Typically, an audit establishes the current conditions of the mill and identifies areas of possible improvement. This usually involves a full day for monitoring and collecting samples during mill operation, followed by a crash stop (i.e., without prior stopping of mill feed) for internal inspection. The audit provides an objective assessment of:

 

The first chamber volume loading, length, media grading, void filling, liner step, intermediate diaphragm setting and slot conditions

Second chamber volume loading, media grading, void filling, ball/liner coating, outlet diaphragm slot conditions

Total mill throughput, circulating load, separator operation, mill ventilation, temperature, water cooling and exhaust dew point

Utilisation and optimisation of cement additives

During the audit, data is collected for mill operation, including:

 

Feed rate and composition

Gross mill power

Mill speed

Circulating load

Circuit temperature

Ancillary power

Mill ventilation

Separator settings

Product fineness

Principal chemistry

Samples are taken of feed, product and the separator circuits for subsequent evaluation of circulating load and separator efficiency, product performance and possibly feed grindability. It is also sometimes useful to perform a mill retention test.

 

In each chamber of the mill, measurements are taken of the effective diameter, length and height above the charge and powder level relative to the media. The average powder level is estimated by considering the chamber in segments, depending on the variation in the powder level across the length and diameter. The diaphragm slot pattern and average width, together with an overall estimate of the percentage of blockage, is noted. The presence, location and degree of coating is also noted.

 

The media grading is evaluated in a number of ways.

 

Measuring a sample of 40-60 balls in chamber 1 and noting in standard sizes

Removing 5-10 kg samples from chamber 2 and sorting to determine the average size (It is often helpful to take three or more samples to assess media classification, along the media length.)

Material samples along the mill length provide an important guide to chamber 1 effectiveness, while samples in chamber 2 length can identify problem areas such as buildup of nibs (3-5mm or larger clinker pieces). Data collected from these and the circuit samples can help determine the main parameters influencing cement grinding efficiency, such as media loading and grading as well as circulating load and separator efficiency.

 

With a cement additive business spanning more than 80 years, GCP Applied Technologies draws upon a database of more than 400 audits of cement grinding systems conducted during the last 25 years for our clients. Such a mill audit requires mutually agreed upon targets and benefits and pre-audit preparation for safety (including isolation, access, confined space working, etc.), personnel, sampling, equipment, as well as mill availability (both in operation and stopped).