Selection and Sizing of Autogenous and Semi-Autogenous Mills. Pilot Plant Testwork Basis, страница 7

The net result from a testwork program as input to design criteria and mill sizing will include a range of applied net power draws based on:

•         Design values for specific power consumption for primary grinding and operating criteria in terms of ball charge volume, top ball size, total mill charge volume, mill speed, ore specific gravity, mill charge density, pulp density, feed sizing, and product sizing

•         Design values of specific power consumption for secondary grinding, ball charge volume, mill speed, pulp density, and product sizing, and (for pebble milling) total mill charge volume, mill charge density, pulp density, and pebble consumption

•         Criteria, similar to that preceding, which represents a practical range for variation in ore grindability for primary milling and, separately, secondary grinding, both to indicate mqx'rmim and minirr)iim valllCS

•         A decision to use fixed speed or variable speed mills.

PRIMARY MILL SIZING AND ASPECT RATIO

Given that the testwork program and process simulation exercises have defined the necessity for a single-stage or, more commonly, a two-stage grinding circuit and ranges for the applied net power draw in the primary mill, it is necessary for the grinding circuit designer to be in possession of a method for relating mill size to mill power draw for the range of operating conditions which are detailed in the mill purchase specification.

Traditionally, mill suppliers have provided data for net mill power draw in tabular and graphical form for a mill size, which is:

•         Either specified by the engineer-of-record to a specified range of operating conditions, mill speeds and power draws that are scaled up from testwork

•         Or is selected by the prospective supplier to satisfy the same range of specified operating conditions, mill speeds, and power draws.

More often than not, the predicted power draws from different mill suppliers do not agree with those of the engineer-of-record for a specified mill size and, alternatively, the mill size selected by a supplier does not agree with that calculated by an engineer-of-record for agreed power draws. These disagreements have arisen due to differences in approach that the various vendors and design parties have adopted when surveying various mills and interpreting their power draw, resulting in differences in calculating expected power draws. If one of the vendors has information on a similar ore type in their database, then this information is very beneficial to the design team. However the design team could also be inheriting the vendor's design prejudices as well, so care needs to be taken when assessing vendor supplied power draw information. The answer usually lies in a commercial compromise with respect to both mill size and motor power, usually in comparison with an installed list, which may result in a limitation on the operability of the mill. Such a limitation could take the form, with a gearless drive for instance, of:

•         Selection of a mill with a diameter which is inadequate with respect to drawing power but, more importantly, also with respect to passing pulp or pebbles; i.e., having sufficient grate open area

•         Selection of a mill with other than the optimal aspect ratio; i.e., the effective grinding length is either too long or too short to be compatible with the selected range of operating conditions. This length obviously affects the calculation of the effective mill volume, but not nearly so much as does the mill diameter

•         Selection of a motor with less than the required torque output over the desired mill speed range; i.e., selection of a motor with other than the optimal base, or rated design speed, will leave the motor with insufficient torque output at mill speeds that are higher than the rated design speed, even if the rated design power of the motor is adequate. This aspect of mill/motor selection is critical for grinding harder, more competent ores at the desired feed rates.

These points will be discussed and illustrated in more detail later in the paper.