How to Prevent Caking of ATH Powder After Drying

ATH (aluminum hydroxide gibbsite) powder caking after drying arises from multiple root causes: residual free moisture, surface hydroxyl‑hydrogen bonding, fine particle high‑surface‑energy agglomeration, pressure compaction during storage, temperature‑humidity cycling, and incomplete drying or over‑drying‑induced surface activity. Caking is especially severe for ultrafine / sub‑micron ATH grades. Caking can cause feeding difficulty, poor dispersion in polymer, inconsistent D97, and downstream cable compound defects. Below are systematic industrial preventive measures.

1. Strictly control residual free moisture after drying (primary factor)

Even tiny excess free moisture promotes hydrogen‑bonding between ATH surface hydroxyl groups and forms solid bridges between particles.

1. Control final free moisture: ≤0.2 % for cable‑grade ATH; ideally 0.05‑0.15 %.

Distinguish: free surface moisture (must be removed) vs crystal water (must retain, LOI ≥34‑35 %).

2. Follow optimal drying temperature window: particle temperature 100‑120 °C, avoid incomplete low‑temperature drying, also avoid excessive over‑heating which increases surface activity.
3. Eliminate dryer dead zones: material hold‑up leads to partially under‑dried lumps mixing into finished product. Regularly clean dryer internals.
4. After drying, implement effective gas‑solid separation; prevent wet tail‑gas condensation contacting hot powder inside cyclone / bag‑house. Cold spots inside dust collector cause local moisture re‑condensation and seed caking.

2. Proper powder cooling before packaging (very frequently overlooked)

Hot ATH powder packed directly into sealed bags will release heat inside package; internal air cools and water vapor condenses on powder surface, triggering severe caking inside bags.

1. Install powder cooling screw / fluidized‑bed cooler after drying. Cool powder down to ambient temperature +5‑10 °C (≤45 °C) before packaging.
2. Do not pack warm powder. Even low‑moisture hot powder will cake during storage after sealed packaging.
3. Avoid cold‑wall condensation inside cooling equipment; keep cooling‑wall surface temperature above dew‑point of process gas.

3. Modify particle surface properties

Dry‑surface coating / surface passivation

For cable‑grade ATH, small‑dosage surface treatment reduces surface hydroxyl activity, weakens inter‑particle hydrogen bonding.

1. Silane coupling agent (for polymer composite application): low dosage 0.3‑0.8 %, applied in high‑speed mixer or inline coating unit post‑drying. Besides anti‑caking, also improves compatibility with polyolefin matrix.
2. Minor inorganic anti‑caking agent: very fine fumed silica or alumina, dosage 0.1‑0.3 %. Forms isolation layer between ATH particles.

Note: Dosage must be strictly controlled, excess additive will degrade electrical properties of LSZH cable; must match end‑specifications.

For wet‑process ultrafine / sub‑micron ATH

Add appropriate dispersant in bead‑mill slurry stage (polyacrylate type). It remains on particle surface after drying, reduces hard agglomeration tendency. Select dispersant compatible with downstream polymer requirements.

4. Optimize particle‑size distribution

1. Excess ultra‑fine fractions (over‑grinding) drastically increase caking tendency. Prevent over‑grinding; control volume fraction <1 μm fines. Higher specific surface area = stronger inter‑particle attraction and easier caking.
2. Reasonable particle‑size gradation: appropriate proportion of coarse‑fine mixing reduces void fraction and contact points between ultra‑fine particles.
3. Avoid excessive circulating load in closed‑circuit grinding which amplifies super‑fine content.

5. Control packaging and storage conditions

1. Packaging material: use thick‑weight inner PE moisture‑proof bag + outer woven bag. For long‑term storage or humid climate, use aluminum‑laminated moisture barrier bags to block ambient moisture ingress.
2. Fill nitrogen sealing (optional high‑end grade): reduce residual moist air inside package.
3. Stacking pressure control: limit stack height of bagged ATH. Heavy stacking pressure forces particles into close contact and accelerates compaction‑caking. Typically stack ≤8‑10 layers.
4. Warehouse environment: keep relative humidity RH <65 %, temperature stable. Avoid large temperature‑humidity fluctuation; prevent dew condensation on bag surface. Store off‑pallet, avoid direct contact with cold damp concrete floor.
5. First‑in‑first‑out inventory management; limit storage cycle, avoid long‑term stockpiling. Caking risk increases with storage time.

6. Post‑treatment: de‑agglomeration after drying

Even if caking prevention measures are applied, soft agglomerates may still appear for ultrafine ATH.

1. Install post‑drying de‑agglomerator / turbo‑sifter before packaging. Breaks soft agglomerates before bagging. Cannot eliminate hard caking caused by excessive moisture or thermal condensation.
2. If hard caking already formed inside bags, it cannot be restored back to original primary particle size by simple sifting. So prevention is far better than post‑remedy.

Common troubleshooting table

Summary of key priority actions

1. Control residual free moisture ≤0.2 %.
2. Fully cool powder below 45 °C before packaging.
3. Avoid over‑grinding to reduce excessive ultra‑fine fractions.
4. Apply suitable surface passivation (silane etc.) if product specification permits.
5. Moisture‑barrier packaging, limit stacking height, stable low‑humidity warehouse.

Important note: Hard caking formed by water‑bridge solid‑bridge cannot be eliminated by downstream grinding; all control must be implemented at drying‑cooling‑packaging stage.