Chemical Dosing Tank Mixer for PAC: Does the Solution Need Constant Stirring?

A chemical dosing tank mixer on a PAC (polyaluminium chloride) tank does its real work during batching. Run it while powder or liquid product is added to water, keep it running until the batch is fully dissolved and uniform, then stop it. The finished solution can stand in the tank while the metering pump draws from it; PAC does not separate or lose strength just because nobody is stirring. Two cases call for extra mixing. One is solid PAC with a high insoluble content, where residue settles and can block the pump valves if the suction picks it up. The other is a very large tank holding several days of solution, where a timed intermittent stir keeps the concentration even. This guide is for water and wastewater operators choosing and running a PAC tank mixer.

Why PAC does not need continuous stirring

PAC is an inorganic polymer coagulant that forms a stable solution in water. It is not a suspension like lime slurry, so nothing useful settles out on standing. A chemical dosing tank mixer exists to dissolve the product and even out the concentration, and once that is done the job is over.

Running a chemical dosing tank mixer all day has real costs:

1. Bearings and gearboxes wear, shortening mixer life.
2. A deep vortex pulls air into the solution, the metering pump picks up bubbles and the flow becomes unstable.
3. It wastes power, which adds up on plants with several tanks.

When a chemical dosing tank mixer should run more often

Whether more mixing is needed depends on the product and the tank:

• Solid PAC with high insolubles. Some powders leave a sludge that builds up on the tank floor. Let the batch settle after mixing, keep the foot valve or suction inlet well clear of the bottom and drain the sludge regularly. That works better than endless stirring.
• Large tanks holding several days' supply. Set intermittent mixing, a short run every few hours, so the top and bottom stay at the same strength.
• Cold weather. Cold water dissolves powder slowly. Extend mixing time during batching and confirm there are no lumps before stopping.

Liquid PAC diluted on site rarely needs stirring after it has been blended.

Setting mixing time

The common approach ties the chemical dosing tank mixer to level control:

• The mixer starts when the fill valve opens and powder is added.
• After the tank reaches set level and all chemical is in, it keeps running for a fixed time.
• A timer then stops it and the tank switches to storage or dosing.

There is no universal mixing time; it depends on product solubility, water temperature, tank volume and mixer power. A simple field check is to take one beaker from the top and one from the bottom after stopping. If color and clarity match and there are no particles, the batch is mixed. Write the confirmed time into the operating procedure and set it on the timer relay.

Choosing a chemical dosing tank mixer

Four things matter: wetted material, speed, impeller type and shaft length.

Material. A chemical dosing tank mixer on PAC sits in an acidic solution containing chloride, which attacks carbon steel. Wetted parts are usually stainless with a protective lining, or plastic-lined or FRP shafts and impellers; confirm against the actual concentration.

Speed and impeller. Dissolving PAC calls for a medium to high speed propeller or pitched blade turbine that disperses powder fast. Polymer make-down needs a slow mixer to avoid shearing the chains, so do not swap mixers between PAC and PAM tanks.

Shaft length. Place the impeller in the lower part of the tank so it lifts powder from the floor, but not right on the bottom.

Mounting. The bridge on top of the tank must hold the chemical dosing tank mixer rigidly. A flexing support shakes the mixer and ruins the gearbox and seals. Fit a motor guard and choose a suitable ingress rating for humid dosing rooms.

Mixer and metering pump pitfalls

• The mixer runs while the pump draws, the vortex sends bubbles down the suction line and the flow jumps around. Keep the suction well away from the impeller, or separate the batching tank from the day tank.
• A single tank forces the pump to stop during batching. For continuous dosing use two tanks that alternate, or one batching and one day tank.
• A foot valve sitting on the floor of the tank sucks in sludge and blocks. Raise it clear of the bottom and add a strainer.

Lock out power before servicing the chemical dosing tank mixer, never reach into a tank that holds solution, and wear gloves and goggles.

Conclusion

PAC tanks do not need constant stirring. Mix until dissolved and uniform, then let the solution stand; with dirty powder or large tanks, use timed intermittent mixing plus regular draining. Choose a chemical dosing tank mixer with corrosion-resistant wetted parts and a medium to high speed impeller, and interlock it with level and the metering pump. More detail is in how a PAC dosing system works, PAC and PAM dissolution tips and PAC dosing pump selection. Equipment is listed under dosing skids and dosing accessories. Send your chemical, tank size and feed rate and contact our engineers.

FAQ

How long can PAC solution stand before use?

Usually several days, because PAC solution is stable. Cover the tank against dust and protect it from freezing in winter.

Can PAC and PAM share one mixer?

It is not recommended. PAM needs gentle low-speed mixing, and the high shear used for PAC breaks polymer chains and weakens flocculation.

Does a faster mixer dissolve PAC faster?

Too much speed creates a deep vortex, entrains air, splashes and overloads the motor. Size the chemical dosing tank mixer to the tank and product.

How do I deal with sludge at the bottom of the tank?

Open the drain valve regularly and clean the tank when needed. Keep the foot valve raised so it does not pick up sludge.

Can the mixer be run manually?

Yes. Small plants often use a manual start with a timer. Interlocking with tank level avoids someone forgetting to switch it off.