Feature

Online Particle Size Analysis Improves Reliability

Posted on 27 January, 2010 | Tags: Process Optimization

Problems can often arise in case of milling of metal granulates as a result of poor control of mill filling. Conventional techniques usually result in over filling of the mill that affects the operational efficiency of the mill. One of the ball mill operators was facing this problem until the installation of online particle size analyzer from Malvern Instruments.

On-line analysis is becoming increasingly widespread for the control of milling processes because moving process analysis from the lab into production can generate considerable cost savings. Conventional quality control methods concentrate on the identification and exclusion of out-of-spec product, whereas real-time and on-line process analysis enable to readily identify unsatisfactory plant conditions and hence improve the process operation.
On the other hand uncontrolled operation is very much characterized by the personal views of the operators on how to run the mill, but with a reticence to push the equipment to the limit. On top of that, modern processes are run with fewer staff, each looking after multiple operations.  
 In case of milling of metal granulates and foil waste in a ball mill, as a result of poor control of the mill filling, problems can often arise. The weight of metal powder is almost negligible as compared with the weight of the mill and the milling balls. Due to the lightweight of metal powder, it is not appropriate to use conventional methods such as acoustic measurement technique and weigh cells to measure the fill level. Using conventional techniques often results in over-filling of the mill that leads to erratic feed rate to the classifier, which results in poor classifier operation.

Case History
To address the issues with the performance of ball mill one of our clients installed Insitec, a rugged on-line particle size analyzer an automatic on-line particle size analysis system designed for use in process environment. This system was integrated with the existing plant equipment to derive maximum benefit. The aim of the project was to significantly reduce the specification span, particularly in mid-range particle sizes and ultimately have better product quality, improved processability and more efficient plant utilization.

The Process
With the aim to make product with the required specification, material from the mill is pneumatically conveyed to an air classifier. The coarse fraction rejected by the classifier is recycled to the mill. The fines are fed to a sieve and ultimately to various storage containers. The inert gas is compressed and then recycled after filtration.
Prior to installation of an on-line analyzer, product flow around the system was controlled manually using a series of flapper valves. Every few-hours final product samples were extracted and analyzed in the lab to measure particle size distribution with the help of laser diffraction instrument.  Due to working pattern in shifts, this analysis was often not carried out until the following day. The resulting time lag was a real problem because the plant operator did not have required information to control the proper particle size for the final product.
Some control of product quality was achieved by varying the classifier's speed of rotation. However, natural variations in raw material feed and especially the rapidly changing material flow rate from the mill to classifier resulted in large fluctuations in classifier operation. As a result, the product had a relatively broad particle size distribution.

Testing Laser Diffraction
A laser diffraction on-line particle analyzer was installed in the pneumatic conveying pipe between the classifier and the filter. As swirling and segregation are also problems in this area, a flow conditioner was installed after the classifier in order to enable reliable and representative sampling. The flow conditioner is a simple steel cross whose length is approximately three diameters.
At about one pipe diameter distance after the flow conditioner, a gas venturi continuously extracts a sample, disperses it and passes it to the measurement zone of the analyzer.
A laser diffraction pattern is measured with an update period of 15 seconds and converted by the associated software into a particle size distribution. As a result, relatively rapid changes in particle size can be tracked effectively. Particle size data, in the form of rolling averages is fed back into the process control system using a Modbus RTU protocol.
The aim of the initial trial was to develop an understanding of the current situation and work out an appropriate strategy to prevent the frequent overfilling of the mill and achieve a narrow specification of the product, without compromising process efficiency. Tests were carried out over several days to get a picture of mill performance in the absence of any automated control. These showed very large fluctuations in particle size distribution, which operator took a minimum of 15 minutes to rectify. During the unsteady operation, particle size rose by as much as 50 percent of the original value and the standard deviation in Dv(50) was up to 20 percent of the target value. The coarse fraction, Dv(90) value, occasionally rose to 150 percent percent of its initial value. While homogenization of the material during subsequent processing and transportation reduced the impact of these fluctuations on final product quality, the finished material still had a very broad particle size distribution.
The occasionally induced disturbances generate fluctuations in the amount of recirculated material into the mill. Even at constant raw material feed and constant classifier speed, changes in the final product fines are visible; with increasing amounts of recirculated material the product fines decreases and vice versa. The surveyed fluctuations are higher at high raw material feed rates and lead in the worst case to overfilling of the mill.
In 1978 Hiettler1 explained this behavior and developed a control strategy that took it into account. Since most process fluctuations are caused by an erratic feed rate from the mill to the classifier it was suggested that particle size should be controlled by controlling the raw material feed rather than the classifier speed. The theory is that with careful monitoring of the mill and the classifier, the particle size distribution should remain constant using a constant classifier speed and uniform material loading.
After Installation of On-line Particle Sizing System
 Taking into account the principles outlined above, an automated control system was designed and installed with recycle mass flowmeter as the set point and classifier speed as the variable parameter, which automatically varies within limiting values. When the closed-loop control system is active, throughput is increased if the particle size falls short of the target size, and decreased if it is too high. The classifier speed regulates itself automatically. This works for all ball-milling processes with closed circuits.
In our case the strategy was slightly modified to allow the raw material feed to fluctuate between two given limits. With the closed-loop control system now in place, material feed varies between these two levels but particle size remains steady. The standard deviation in Dv(50) has been reduced to 10 percent of the original value. Dv(90) and Dv(95) values show only small variations when the maximum load limit is reached. Mill throughput is now higher and operators are able to run the mill more efficiently, closer to its performance limits.
The installation of the online particle sizing system, together with associated particle concentration/loading measurement, after the classifier, has facilitated the rapid diagnosis of process problems. Automatic control of the raw material feed has been enabled and a 75 percent reduction in the original width of specification has been achieved. Classification is now more selective, resulting in a more effective product split and a narrower particle size distribution. The once routine lab analysis now serves only to check the product quality prior to release to customers.

Jim Scotland  is Regional Sales Manager, North - Process Systems, Malven Instruments Ltd India
Contact: soloni.gosalia@malvernaimil.com