SPM-8100 Ball Mill – Simtronics SPM Series

SPM-8100-Overview-Black-Image

SPM-8100-Feed-System-Black-Image

SPM-8100-Ball-Mill-Classifier-Black-Image

SPM-8100-Product-Recovery-Air-System-Black-Image

SPM-8100-Exhaust-System-Black-Image

Process Description

Overview of Ball Mill

Simtronics’ Ball Mill simulator consists of the following equipment:

Feed Screw Conveyor
Feed Bin
Ball Mill
Oversize Classifier
Cyclone
Powder Product Bin
Air Circulation Fan
Bag Filter
Exhaust Fan

The Feed Screw Conveyor, X-101, transfers solid feed particles having an average diameter of 0.5 inches to the Feed Bin, V-101. The Feed Bin then transfers feed into the feed hopper of the Ball Mill, X-102, by gravity using a rotary valve.

The Ball Mill employs heavy, hard metallic balls which tumble within the mill as it rotates. The mill is rotated at a sufficient speed to cause the balls to be lifted up the wall of the cylindrical mill to a point where gravity will pull the balls down toward the centerline of the cylinder. The falling action of the balls breaks the feed material into smaller pieces. The dimensions of the mill, the speed of the mill and the diameter of the balls determine the average size of the powder that will be produced. Air sweeps through the mill to carry the produced powder out of the mill to the Oversize Classifier, V-102.

The crushed larger diameter particles swept out of the Ball Mill are separated by the Oversize Classifier, V-102, and returned to the Ball Mill from by gravity so they can be re-crushed to on-specification size. The balance of the produced powder swept from the mill exits the Oversize Classifier, V-102, with the air leaving the Oversize Classifier, V-102, and is recovered in the Cyclone, V-103. Product powder is routed from the Cyclone, V-103, by gravity to the Product Bin, V-104. The product powder has an average diameter of about 500 microns.

After the Cyclone, V-103, the circulating air is blown back to the Ball Mill by the Circulation Fan, J-101. A portion of the air from the outlet of the Circulation Fan, J-101, is drawn off to the Bag Filter, F-101, to remove dust and is pushed out to atmosphere by the Exhaust Fan, J-102. The use of the Exhaust Fan, J-102, maintains vacuum conditions throughout the unit so that powder and dust do not escape from any openings in the unit. Dust removed from exhaust air is collected in the Dust Bin, V-105.

Feed Screw Conveyor, X-101

Roughly ground feed from a semi-autogenous grinding (SAG) mill is fed to one end the Feed Screw Conveyor, X-101, where it is conveyed horizontally by screw action to the other end of the conveyor. The screw blades within the conveyor are connected to a shaft which is turned by an electric motor that uses a variable frequency drive (VFD) electrical system to control the rotational speed of the conveyor’s shaft. The conveyed feed exits the far end of the conveyor and drops by gravity into the Feed Bin, V-101, through a short circular duct. The normal feed rate through the conveyor is 40 T/H. The conveyor and the transfer duct are designed to keep rain from entering the feed system. The feed enters roughly at ambient temperature.

Feed Bin, V-101

The Feed Bin, V-101, is conically shaped and normally stores only about one minute’s worth of feed. The relatively low cross-sectional area of the bin keeps a consistently vigorous flow of material throughout the bin to avoid agglomeration of the feed within the bin in case it is somewhat wet. Feed material exits the bottom of the conical section of the bin into the Feed Standpipe which sits above the feed hopper to the Ball Mill, X-102. The main purpose of the Feed Bin, V-101, is to provide a consistent height of feed material in the feed standpipe to feed the rotary valve at base of the standpipe. This ensures smooth feed flow into the Ball Mill, X-102. The average size of the feed is ½-inch.

Ball Mill, X-102

The Ball Mill, X-102, consists of 5 main parts:

Feed Hopper
Outside cylinder
Balls (inside the cylinder)
Motor
Turning gear & drive

Feed material from the Feed Standpipe from V-101 flows into the feed hopper of Ball Mill, X-102, where it combines with oversize recycle material flowing into the feed hopper from Oversize Classifier, V-102. The normal flow rate of oversize recycle is 8.1 T/H. Combined feed and recycle then flow by gravity into the center line of the Ball Mill where it is ground into smaller particles.

The outside cylinder of the Ball Mill, X-102, is made of steel and is thick and heavy (around 100 tons) to withstand the weight and crushing action of the hard, metallic balls within the cylinder. The cylinder is 9 feet in diameter and is 15 feet long. The cylinder is lined with a durable rubber composite material which is designed to be replaced after it is worn over time from the crushing action. The balls within the cylinder are 3 inches in diameter. The cylinder is 40% full of balls on a volume basis. The weight of the balls is around 50 tons, making the total rotating weight of the Ball Mill around 150 tons without any crushed material.

The cylinder of the Ball Mill, X-102, is rotated at 20 RPM. At this speed, the balls will be moved along the bottom of the cylinder at a velocity that is sufficiently high to lift them up, pressed to the cylinder as it rotates. As the balls reach the peak of the rotation, gravity overtakes the centrifugal force of the cylinder and they fall back toward the centerline of the cylinder and crush the feed material (see Figure 1 below). If the rotational speed of the cylinder is too slow, the balls will not reach a high enough point for effective crushing. In this case the balls will roll backward over the rising balls toward the bottom without accomplishing much crushing of feed material. If the rotational speed is too high, the centrifugal force on the balls is so high that the balls will hug the cylinder and will not fall back to the centerline. Therefore, very little crushing will occur in this situation.

The Ball Mill is swept with circulating air from Circulation Fan, J-101. The air enters the feed side of the cylinder through a specially engineered set of openings in the side to distribute the air into the Ball Mill, X-102. The air is maintained at such a rate that sweeps out the smaller particles created by the crushing action of the balls. Unswept larger particles remain in the cylinder and are crushed as they move along the cylinder from the feed end to the outlet end. Eventually, all the feed is crushed to a size small enough to be swept out of the Ball Mill, X-102, and into the Milled Powder Liftpipe. The velocity of air in the liftpipe is high enough to transport all the crushed powder to the Oversize Classifier, V-102.

The balls are kept within the cylinder by a retainer plate on the outlet end with many circular openings smaller than the balls to permit air and powder to exit the cylinder into the Milled Powder Liftpipe. Because the cylinder is oriented horizontally, the balls distribute fairly evenly along the cylinder’s length as the balls tumble. To improve distribution of the balls within the cylinder, an intermediate plate that is identical to the outlet retainer plate is installed mid-cylinder. This also permits the use of two sets of different size balls within the Ball Mill, X-102, to optimize crushing performance, depending on the characteristics of the feed and the desired product size. The diameter of the balls will effectively determine the average size of the powder particles produced by the Ball Mill, X-102.

A sealing system between each end of the rotating cylinder and the stationary feed and outlet assemblies minimizes air infiltration into the Ball Mill, X-102, which operates at a slight vacuum to prevent dust and powder from escaping through the seals.

The electric motor that drives the Ball Mill, X-102, is outfitted with a variable frequency drive (VFD) in order to control the rotational speed of the cylinder. The motor normally consumes 432 KW of electric power. The motor drives a gear that turns the cylinder of the Ball Mill, X-102, via a drive track on the outside of the cylinder. Owing to its high weight, the unit has special bearings and a lubrication system (not simulated) to allow reliable rotation of the very heavy cylinder over long periods of time.

Oversize Classifier, V-102

The Oversize Classifier, V-102, is a specially designed vessel that separates larger diameter particles entrained in the air flowing up the Milled Powder Liftpipe. The air/powder stream enters the bottom of the Oversize Classifier, up a concentric pipe, and then distributes evenly into the outer housing of the vessel. This reduces the velocity of the air compared to the liftpipe, causing the heaviest, larger diameter particles entrained in the air to quickly separate from the air and fall to the bottom part of the outer housing where the oversize particles are collected and routed back to the Ball Mill, X-102, by gravity in the Coarse Powder Standpipe.

The balance of the powder and air continue flowing to a rotating cone-shaped wheel outfitted at the top of the housing. The wheel is vertically mounted and has slots with specially engineered flaps that impact the entrained particles reaching the wheel. The larger particles are deflected farther from the wheel to the outside of the outer housing where air velocity is lowest. These larger deflected particles fall to the bottom part of the housing for collection and recycle to the Ball Mill, X-102. The balance of the particles remaining entrained in the air make their way through the slots in the rotating wheel and are collected in the outlet assembly which connects to the air outlet pipe of the Oversize Classifier for transport to Cyclone, V-103.

Cyclone, V-103

Entrained on-spec powder and air leaving the Oversize Classifier, V-102, tangentially enter the Cyclone, V-103, to separate nearly all of the entrained powder from the air. The collected powder is routed to the Product Bin, V-104, out of the bottom of the Cyclone, V-103. The Cyclone, V-103, is shaped like a cone to route the incoming air into a cyclonic flow pattern, or vortex, within the vessel. The solid particles entering the cyclone will bounce off the internal conical wall and lose velocity. The solid particles are carried to the bottom of the Cyclone, V-103, as the air vortexes first down and then up to the top of the Cyclone, V-103. The cleaned air is collected in a plenum and routed to the outlet nozzle at the top of the Cyclone. The outlet air is piped to the Circulation Fan, J-101, for recirculation to the Ball Mill, X-102. The powder is collected at the base of the internal cone and routed to the Product Bin, V-104, by gravity.

Correctly designed cyclones are highly efficient at separating most of the small diameter solid particles from gases such as air. Removal efficiencies commonly exceed 99%. Any entrained particles leaving with the air from the Cyclone, V-103, will be recirculated back through the Ball Mill and then the Oversize Classifier and will eventually be removed in the Cyclone.

Product Bin, V-104

Product Bin, V-104, is a large vessel that intermediately stores product powder that flows by gravity from Cyclone, V-103. It has a conical shape leading to a narrower base. This ensures the powder does not pocket when stored. The Product Bin, V-104, can hold a maximum of about 65 tons of powder, or roughly 42 minutes of design production. The Product Bin, V-104, is elevated well above grade so it can be emptied by gravity into a waiting truck or train car below. A motor-operated movable set of dampers located at the base of the vessel is provided to unload the Product Bin, V-104, as needed to keep it from overfilling.

Circulation Fan, J-101

Circulation Fan, J-101, circulates air around the unit through the Ball Mill, X-102, Oversize Classifier, V-102, and Cyclone, V-103, for the transport and separation of powder produced by the Ball Mill. The fan is motor driven and is outfitted with a variable frequency drive (VFD) motor to permit speed control of the fan. Circulating air also helps cool the powder while moving through the unit because the powder picks up some heat from crushing in the Ball Mill, X-102. None of the equipment in the unit is insulated so as to allow the heat from crushing to be dissipated to the atmosphere.

In order to maintain slightly vacuum conditions in Ball Mill, X-102, a small flow of air is taken off from the discharge of the Circulation Fan to the Bag Filter, F-101, to remove entrained dust before it is exhausted to atmosphere. To replace this exhaust air, fresh ambient air is admitted to the suction of the Circulation Fan though air damper, HV-105.

Bag Filter, F-101

Exhaust air taken from the discharge of Circulation Fan, J-101, is filtered in Bag Filter, F-101, to remove fine dust in the exhaust air. The bag filter uses a fine fabric filter inside the filter housing to trap dust particles while exhaust air is permitted to pass through the fabric. As dust accumulates on the outside of the bag filter, it falls off and is collected on the conical bottom section of the filter housing. The collected dust flows by gravity into the Dust Bin, V-105.

Exhaust Fan, J-102

The filtered air from the Bag Filter, F-101, is blown to atmosphere at a safe location by Exhaust Fan, J-102. The fan is motor driven and is outfitted with a variable frequency drive (VFD) motor to permit speed control of the fan. The exhaust fan produces the necessary air flow to maintain vacuum in the air circulation loop which thereby keeps powder and dust contained within the equipment and lines of the Ball Mill unit.

Dust Bin, V-105

Dust Bin, V-105, is a small vessel that stores collected dust from that flows by gravity from the bottom of Bag Filter, F-101. It has a conical shape leading to a narrower base. This ensures the dust does not pocket when stored. The Dust Bin, V-105, can hold a maximum of about 0.6 tons of dust. The Dust Bin, V-105, is elevated well above grade so it can be emptied by gravity into a disposal truck below. A motor-operated movable set of dampers located at the base of the vessel is provided to unload the Product Bin as needed to keep it from overfilling.

Instrumentation

Feed Screw Conveyor, X-101

Switch HS-101 operates the motor of Feed Screw Conveyor, X-101, which is outfitted with a variable frequency drive (VFD). Speed controller SIC-101 controls the shaft speed of Feed Screw Conveyor, X-101, by adjusting the control signal to the VFD. The normal speed of Feed Screw Conveyor, X-101, is 45 RPM. A gearbox is installed between the motor and the shaft of X-101 to reduce the shaft speed from the motor speed, which operates at 3600 RPM at full speed (100% output of SIC-101). Note that the VFD only controls from 50% of maximum speed at 0% output of SIC-101 to 100% speed at 100% output of SIC-101. Therefore, the controllable speed range with SIC-101 is from 30 to 60 RPM. The weight flow of feed material transferred by X-101 is directly proportional to its shaft speed. At design, Feed Screw Conveyor, X-101, feeds 80 T/H of feed at 45 RPM. Therefore, the minimum feed rate is about 53 T/H at 30 RPM.

Feed Bin, V-101

The level of feed in the Feed Bin, V-101, is measured by 2 different types of instruments. LI-101A uses a load cell which is a force-measuring device. The load cell measures the force exerted by the weight of the Feed Bin, V-101, vessel and its contents. The weight is then converted into a level indication knowing the weight of the Feed Bin, V-101, its geometry and the bulk density of the feed material it contains. The second instrument LI-101B uses a laser instrument mounted at the top of the Feed Bin, V-101, which determines the depth of the top of the feed material in the Feed Bin, V-101. This is directly converted into a level reading in % which is consistent with the calibration of the load cell.

Switch LS-101 selects which of the two Feed Bin level instruments, LI-101A or LI-101B, is used for the PV of level controller LIC-101. LIC-101’s output adjusts the speed of rotary valve LV-101 located at the base of the Feed Standpipe near the feed hopper of Ball Mill, X-102 (see Schematic #4). The flow of feed through the rotary valve is proportional to the speed of the valve.

Feed Standpipe

AI-101 is an analyzer which measures the average particle diameter of the feed material in the Feed Standpipe. TI-101 measures the temperature of feed in the standpipe and WI-101 is a weight flow meter that indicates the flow of feed material into the feed hopper of the Ball Mill, X-102.

Ball Mill, X-102

Switch HS-102 operates the motor of Ball Mill, X-102, which is outfitted with a variable frequency drive (VFD). Speed controller SIC-102 controls the rotational speed of the Ball Mill, X-102, by adjusting the control signal to the VFD. The speed sensor for SIC-102 is on the shaft that connects motor to the turning gear on the Ball Mill, X-102. This speed signal is calibrated to indicate the turning speed of the cylinder of the Ball Mill, X-102, on SIC-102. The Ball Mill, X-102, normally turns at 20 RPM. Note that the VFD only controls from 50% of maximum speed at 0% output of SIC-102 to 100% speed at 100% output of SIC-102. Therefore, the controllable speed range with SIC-102 is from 13.3 to 26.7 RPM. The electric power used by the motor of the Ball Mill, X-102, is indicated on JI-102.

Interlock I-102 protects the Ball Mill, X-102, and its auxiliary equipment from damage in case of a mechanical problem with the motor or the lubrication system of the bearings (not simulated). XA-102 is a general pre-trip alarm indication that alerts the operator to potential mechanical trouble in these systems (e.g. low lube oil pressure). XAH-102 is a trip alarm indicating a severe problem in these systems. When XAH-102 is in the TRIP state, HS-102 is locked by I-102 in the STOP position and the motor of X-102 is stopped. After the trip condition, as indicated on XAH-102, returns to the OK state, HS-102 will be unlocked by I-102 and the motor can be started.

TI-102 indicates the temperature of the air/powder mixture in the Milled Powder Liftpipe.

Oversize Classifier, V-102

Switch HS-103 operates the internal wheel within V-102. PDI-102 indicates the pressure drop across the Oversize Classifier. TI-103 indicates the temperature of powder in the Coarse Powder Standpipe which returns oversize (coarse) powder to the Ball Mill, X-102, feed hopper. WI-103 indicates the weight flow of coarse powder to the Ball Mill, X-102. AI-103A through AI-103E indicate the distribution of various particle size classifications, in weight %, of the coarse powder as follows:

AI-103A – 50 micron average
AI-103B – 100 micron average
AI-103C – 300 micron average
AI-103D – 750 micron average
AI-103E – 1500 micron average

These five readings are used to calculate the average particle diameter of the coarse powder which is indicated on AX-103.

Product Bin, V-104

The level of powder in the Product Bin, V-104, is measured by 2 different types of instruments. LI-104A uses a load cell which is a force-measuring device. The load cell measures the force exerted by the weight of the Product Bin, V-104, vessel and its contents. The weight is then converted into a level indication knowing the weight of the Product Bin, V-104, its geometry and the bulk density of the feed material it contains. The second instrument LI-104B uses a laser instrument mounted at the top of the Product Bin, V-104, which determines the depth of the top of the feed material in the Product Bin, V-104. This is directly converted into a level reading in % which is consistent with the calibration of the load cell.

Switch HS-106 is used unload the powder in V-104. Normally, powder is not being unloaded.

Circulation Fan, J-101

Switch HS-104 operates the motor of Circulation Fan, J-101, which is outfitted with a variable frequency drive (VFD). Speed controller SIC-104 controls the shaft speed of the fan adjusting the control signal to the VFD. The Circulation Fan, J-101, normally operates at 2909 RPM. Note that the VFD only controls from 50% of maximum speed at 0% output of SIC-104 to 100% speed at 100% output of SIC-104. Therefore, the controllable speed range with SIC-104 is from 1800 to 3600 RPM.

FIC-104 controls the air flow rate from J-101 to the Ball Mill, X-102. The output of FIC-104 adjusts the setpoint of Circulation Fan, J-101, speed controller SIC-104 to keep the air flow to the Ball Mill, X-102, at setpoint.

PIC-104 controls the pressure of air to the Ball Mill, X-102. The output of PIC-104 adjusts the setpoint of Exhaust Fan, J-102, speed controller SIC-105 to maintain the air pressure at a slight vacuum.

TI-104 indicates the temperature of the air flowing to the Ball Mill, X-102.

HIC-105 adjusts the position of the fresh air louver HV-105.

Bag Filter, F-101

PDI-105 measures the pressure drop across the Bag Filter, F-101. WI-105 indicates the weight flow of dust flowing from the Bag Filter, F-101, to Dust Bin, V-105.

Dust Bin, V-105

LI-105 indicates the level of dust collected in Dust Bin, V-105. Switch HS-107 is used unload the powder in V-105. Normally, powder is not being unloaded.

Exhaust Fan, J-102

Switch HS-105 operates the motor of Exhaust Fan, J-102, which is outfitted with a variable frequency drive (VFD). Speed controller SIC-105 controls the shaft speed of the fan adjusting the control signal to the VFD. The Exhaust Fan, J-102, normally operates at 2176 RPM. Note that the VFD only controls from 50% of maximum speed at 0% output of SIC-105 to 100% speed at 100% output of SIC-105. Therefore, the controllable speed range with SIC-105 is from 1800 to 3600 RPM.