Communication Circuits - Milling Overview

The generic continuous milling circuit is similar for both wet and dry processes, as well as for mills of capacities from 1kg/h to 200t/h. Wet milling can produce finer sizes than dry grinding, since less energy is required. In both wet and dry, a continuous stream of coarse feed material enters the grinding zone and a distribution of finer sizes mixed with unground material exits from it. This intermediate is separated into in-specification product and coarser material, which goes for regrinding. The exiting product can then be further classified to remove super-fines. The size reduction step is carried out by a variety of means, such as ceramic balls, or turbulent air jets.  Insitec systems can be used in many situations, including on pharmaceutical airjet mills and cement ball mills:

Generic milling set up - where "real time sizing" can help

In the case of a dry comminution circuit, there are a number of positions where the installation of an on-line particle size analyzer will enable closer control of the milling process. The final choice of installation point will depend largely on the process and its individual requirements.

A. Process feed stream
Useful for quality control of feed materials, where variation may have adverse effects on the performance of the main unit processes.

B.  After the dynamic classifier
Downstream of the main unit process this can be especially helpful in process start-up. Locating downstream of a dynamic classifier may mean that there is a strong swirl in the powder stream. This can be neutralized using a flow conditioner to give a uniform powder stream for sampling.

C.  Final product stream / below classifier
The most widely adopted sampling point since it gives both quality control of the final product and a real-time view of the entire process performance, allowing optimization of activities.

D. Superfines stream / below filter
Valuable where the product is collected after a bag house (most cement milling processes, for example), or as a measure of the wastage that results from overgrinding and overproduction of fines. This location also allows the Insitec to monitor and improve filter performance.

In some applications (toner production for example) two on-line particle size analyzers are installed on each line, in positions B and C. This gives real-time yield information on the fines classification step.

Air jet mill example

The following is an example of a flowsheet for a milling system. Here an air jet microniser is shown, complete with feed system, dynamic classifier, cyclone and filter.