Monsoon Storage: Keeping Leachate Polymer Free-Flowing at 90 Percent Humidity

Relative humidity sits above 85 percent for weeks at a time on many sites, and the polymer store is often a converted container with a metal roof and a roller door that stays open while the forklift works. Under those conditions a bag of super absorbent powder starts absorbing before it ever reaches the mixing pad, and the symptom at the pad looks like a bad batch rather than a storage problem.

What Wet Air Does to a Bag

Super absorbent polymer is hygroscopic by design; that is the whole point of it. Powder left in still, humid air takes water slowly through the bag wall, faster through an opened neck, and fastest on the outer 50 mm where the surface area is greatest. A 25 kg bag opened on a Monday, scooped from all week and reclosed with a twisted neck can gain two to four percent moisture in four weeks at 85 percent humidity. That is half a kilo to a kilo of water in a bag that will later be dosed as if it were dry powder.

The consequences are not dramatic, which is why they go unnoticed. The powder cakes against the bag wall and along the fold lines. Poured into the mixer, it wets on the outside and stays dry at the core, producing the translucent lumps operators call fish eyes; each one is a gram or two of capacity that never gets used. Free-flowing powder that has taken on moisture also dusts less, which sounds helpful until you realise the dust was the fraction dispersing fastest and doing the early thickening. Sites in wet climates typically dose five to fifteen percent above the dry-climate figure and rarely know the reason.

A Store That Works in a Wet Season

The building matters less than four details. Pallets at least 150 mm off the floor, because concrete slabs wick moisture and the bottom layer of bags is always the first to cake. A 300 mm gap between stacks and the wall, so air moves and condensation does not run down the blockwork onto the pallet. Nothing stored against the roof, because a metal roof in the tropics reaches 60 °C by day and drops to ambient by night, and the water that condenses on the underside lands on the top tier. And a ridge vent or a pair of louvres, so the container breathes instead of cycling its own moisture.

Inside that building, run first-in-first-out properly. Date every bag on receipt with a marker on the shoulder, not on a label that peels, and put the newest at the back. Keep opened bags in their original polyethylene liner, folded twice and clipped, never knotted; a knot seals the neck but leaves the fold open, and it also makes the bag impossible to reopen without tearing. A 100 g silica sachet dropped into a part-used bag costs almost nothing and buys a week.

The single most useful control is a reference bag. Weigh one unopened 25 kg bag on receipt, label it, store it on the middle shelf, and weigh it monthly on the same scale. A gain under two percent means the store is working; three percent or more means the roof is dripping or the door is standing open, and you have found the problem before it reaches a batch. Record the number in the same book as the batch log, because that is the record an auditor will ask about when the dose creeps.

When a Bag Has Already Taken Water

Do not crush it. Crushing caked polymer creates fines and dust, and fines hydrate too fast, forming a skin that seals the rest of the granule. Break the cake by hand into pieces under 20 mm, screen the lot through a 5 mm mesh to pull out the lumps, and dose the screened powder at 1.1 to 1.15 times the normal figure with a verification test on the first batch. Lumps that will not break under thumb pressure are not worth reclaiming for a load that has to pass a gate test; hold them for a non-critical job such as dust suppression or use them in the next batch as a partial replacement after a pail trial.

Incoming acceptance is worth a two-minute check. Take 10 g from the centre of one bag in twenty, dry it at 105 °C for two hours, and accept if the loss is under 1.5 percent. Batches that fail go back to the supplier rather than into the store, because the problem compounds quietly and shows up months later as an unexplained dose increase.

Storage conditions also affect how a grade behaves on the pad, and grades with coarser particle size tolerate humid handling noticeably better. The options are set out on the leachate solidification grade pages.