Primary autogenous mill feed is either primary crusher discharge or run of mine ore, containing material on each size of the screen analysis below the largest piece in the feed. It is subject to the large variations in size analysis normally found in primary crusher products or run of mine ore. There can also be variations due to segregation in stock pile or bin storage. In feeding primary rock mills there is usually no attempt to distinguish between media size and ore feed size. Variations in size analysis, shape, crushability and grindability of the feed are unavoidable and cause wide variations in the feed rate to the mill.
Critical size buildup is a term frequently used in connection with autogenous grinding, particularly in connection with primary rock milling. It can be defined as a buildup or accumulation in the mill of ore or rock- too large to be broken by the coarse fraction of the mill feed and too small to be effective grinding media for grinding the fine fraction of the mill feed. In primary rock mills critical size material comes from two sources:
- Directly from the mill feed.
- Coarse fraction of the feed either breaking down or wearing down to critical size.
When primary rock milling is followed by secondary rock milling it is the critical size material that is removed from the primary mill to be used as media in the secondary mill.
Reports from some primary rock mill installations indicate operating problems and wide fluctuations in feed rate due to critical size buildup. In some plants it is necessary to add to the mill a 5 to 20% charge of large steel balls to assist the comminution process, particularly in breaking the critical size material. The addition of balls to primary rock mills reduces the benefits of autogenous operation, and has been called “Semi-autogenous grinding.”
The feed to the primary rock mill at Bolidens Vassbo Mine is blended from three bins. If there is an excess of the 3½-in. x 1½-in. fraction (the middle fraction), which is the critical size range, it can be bled off as required to be used as media in secondary rock mills used as regrind mills or it can be crushed and added to the minus 1½-in. fraction. By use of a special dc drive, mill speed can be varied to suit feed conditions.
Primary rock mills are frequently run with a 25 to 30% load. Low loads permit impact crushing of the coarse and intermediate size fractions of the mill feed. Crushing occurs from the impact of the lifters against the ore, impacting of the ore against the liners at the toe of the charge and impact from other pieces of coarse ore. To prevent critical size buildup in primary rock mills, crushing and abrasion of the coarse and intermediate sizes through the critical size range must be at a rate no slower than the rate at which critical size material is generated in and fed to the mill. The load level in the mill has to be low enough to permit impact crushing in the mill.
In all three phases of rock grinding it has been found necessary to have the lowest level discharge possible to prevent cushioning of the grinding action and to minimize any tendency to float the charge.
The primary rock mills installed in this country and Canada have been large diameter short length mills with a length to diameter ratio of approximately 1:3. Dry grinding requires the short length to give distribution of the coarse material throughout the mill. The low load level in the mills is also a result of the L/D ratio and low level discharge. In South Africa, the mills wet grinding run of mine ore have L/D ratios approaching 1:1. These mills carry higher loads and there are no indications of lack of flow through or poor distribution of coarse material in these mills.
The large feed openings required to pass 12 in. and larger ore also restricts the load level that can be carried in primary rock mills.
Reports show anywhere from 5% to 25% higher power consumption per ton of ore ground in autogenous mills than required in a conventional crushing and grinding
circuit when making the same product size from the same feed size.
Liner wear in primary mills is reported to be greater than the wear on crushing surfaces and mill liners in a crushing and grinding circuit. Overall savings in steel consumed are estimated to be 55% to 60% of the cost of rod and sometimes ball wear. Savings in semi-autogenous milling should be less due to ball wear and increased liner wear.
Ores that break rapidly through the critical size range yet contain a sufficient quantity of coarse hard material to serve as grinding media to perform the desired grinding action make the best feed for primary autogenous mills. Uniformity of size distribution, shape, crushability and grindability of primary rock mill feed reduces the operating problems encountered due to critical size buildup.
If not constantly watched, rock mills can become upset very quickly. Automation of primary mills using a combination of mill power and mill sound is recommended to constantly watch the mill operation and adjust the feed rate to compensate for the variables in the mill feed.
Secondary autogenous mill feed is normally rod mill product or an equivalent size from a roll crusher or a primary rock mill. Media, sized and removed from the ore stream ahead of the mill, is selected to have the proper size and weight to grind the mill feed. In secondary rock milling as in ball milling the media is selected for the work to be done. The proper size media is that having the same weight as the proper ball size for the same service. Media is added to the mill continuously with the feed and is considered as part of the total feed rate. While normally added at a fixed ratio to the feed rate the rate of media addition can be changed based upon the power being drawn by the mill. This permits adjusting for changes in the quality of the media.
Secondary autogenous mills are low level diaphragm grate discharge mills with the length being one to two times the diameter. The low level discharge is required to obtain flow through the mill and maintain a pulp that will not float the charge. The mills draw maximum horsepower at between 45 to 50% load level and for control purposes are normally operated at a point slightly below maximum horsepower.
Media addition to secondary rock mills varies from 1½ to 5% of the total feed rate. It is added at a rate to replace media which has worn down and has been ground to product size. With controlled addition of coarse ore for media there is no need to crush this material in the mill. The media wears through a critical size range, but the range is probably small and the quantity small enough so that the wear rate is fast enough to prevent a buildup. The ratio of media size to ore size in terms of particle diameter is larger than that in intermediate autogenous grinding, and may be as high as 30 to one. No reports indicate the need to add balls to supplement the grinding action of the ore media.
Reports indicate the power required by secondary rock mills is the same as for ball mills performing the same work. With proper media load and size, variations in capacity are in line with variations in feed conditions.
Liner consumption is greater than in ball mills. Estimated saving in steel consumption is 65 to 70% of the cost of ball consumption. The expense of rod consumption is still present when rod mills are used to prepare feed.
Secondary rock milling can be applied to any ore or rock containing sufficient material which is sufficiently hard in the required size range to be used as grinding media.
Automation of secondary rock mill circuits helps keep them operating at optimum capacity by adjusting media addition and feed rate to suit conditions in the mill. Media addition can be regulated by mill power with the ore feed rate control based upon circulating load.