Agglomeration is the formation of aggregate by the sticking together of feed and/or recycle materials, and it includes the formation of agglomerate nuclei. The main objective in agglomerating fines being the conversion of ores, minerals and chemicals of undesirable fineness into agglomerates characterised by a size consistency desirable for subsequent use or processing. In metallurgical applications, the unit process of balling aims at achieving highly permeable large aggregates with a built-in ability to withstand large crushing forces. The latter can be achieved by resorting to particular binder additions which confer structural integrity to the ball or pellet after a specified curing period. The ability to form strong pellets then enables metallurgical recovery of finely disseminated metallurgical values by subsequent hydro or pyrometallurgical techniques.
Procedure
- Homogenise the sample, and determine and record its chemistry, size analysis and (when necessary and possible) the specific surface area.
- Where applicable add necessary quantity of binder ex: Portland cement, lime or bentonite.
- For a standard column leaching charge of 25kg, weigh out separately 3 kg for seed ball preparation. Normally, you would require around 10% of overall charge.
- Clean inner surface of balling drum and moisten very lightly. It may be desirable to precoat inner surface of drum with a thin layer of material being agglomerated.
- Start rotation of drum and add small quantities of ore in 50 gram amounts to the drum. With a water spray bottle, ensure a fine adequate wetting of the ore surface. The precise amount of water addition will be arrived at after some practice.
- When sufficient seed balls have formed empty them onto a nest of sieves such that seed- balls only in the range 0.45 to 0.55 mm are accumulated. The minus 0.45 mm balls will be re-fed into the drum, along with fresh fine ore. The object of restricting seed-ball size range is to ensure a small size spectrum with regard to finished green balls.
- Maximise build-up of seed-balls in the above size range. Divide the rest of the charge to be agglomerated into say 3 batches and equivalently the number of seed-balls.
- Introduce seed-balls into the balling drum and allow to cascade in drum. Wet surface lightly with spray.
- Start stop-watch and attempt to add fine ore as quickly as possible over a 3 minute time span maintaining a fine water spray addition.
- Complete ball growth utilizing all of the available fines which have occurred by now.
- Allow rotation of green balls for a further 90 secs and then spread out prepared balls onto a plastic sheet for drying.
- Set aside samples of green balls for moisture, green compression and dry compression testing.