For the settling slurries most mining pumps move, the seal to choose is a heavy-duty single slurry seal in a cartridge, often run with no flush water at all. A pressurized double seal with clean barrier water is needed for solids fine enough to pass between the faces and for pumps that can draw in air. Specified and applied correctly, a seal in abrasive mining slurry typically runs more than 40 months between failures.
Mines move to mechanical seals from gland packing mainly to save flush water, which remote sites often lack. They move from expellers because an expeller seals only while the pump runs and only at low suction pressure.
Slurry Data to Collect Before Choosing
The seal is decided by the solids in the slurry, its chemistry, how the pump runs and what clean water the site can supply. Collect these before asking for a quote:
- Solids concentration and its basis: percent by weight or percent by volume.
- Particle size and hardness: the d50 from a sieve analysis, and how hard the solids are.
- Whether the slurry settles: most mining slurries and tailings settle, while clays and other fine slurries stay in suspension. Which one you have depends on particle size and on the specific gravity of the solids against the carrier liquid.
- Chemistry: pH, chlorides, oxidants such as ferric iron, and temperature.
- Operating conditions: shaft diameter, speed, seal chamber pressure, temperature, how often the pump stops and starts, and whether it can draw in air or run dry.
- Shaft runout and vibration: if either is excessive, specify a seal designed for shaft motion; most compensate for about 1/8 in. radially and 1/8 in. axially.
- Water on site: whether clean water is available at pressure all the time, and whether the process can take the dilution a flush adds.
Single or Double Seal
A mining slurry duty needs a pressurized double seal when any one of these three conditions holds:
- The single seal failed by solids crossing its faces: the Fluid Sealing Association holds that moving to a double improves seal life only when this is how the single seal failed, so establish how it failed before changing the arrangement.
- The solids are fine and stay in suspension: particles small enough to pass between the faces, down to sub-micron clays.
- Air can reach the faces: entrained air, a suction lift or a pump that can run dry.
A coarse, settling slurry in a wide-open seal chamber that meets none of these conditions runs on a heavy-duty single slurry seal.
When the duty calls for a double, specify it pressurized (API 682 arrangement 3) and face-to-back.
Flush and Barrier Water
Many seals designed for slurry need no flush water, and a flushless seal is the first choice where clean water is scarce or unreliable. Silt-laden river or lake water does not make a clean flush. Where clean water exists, a flush still dilutes the slurry, which adds evaporation and reheat costs downstream and, in potash, dissolves the product.
Where a single seal takes a clean external flush (API Plan 32), the flush works through a close-clearance throat bushing at the bottom of the chamber. It has to leave through that clearance at about 15 ft/s, so set the flush by the flow through that gap, not by supply pressure.
A flushed seal needs the throat bushing even in an oversized chamber. A flushless slurry seal can run with the throat open.
A pressurized double needs clean barrier water at a minimum of 15 psi (1 bar) above seal chamber pressure, from a seal pot (Plans 53A, 53B and 53C) or a plant header (Plan 54) kept above the highest seal chamber pressure it serves. Keep the margin near that minimum, because face-to-back doubles accept less differential pressure than other layouts.
Where the liquor carries dissolved solids, the seal can need a water quench on the atmospheric side (Plan 62) to keep crystals from forming where leakage dries behind the faces.
Slurry Seal Design Features
A slurry seal keeps every part that has to move or flex out of the solids, and it sits in a chamber with room for solids to clear. On a quote, look for these features:
- Line-in-line faces: the rotating and stationary faces share the same inside and outside diameters. They are hard against hard, most often sintered silicon carbide on silicon carbide.
- Springs out of the slurry: placed outside the process fluid, or on the rotating face, where centrifugal force throws particles away from them. A single cone spring, which does not clog, is the other way to do it.
- An open dynamic O-ring groove: an open groove on the process side, or a wiper ring, keeps solids from packing around the dynamic O-ring.
- Room around the seal: an oversized seal chamber, which most slurry pump makers offer, with at least 1 in. of radial space around the seal where the pump allows it. A conventional tapered bore is the wrong choice, because the taper directs solids toward the faces.
- A wedge-clamp drive collar: slurry pump shafts are hardened against wear, so set screws cannot bite into them, and the collar has to clamp with wedges.
Specify a balanced seal; in a pressurized double, the inboard faces must be balanced to the barrier fluid.
Pumps that stop and start often collect more solids behind a standard throat bushing at every stop. A bushing with reverse spiral grooves clears the chamber from the moment the pump starts.
Elastomers and Metals for Corrosive Slurry
Seal hardware for corrosive mining slurry is usually duplex stainless steel or high-chrome iron, and super duplex in hot acid-chloride leach liquor. Lining the wetted metal with elastomer is the alternative to a special alloy. The usual elastomer is FKM, with FFKM where FKM is not resistant enough, and the FFKM grade still has to match the chemistry.
In leach service, keep the springs out of the liquid or make them from a Ni-Cr-Mo alloy.
Avoid PTFE wedges as secondary seals in slurry.
Hot caustic attacks both FKM and EPDM. Check the compound against the plant’s own liquor before specifying either.
Cartridge or Component Seal
A mining slurry pump should get a cartridge seal. Slurry pumps are built to have the impeller moved axially as it wears, and a cartridge allows that adjustment without disturbing the seal faces, while a component seal cannot be moved once set.
Order the cartridge for the pump’s actual seal chamber. A cartridge for an oversized slurry chamber is a different build from one that has to fit a box designed for packing.
