Mg slurry pumps with stones need a solids-passing pump

A recovery plant sends a magnesium-based slurry with pebbles and stones to a pump, and the pump plugs at the suction, loses flow, or wears out fast. The request is a single machine that grinds and pumps. That machine exists only for a narrow class of solids. Whether it fits depends on two numbers nobody has yet: how hard the solids are and how fine the product has to be.

Skip these fixes: they fail on stones in Mg slurry

• Buy a grinder pump and call it done. Grinder pumps are common for soft materials. On hard material the cutters are the wear item and the jam point. Plan on a separate grinder and pump for hard solids.
• Upsize a standard centrifugal. A bigger casing does not fix a closed, multi-vane impeller that stones bridge across. Passing large solids is a function of impeller vane count and passage geometry, not casing size.
• Put a rotor-stator pump in-line and expect the target grind in one pass. High-shear roto-stator pumps do pump and grind, but they are normally recycled back to the tank to build the residence time the grind needs. A single pass through the head does not deliver it.
• Treat "Mg slurry" as one material. MgO slaking solids are relatively soft (Mohs hardness usually about 3 to 5). A different, very hard Mg-bearing material is a different grinding problem entirely. Selecting equipment before that is known is guessing.
• Size the pump without flow and head. 10 gpm and 1000 gpm are different machines. No pump type can be selected without duty flow, total head, and solids data.

Separate the two duties: pumping and grinding

Pumping large solids and reducing particle size are different jobs, and each has its own hardware.

• Pumping only (avoid plugging): the goal is to pass solids through unchanged. Solids-handling centrifugals do this by geometry.
• Grinding and pumping together: the goal is size reduction plus transport. Roto-stator high-shear pumps and grinder pumps cover this, with the limits noted above: recirculation for the roto-stator, soft solids for the grinder pump.
• Grinding hard solids: this is a grinding-application problem. Expect ceramic grinding technology, and involve a grinding applications specialist rather than a pump vendor alone.

The first question to settle with Process is therefore: is grinding required for the process, or only to keep the pump from plugging? If only for plugging, drop the grinder from the design.

Collect the data every vendor will ask for

Get these before requesting quotations. Each one changes the pump or grinder type.

Match the symptom to the likely cause

Choose the equipment with this decision path

1. Confirm whether grinding is needed. If the only purpose is to avoid plugging the pump, go to step 2. If a product size is required, go to step 3.
2. Pump only. Use a trash pump style centrifugal with a 2-vane impeller. As a rule of thumb, a 3-inch by 3-inch pump usually handles hard spheres up to about 2.5 inch diameter. Confirm the exact passage on the manufacturer's data sheet. For severe plugging, evaluate an eddy pump, where the impeller is set back from the suction and the eddy currents from the spinning impeller do the pumping. It is much less hydraulically efficient and much more reliable.
3. Grinding needed, solids soft. For MgO slaking-type solids (Mohs about 3 to 5), evaluate a high-shear roto-stator pump on a recirculation loop, or a grinder pump for soft material.
4. Grinding needed, solids hard. Design a separate grinder and pump. Engage a grinding applications specialist and evaluate ceramic grinding technology.
5. Feed protection. When the stones exceed what the chosen pump passes, put the grinder or a size-reduction stage upstream of the pump rather than relying on the pump to cope.

Size the roto-stator recirculation loop for residence time

A roto-stator pump grinds in the shear zone, so grind quality tracks how many times the batch passes through it. The relationship, as a general engineering estimate:

turnover time = tank volume / recirculation flow
passes = process time / turnover time

Use these as planning numbers only. Required passes come from a trial on the actual slurry against the target size, not from a formula.

• Feed the loop from a tank so the slurry returns to the same inventory; a once-through line gives one pass.
• Size the pump for the loop flow and head at the slurry density, not for the downstream transfer duty alone. Add a separate transfer path if the ground product moves on.
• Sample the loop at intervals to find where particle size stops improving; that sets the run time.

Trial the slurry before committing to a machine

Ask the vendor for a test on a representative sample, with the stones included. A clean-water curve says nothing about grinding performance or plugging.

1. Sieve or size-analyze the incoming slurry and record the largest stone and the distribution.
2. Run the candidate pump at the design flow and head. Log flow, discharge pressure, and motor current.
3. Watch for plugging events: flow dropping while motor current falls or spikes.
4. Sample the discharge (or the loop over time for a roto-stator) and compare against the target size.
5. Inspect the impeller, cutters, or rotor and stator for wear after the run, since abrasion drives the running cost on hard solids.

Verify the installed system holds up on real slurry

• Passage: the largest stone you measured passes the pump without a plugging event across a full run.
• Grind: discharge samples meet the target size the process requires. If the requirement was never confirmed, the grinding stage may be unnecessary.
• Trend the pump: log motor current and discharge pressure. A steady drift downward in head at constant speed points to impeller or liner wear; repeated current spikes point to partial blockage or cutter jamming.
• Wear interval: record hours to first inspection of the wetted parts and compare against the vendor's expectation for the slurry hardness.

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