Things to consider when ramping up production through an existing mill
In mining, productivity is king. From the initial blasting phase to the final step in the processing of ore, even small inefficiencies in the operation can add up. The same holds true when it comes to the comminution circuit. Bottlenecks in the system can prevent crushing, screening and milling equipment from operating at their optimum performance levels. But first, the bottlenecks needs to be identified, which is sometimes no easy task. MINING.com spoke to the experts at Metso to get some answers on how to get the best production out of your mill. The responses are by Matt Gallimore (MG), Global Director, Process Equipment, Jeff McKay (JM), Global Manager, Expert Systems, and Chris Wyper, Director, Pumps at Metso.
The plant is running at full capacity, or close to it, but the mine owner wants to ramp up production. What diagnostic tools are available to measure/ track process optimization, and how do they work?
JM: Operational assessments can be blurred due to uncalibrated instrumentation, controls that are not used properly and historian data that is missing some key information. Getting these addressed is a solid first step to measuring optimization work.
An Advanced Process Control (APC) tool such as Metso’s OCS 4D would be a good tool. We utilize an APC approach called “Dynamic Constraint Control” which identifies the current limiting aspect of the plant and adjusts production to try to stay close to that limitation. The same tool quantifies the percentage of time each potential constraint actually limits the plant production. This combined control and diagnostic tool then provides hard data on where to place capital investment and what the expected improvement will be once the constraint has been removed.
What are some of the ways of optimizing the plant, that will increase your productivity? Other than being more productive, what other advantages are there to optimization?
MG: Generally, to get the best assessment of the operation, instrumentation must be calibrated, and controls properly tuned. If these first steps are not done, the entire study is in jeopardy.
Many plants believe they are running at near full capacity. There are plenty of examples where the bottlenecks in the system may not be not well understood. Looking at which equipment is near peak operation and what percentage of the time may be a starting point. However, too many other things may be masking the root cause. For example, a pump that is the bottleneck may be a symptom of poor classification, grinding mill performance or changes in ore characteristics. None are directly related to the pump.
A good audit of the circuit, coupled with a review of historical operating data can uncover opportunities for improvement. Beyond sampling the ore, and mass balancing, there are simulation and optimization tools to project a more optimal operating condition for the equipment. Mine to mill assessments (JKSimMet, Metso VPS software) can highlight economical ways to improve capacity by improving blasting techniques or crusher operation. Discrete element method (DEM) tools such as Mill DEM Modeling Suite can assess liner effects on charge. Such tools can highlight potential adjustments to ball charge, liners, classifier operation, etc.
The study of audit data can also lead to a range of recommendations for improved automation, better operational practices such as ore blending, and changes to the flowsheet.
The goal of optimization varies with what is to be optimized. The objective can be a higher production rate, lower production costs, or maximizing net present value. The goal drives the recommendations and selected actions.
Changes to plant operation have been shown to increase equipment availability and reduce opex in addition to improving the production rate.
JM: Use of a self-quantifying control strategy such as Dynamic Constraint Control, within a solid operating platform like OCS-4DTM becomes the basis for justifying audits and analysis of suspected bottlenecks as it provides the necessary data to support the auditing effort and expense.
Where are the typical bottlenecks/ problems in the comminution circuit?
MG: Bottlenecks may be found in the equipment for comminution, transportation or classification. Causes may be related to ore, operations or equipment design. Often the cause blamed for a bottleneck may not be the root cause. The benefit of a quality study is properly identifying both the root cause and corrective actions.
Having run the efficiency tests, you find out that in fact all the equipment is working well together. There are minimal efficiencies to be gained. What questions do you need to ask yourself before going down the road of upgrading the mill?
MG: There are some basic process questions like:
- Is the circuit properly configured?
- Can we get a machine that is not the bottleneck to do more work after the bottleneck has been identified and solved
- Can we get higher equipment availability?
- Can we adjust the inputs to the circuit (finer feed/ore blending) to help the mill?
- There are controls questions: Does the plant have the right instrumentation? If not, could more instrumentation and an advanced process control system (such as Metso OCS 4D) provide a performance boost?
JM: Every plant always has a bottleneck. That is, every process has a stage or piece of equipment that stops the operation from producing more. Plants continue to expand throughout their lives or are shut down. Reductions in production are virtually never an economic solution as cost per ton goes up not down in smaller plants. Expansion is driven by value per tonne of the mineral processed and stops when the cost to expand is no longer supported by the value per tonne of mineral.
If you decide that an equipment overhaul will get you the desired production increase, you can either upgrade existing equipment or replace it. What are the considerations in deciding whether to upgrade or replace?
MG: A lot depends on the type of machine and duty.
A gyratory crusher can sometimes be upgraded in place. This was a driver in the design of the Metso SuperiorTM MKIII primary gyratory crusher, which gives more capacity for the exact footprint of either MKI or MKII but provides superior opex.
In the case of a cone crusher, many Symons cone crushers have been replaced by Metso HP cones that fit in the footprint for seamless upgrades.
If SAG mills are the bottleneck, the addition of a pebble crusher may be an option, but replacement is rarely done; rather, additional lines are added and the older equipment continues to be used until is no longer sustainable.
For ball mills, replacement is less common. Often, after optimizing ball charge, liners and classification, the attention is to crush finer or add additional grinding, parallel to or after the existing mill. Parallel addition involves a significant step change in production. Additional grinding in series may provide more incremental benefits: higher tonnes per hour (tph) but maintaining or improving fineness of grind. In the case of adding in series a more efficient comminution device should be considered. A stirred media device such as a Vertimill® or SMD® can provide extra grinding capacity more efficiently than conventional ball milling.
What are some of the considerations for pumps and pumping systems, in making the circuit more productive?
CW: A good plant design will allow plenty of power for the mill pumps. These typically operate on variable speed drives, so the speed and hence the power draw will vary depending on many process variables. It is worthwhile to examine some historical data on power drawn, to estimate how much extra power would be available for tonnage increases. Although engineering data sheets often state a minimum, nominal and maximum value, these can be an oversimplification. Of more use is a point cloud type plot, that shows the flow and pump pressure as a function of time. This can be very useful for the pump and cyclone supplier in deciding the optimal equipment sizing based on real world data, too.
It is widely recognized that high inlet velocities combined with large, sharp material is a significant cause of impeller wear. Prior to increasing the pump flow the inlet velocity should be checked with the supplier. In general 4.5 to 5 m/s is considered a maximum limit for abrasive duties. Many pump suppliers are able to provide a variety of impellers and inlet liners for the same casing that can help to reduce inlet velocity without sacrificing efficiency or requiring an expensive pump upgrade.
Increasing the pump duty will usually increase the power transmitted through the gearbox. This increases the amount of heat that must be dissipated. A gearbox that is sized marginally for air to air cooling may overheat at the higher continuous duty. Consideration must be given to the cooling capacity of the lubrication system, particularly at higher ambient temperatures and altitudes.
The pump gland seal water system should be sized to adequately deliver a constant flow of gland water under all operational conditions. This applies to the pump duty, including any increase in head due to the tonnage increase. It should also be checked that the gland seal water system is adequate when other demands are placed on it, such as hose down or flushing.
The rate of abrasive wear on surface is approximately proportional to the fourth power of velocity (if you double the speed, the rate of material loss increases 16-fold). If the increase in throughput is substantial then it will be necessary to re-examine the pipe sizing to minimize friction losses and wear. Of course, if there is a large variation in flow (for instance due to large variable in ore grindability) then the minimum velocity to prevent settling should be a factor under consideration.
For large increases in throughput the volume of sumps may need to be increased to allow stable process operation. Typical residence times for a mill sump are in the order of 30-60 seconds. If the residence time is too small then level control may be difficult.
The live volume of floor sumps should be calculated to take into account the maximum inflow caused by a plant crash stop. This may include the mill static overflow and any dump valves to empty pipes and sumps. If sump size is increased or the mill volume changed then the sumps may be undersized. In this case they can be deepened, enlarged or additional sumps created to deal with the volume. Typically mill sumps should be separated from the other sumps in the plant due to the possibility of mill balls entering the sump.
A well-designed plant should have enough redundant cyclone capacity to cope with routine maintenance of at least one cyclone even at maximum recirculating load. If the tonnage or recirculating load increases then this will reduce the number of available cyclones. It is tempting to use the “maintenance” cyclone for additional capacity but this can result in a frustrating lack of availability. If the cyclone clusters are currently under manual control then any upgrade is an ideal time to consider the benefits of automatic control.
How can you design the circuit so that it’s flexible and can be adjusted if the mine plan changes, considering that the circuit should run seamlessly?
MG: The plan can be simple or complicated depending on ore variation and operational considerations. Ideally one would want a plan that seamlessly allows transition of work load to the equipment that can best handle it. In the case of a SAG – ball mill circuit, one type of ore may be SAG limited and another ball mill-limited. The challenge is setting up a plan to get the most effective work out of the equipment and thus highest productivity. In some cases, the ball mill cyclone coarse split can have some of its flow directed back to the ball mill when ball mill-limited. When SAG limited, the cone crusher closed side setting can be decreased and/or sent to the ball mill circuit. Definitely, advanced process control strategies should understand the current limit and make changes that help balance the load and maximize utilization of circuit assets.
If the production increase entails using different ore, how do you know if your existing comminution circuit can handle it? How much of a factor is ore hardness in having to optimize, upgrade or replace current equipment?
MG: There are several techniques to assess how the comminution circuit can handle ore variation. One of the challenges may be in the regrinding circuit as there can be very high fluctuations in mass flow when there are swings in ore grade.
Ore hardness can be a decisive factor depending on ore variability. In cases where there is minimal variation, bottlenecks do not occur very often. However, with highly variable ore, the operational philosophy should be considered. Ideally it would extend back to the mine and blend mine and concentrator production planning. Again, optimizing control logic needs to be intelligent enough to handle ore type variations by looking at multiple constraints and rates of change of key variables in making control decisions.
Obviously, budget is a critical factor in deciding how the mill is upgraded. Does spending more (ie. equipment replacement) always get you better results, in terms of productivity and maintenance costs? Can you think of an example where a lot of money was spent on new equipment, but the production goal wasn’t reached, and why that was the case?
JM: A proper assessment should identify problems and present recommendations based on capex, opex and net present value. A plant workforce can then select the goal they wish to pursue. If the assessment was done poorly or with inadequate data, then the implementation risk is higher than if a proper study was conducted.
In cases where the proper legwork wasn’t done, the root cause of the bottleneck may not have been identified. Then equipment added to the flowsheet or modifications to the equipment will not deliver the desired improvements.
Also, if equipment is added or modified and another device was nearly at its maximum, it can become a constraint and result in a disappointingly small increase in production. By failing to identify or address that constraint, the project fails to achieve its goals. Capital needs to be invested judiciously to achieve satisfactory return on investment. In general, investing no money into improvements assures no improvement; however, simply investing money without thought to what the potential effects and secondary bottlenecks may be, will not yield acceptable or desirable benefits.
We have been involved with clients who have installed advanced process control from us in their comminution circuits and it has driven production up, only to be restricted after little or no gain, because the mine had insufficient capacity to keep up with it. Concentrators often increase SAG mill ball charge in order to increase hard rock production, only to have it wasted when the mine switches to a softer ore and the SAG mill is not a constraint.
We have also had clients who were able to use the constraints limiting a process, to increase pump motor sizes and who enjoyed a substantial production increase based on this relatively inexpensive solution. Placing good capital in intelligently directed areas is a formula for success.