In lime slurry neutralization, how do you solve pH electrode fouling and feedback lag?

Conclusion first: when the lime slurry neutralization section cannot hold pH, nine times out of ten it is not that the pH meter is inaccurate — it is “the electrode seized by calcium scale” stacked with “the dosing point far from the measurement point”. Immersed in calcium-hydroxide-saturated slurry, the membrane grows a calcium carbonate film within two or three days and response stretches from seconds to minutes; meanwhile neutralization is a fast, strongly exothermic reaction and the pH curve is nearly vertical near the equivalence point — by the time the measurement side sees the change, the tank went over-alkaline long ago. Solve the two sides separately: the electrode side handles scaling and placement, the loop side handles lag. Do not just fight it with PID tuning.

Root causes

Membrane fouling. Ca²⁺ in the lime slurry meets carbonate in the water and precipitates CaCO₃ as a dense shell on the glass membrane and junction. The shell is not an insulator, so the reading does not die at once — it turns “sluggish”: after dosing alkali the reading climbs slowly, the operator sees the lag and increases dosing, and the loop overshoots all the way.

Junction clogging. Calcium scale deposits in the porous ceramic junction faster than on the membrane, because the pores are small. The symptom is an electrode that seems “dead” — motionless even in standard solution.

Feedback lag. Between the dosing point and the pH measurement point lie several meters of pipe or an agitated neutralization tank — the mixing time is pure dead time. That lag combined with the steep pH curve at the equivalence point makes a classic difficult loop: more gain oscillates, less gain drifts.

Poor sample representativeness. A probe mounted on the tank-wall return line measures a local pocket, not the bulk.

Diagnosis

Read the trend, not the instantaneous value. Zoom the pH trend to a 30-minute window: “slow ramp + sudden jump” is the classic scaling-plus-lag signature; “high-frequency small sawteeth” is bubbles or agitation interference. Then do one more thing — insert a spare new electrode temporarily beside the running one; a gap above 0.3 means the running electrode has scaled. Third, relate valve opening to pH: normally a valve change should move pH immediately; if the valve must reach 80% before pH budges, lag is the dominant cause.

Remediation

Emergency. First, manual dilute-acid membrane cleaning: pull the electrode, soak 5 minutes in 5% hydrochloric acid — the scale fizzes off — rinse, recalibrate; buys three to five days. Do not soak the junction in acid for long; it damages the reference. Second, temporarily change dosing from continuous to “small pulses + pause and observe”, using the pause to give the measurement side time and manually cut the loop gain. Third, change the pH target from “pinned at 7.0” to a “6.5–7.5 band”, allowing fluctuation so the loop stops hunting at the equivalence point.

Systematic retrofit. First, automatic cleaning: ultrasonic or spray type, set every 2–4 hours, catching scale at formation. Second, an anti-scaling electrode structure — flat membrane, removable guard, large-aperture junction. Hamilton’s process electrodes use the patented Single Pore single-aperture structure: a simplified reference path with anti-clogging pore design, and with a cleanable guard the maintenance interval stretches markedly in heavily scaling neutralization tanks. Third, relocate: move the pH point far enough downstream of the dosing point and after a static mixer so the mixture is homogeneous before measurement — generally at least 30 seconds of mixing time. Fourth, feedforward: compute the lime demand from inlet flow and inlet pH by mass balance, and let pH feedback only trim — the fundamental fix for lag.

Selection comparison

Option	Anti-scaling	Response	Suitable	Not suitable
Ordinary glass electrode + ceramic junction, plunged into tank wall	Poor — fouled in 2 to 5 days	Fast initially, collapses once scaled	Clean water, low-hardness neutralization	Lime slurry, high hardness, high alkalinity wastewater
Flat membrane + large-aperture junction + removable guard	Fair — interval stretches to 2 to 4 weeks	Fast	Moderate-solids neutralization tanks	Undiluted supersaturated lime slurry
Hamilton process electrode (Single Pore) + auto cleaning	Good — long-term online with ultrasonic or spray	Fast, stable reference	Long-period unattended neutralization loops	Verify membrane material and temperature ceiling per medium
Sample conditioning + analyzer cabinet (measure after filtration)	Good, but sampling lines clog	Extra lag introduced	Multi-point centralized monitoring	Dosing points needing fast closed-loop control

Engineering implementation

1. The technical specification must require “electrode with automatic cleaning device” — do not write “optional”.
2. State the minimum distance between pH point and dosing point and the mixing method in the piping layout requirements.
3. Require the supplier to provide scaling and cleaning-interval data under saturated lime water.
4. The control system specification must state a feedforward-plus-feedback strategy; pure PID is not acceptable.
5. The electrode guard must be replaceable online with isolation valves, so cleaning does not stop the plant.

Common mistakes

Mistake 1: cranking the PID integral time. The lag remains, and more integral means more oscillation.

Mistake 2: reinstalling after an acid soak without recalibrating. A washed membrane’s slope changes — skipping calibration just swaps in a new bias source.

Mistake 3: mounting the pH point directly below the dosing point. Locally over-alkaline, the reading is always high, dosing gets pushed down, and neutralization ends up incomplete.

Mistake 4: believing steadier lime concentration means easier control. Steady concentration with fluctuating flow still lags — control the alkali entering the tank, not the pump frequency.

Mistake 5: ignoring temperature compensation. Neutralization is exothermic; a tank temperature swing of ten-odd degrees with wrong compensation biases the pH reading itself by more than 0.2.