A typical roll compaction system is configured with an initial feeding device, roll compactor, mill, screener, and conveyor. Material enters the compactor (via twin feed screw design), is formed into compact, milled, and screened to separate recycle from the final product. The fines & overs are blended with fresh feed and recycled back into the compactor. At times, a secondary mill for regrinding overs, also known as a regrind loop, is beneficial to increase the system’s overall production rate.
In an ideal situation, a roll compaction system produces a first pass yield of 50% or greater. When a single mill compactor system produces 35% yield or less, a secondary milling circuit is strongly recommended in order to increase the yield to a minimum of 45-50%.
Secondary milling is common in systems with high production rates or when the objective particle size distribution (PSD) is narrow. The expectation of an additional mill should be to increase the total product yield between 8 – 20% depending on the system, the integrity of the feed material, and PSD required. Addition of a secondary mill is economically efficient when it accomplishes two main objectives:
(1) Applies a larger force to the particle than the primary mill
(2) Creates a particle size that compliments the objective PSD
In a roll compaction system, ‘overs’ are considered any particles larger than the top screen size of a screener. Overs are known to create excess heat and lower the total system yield when recycled through a compactor. When a system experiences these conditions, a secondary milling circuit is a cost effective solution with quick return on investment.
– Particle Properties –
Overs that have ‘survived’ the primary mill settings are now harder, smaller, more uniform, and require a greater energy to fracture the granule. Using the same blade type, rotor speed, and screen in the secondary mill will be of little to no benefit as the overs have already been exposed to this force and did not reduce to the objective PSD during primary milling. To optimize the performance of the system, a secondary mill should be configured with a different blade type, speed, and screen. Using the same mill settings in the regrind loop would allow the overs to accumulate within the circuit, causing the secondary mill to operate at a high amperage and overload the top screener deck. To produce the highest product yield from the secondary mill, a greater force must be placed on the particle in order to fracture it down to the desired particle size. IPA typically supplies secondary milling circuits with the following: (a) impact blades, (b) a minimum of one screen size smaller than the primary mill, and (c) a rotor speed on average 30% higher than the primary mill speed. By changing the settings of the secondary mill, the overs can be efficiently milled into acceptable product.