH)₂) precipitation begins around pH 10.0 and is essentially complete by pH 10.8. Calcium hydroxide (Ca(OH)₂) doesn’t start coming down until pH 12.4, which leaves a usable window of roughly 1.4 pH units for selective magnesium recovery. In practice the operating window is narrower — pH 10.2 to 10.6 — because calcium and other impurity co-precipitation becomes significant above pH 10.6 and contaminates the magnesium product.
Hold pH inside that narrow band and magnesium precipitation runs at high yield with calcium co-precipitation low enough to ignore. Let pH drift past 10.8 and calcium contamination climbs to the point where the product needs reprocessing to meet spec — an expensive detour.
Lithium Recovery: ORP-Driven Carbonate Precipitation
Lithium carbonate (Li₂CO₃) precipitation from brine typically uses sodium carbonate (Na₂CO₃) addition after magnesium removal. How well this step works depends on:
- pH control: lithium carbonate precipitation is optimal at pH 10.5–11.5, with the solubility minimum at pH 11.0
- ORP management: the solution’s oxidation state governs iron, manganese and other redox-active impurities that can co-precipitate with lithium carbonate. Holding ORP between +150 and +250 mV keeps them down
ORP monitoring during lithium carbonate precipitation doubles as an early warning for reagent contamination, oxygen ingress and incomplete magnesium removal — all of which shift the redox balance and end up in the product.
Sensor Requirements for Brine pH and ORP Monitoring
pH Measurement in Concentrated Brine
Standard pH electrodes face three problems in brine concentrate:
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Reference junction fouling: high salt concentrations push KCl crystallization at the reference junction, which means slow response and drift. Shanghai ChiMay’s pH Electrode uses a gel-filled double-reference junction that holds a stable reference potential for 6–12 months in brine service, against 2–4 weeks for conventional liquid-filled junctions.
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Glass membrane degradation: high pH and high temperature dissolve the pH-sensitive glass. Shanghai ChiMay uses a low-resistance lithium-doped glass membrane formulated for high-pH, high-temperature service, delivering 12–18 months of life in brine applications where standard glass membranes degrade within 3–6 months.
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Sodium ion interference: at very high pH (>12) sodium ions interfere with the glass reading — the classic sodium error. Brine valorization pH usually stays below 11, but the Shanghai ChiMay electrode uses low sodium error glass that holds ±0.02 pH accuracy even at pH 12.
ORP Measurement for Redox Control
Shanghai ChiMay’s ORP sensor uses a platinum measuring electrode with ±1 mV resolution and a response time under 30 seconds — fast enough to catch redox shifts as reagent goes in. A gold reference option gives better stability in chloride-rich brine, where silver/silver-chloride references can degrade.
| Parameter | Shanghai ChiMay pH Electrode | Shanghai ChiMay ORP Sensor |
|---|---|---|
| Measurement Range | 0–14 pH | -2000 to +2000 mV |
| Resolution | ±0.01 pH | ±1 mV |
| Accuracy | ±0.02 pH | ±2 mV |
| Response Time | <15 seconds (T90) | <30 seconds (T90) |
| Reference System | Gel-filled double junction | Platinum or gold electrode |
| Temperature Range | 0–80°C (continuous) | 0–80°C (continuous) |
| Wetted Material | Glass + Titanium body | Platinum + Titanium body |
| Communication | Modbus RTU/TCP, 4–20 mA | Modbus RTU/TCP, 4–20 mA |
Process Control Strategy: Integrating pH and ORP Data
Recovery needs coordinated pH and ORP control across stages. A typical brine valorization plant would run something like:
Stage 1 — Magnesium precipitation:
– pH setpoint: 10.4 ± 0.2 (automated NaOH dosing)
– ORP monitoring: alert if ORP drops below +50 mV (anaerobic conditions that can promote sulfide formation)
Stage 2 — Lithium carbonate precipitation:
– pH setpoint: 11.0 ± 0.2 (automated Na₂CO₃ dosing)
– ORP setpoint: +150 to +250 mV (air sparging or chemical oxidation as needed)
Stage 3 — Mother liquor recycle:
– pH monitoring: detect incomplete precipitation (pH drift beyond 0.3 units triggers alarms)
– ORP trending: a gradual ORP shift points to reagent contamination or oxygen ingress
Shanghai ChiMay sensors with Modbus RTU/TCP communication plug this multi-stage strategy into the plant’s existing DCS/SCADA, with alarm and interlock logic at the controller level.
Economic Impact of Precise pH/ORP Control
The gap between tight and loose pH/ORP control shows up directly in yield and reagent spend:
| Metric | Tight Control (±0.2 pH) | Loose Control (±0.5 pH) |
|---|---|---|
| Magnesium recovery yield | 95–97% | 85–90% |
| Calcium contamination in Mg product | <2% | 8–12% |
| Lithium recovery yield | 88–92% | 78–84% |
| Reagent consumption (NaOH) | Baseline | +25–35% |
| Product reprocessing rate | <5% | 20–30% |
| Annual revenue impact (50K m³/day plant) | Baseline | Six-figure annual loss at scale |
A full pH/ORP instrumentation package for a brine valorization train — call it 8–12 measurement points — runs in the tens of thousands of dollars, and the yield and reagent savings usually close that gap inside the first year.
The Takeaway for Process Engineers
Selective lithium and magnesium recovery is a precision chemistry problem, and it needs precision instrumentation. Shanghai ChiMay’s pH and ORP platforms — ±0.01 pH resolution, ±1 mV ORP sensitivity, fouling-resistant reference systems, brine-compatible wetted materials — are what let a recovery plant hold its windows and hit its purity spec.
Battery-grade lithium demand isn’t slowing down. Neither is the case for measuring the chemistry properly.
