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Grinding Mill Control Types (17 replies and 1 comment)
Is your interest primarily in SAG/AG mills? Can you be a bit more descriptive re use of sound control? Was it in conjunction with the other commonly used variable you mentioned?
Yes, primarily SAG Mills. The idea was that the mill was set to a certain Db rating and the mill would slowly fill till it hit that rating and then keep the feed constant at that rating. Didn't work that well as it always underfed the mill. May have something to do with the calculations that worked out the ratio!
I found the control Philosophy so that I can describe it more accurately.
The required acoustic level is entered by the control room operator as a set point into the acoustic controller. The acoustics of the mill charge motion is measured by a device mounted adjacent to the SAG Mill shell. The resulting signal is transmitted to the acoustic indicating controller which, in turn, sends a remote set point to the SAG mill speed controller. The speed of the SAG mill is then automatically adjusted (within set limits) to maintain the required acoustic level.
Sound provides additional information on what is happening in the mill with the grinding media and mill load. It's particularly useful in conjunction with the traditional measurements you mention (total load from bearing pressure, load cells and power draw).
Wow, good comment, I believe you are talking about three parameters, Sound, weight and power. Now a day’s those three parameters are fully controlled by manter cub, and it has reduce the work of control room operator.
The problem with sound measurements, and other set points is the optimal operating RPM, filling, et al varies with mill liner geometry. Typically, new liners cast the charge trajectory much further and therefore register higher sound pressure readings. The RPM is limited by the trajectory. Later as the mill liner geometry has softer worn lines; the charge is not cast as far and thereby can support an increase in RPM. In the latter case, the charge cast will be more uniform. Typically, the mill throughput will increase between 6 and 9%, if you have a variable speed mill.
Liners are often designed with this in mind. The trend in mill liner shapes has changed from olden days when the liner/lifter pitch set = 2 x mill diameter in (ft.). Today, the liner pitch set should be half that ancient value or less. Most mills are handicapped by the bolt pattern set by the old design philosophy. Cadia 40 ft. mill original lifter set = 78 rows. Today the lifters are 26 high and 26 low rows. The original lifter height above the inner liner floor was about 150 mm. Today it is about 440 mm. The later improve liner life by about 47% when considering total tons ground. Work Index (kW-hrs/ton) was reduced by a few %, meaning more throughputs are possible. However, more aggressive liner geometry would be required to capture this benefit. Published proof is in SAG 2006 article on Cadia written by Steven Hart and the undersigned with Craig Faulkner of Bradken.
What about the future? Can throughput and liner life be increased without penalizing kW-hrs/ton? We say yes. How much? Substantial! This can be said for all AG/SAG mills with variable speed control and uni-directional rotation. Bi-directional rotation can also be improved, but, by a lesser amount due to bolting limitation and casting sizes. However, all things are possible.
In short, all AG/SAG mills are under-utilized for their size. Why? Good question.
We have supplied microphones to sites to 'monitor' the mill. I don’t believe it can be used to 'control' the process or operation of the mill. These systems are more common in dry grinding applications such as cement mills where the operation is perhaps a lot more stable. In wet mineral mills, to me, there is too much going on to obtain any meaningful control parameters.
Sound, pressure (weight) and power, along with measured charge volume and % solids are measured. These measurements are used to control the variables in relation to the "model" for a particular ore. Is it the best system? Use of multiple microphones around the shell adds extra dimensions of measurement to determine the toe of the charge. It helps! Is there a better system out there with variables being Feed, Water, Mill Speed?
Microphones are quite noisy instruments beyond establishing the onset of ball impact trajectory. Once balls hit liners the typical microphones are not set or equipped to feedback a noise pattern than can be calibrated to the over-throw of balls beyond the toe.
In addition, the location of the microphone, along the mill axis, can alter the reading. Any microphones near the feed or discharge ends will have added noise due to charge throw of the end cone wear bars.
All these points change with shell liner wear. They can be calibrated with PI data to improve the set points. It does take time to convince the operator how to accomplish the best practice. Changing ball to ore ratio will also change the toe position along with RPM.
Agreed, after monitoring and viewing the results of a SAG optimization that resulted in significant throughput increases I noted that the stability of the mill remained similar to the circuit prior to the changes. The mill could still become unstable and require a grind out with a change in feed (etc.). Particle size analysis on the feed can help, but will not indicate a change in ore WI. I am wondering if other systems have been developed out there for SAG mill control. (This question ties into a project from 2010)
Take a look here as an example: The benefits of using SmartEar at Pueblo Viejo:
One missing component in the mix is the correct ball to ore ratio for maximum comminution where you cannot increase or decrease power without reducing throughput. Both kidney size and correct mix are critical to achieve optimal performance. Getting feedback on ball contacts does not provide this condition. Although, it does help to minimize ball and mill liner breakage! Mill RPM can be maximized to reach the limit of noise.
We are San Cristobal Mine in Bolivia; we are working with sound control in 36’ SAG mill, the sound control is very good in term of throughput, liners protection and power consumption.
The required sound level (DB) is entered manually as a set point into the Expert System, the sound of the mill charge motion is measured by 4 microphones adjacent to the SAG Mill shell, and the resulting signal is transmitted to the Expert system which, in turn, sends a set point to the SAG mill tonnage and speed controller. The expert system automatically adjusted the tonnage and speed (within set limits) to maintain the required sound level according some rules.
The cement guys should step in here, as they have been using it for a long time now, whereas we minerals guys are a bit behind. Obviously, they are dry grinding whereas we are almost completely wet grinding. I know of a number of sites that have introduced sound as an input to an expert system with mixed success. The greatness gain has been in liner protection on large SAG mills.
Yes, the system is in use since decades in the cement. From my experience the system give a good indication about what's going on inside as long as the mill is a single unit in the building. With two (or more) mills in the same building forget it. There are other systems available that are installed directly onto the shell and control filling level and temperature. The results are much better because the influence of surrounding machines is reduced.
In my experience mill sound is important but is only one parameter that should be used for mill control. It is complimentary to the other common control actuators such as speed, feed rate, mill mass and density and not a substitute. However, with the right type of advanced instrument mill sound can be extremely useful in helping to control the load dynamics (ball trajectories and impact zone) to increase ore reduction, reduce energy consumption and improve liner life through prevention of ball on liner strikes. Together with mass and power it helps the operator keep the mill at peak power without topping over the peak of the curve and prevents overfilling while maintaining protection of the expensive protective process lining. Modern SAG mills that have variable speed and mill mass measurement capability are more often including mill sound into their control strategy and in some cases integrating this into the advanced process control enabling this input to influence the automatic process control. If you have the contacts I believe that Glencore's Antapaccay operation in Chile is a very good example to learn from on how this can be done well.
Having used weight control and power draw to control Mill operations, using Mill Sound to run a mill was a new one for me. I found it unusual and not that effective. Has anyone else used that style of operation control to run their mill and did you find it more effective than the more traditional way.