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Title: Increasing Capacity of Existing Limestone Grinding Systems While Reducing Grind Size 

Authors: Mr. Gary Harper, Tri-State G&T Association 

Ms. Melissa A. Hagan, Burns & McDonnell 

Mr. Paul Dyer, Burns & McDonnell 

Mr. William J. Breuer, Burns & McDonnell 

ABSTRACT 

Increased limestone grinding capacity may be required at existing FGD systems in response to changes in coal sulfur 

content, increasingly stringent limitations on SO2 emissions, and escalating SO2 allowance prices. In a 

conventional wet grinding circuit the capacity can be easily increased, but at the expense of particle size. The 

resultant coarser grind can decrease limestone utilization, leading to operating problems. The challenge faced by the 

FGD system operator is how to increase limestone slurry production capacity while maintaining or improving the 

fineness of the limestone grind achieved. This paper presents the results of a successful full-scale application of a 

novel approach to achieving this goal by placing a new vertical ball mill in series with an existing conventional 

horizontal mill. Advantages of this approach are documented, including minimal disruption to operations and 

installation with minimum outage time. Grinding system test results before and after the modifications are presented. 

INTRODUCTION

This paper describes an alternative method to increase limestone reagent production for existing FGD systems while 

preserving existing in-place grinding assets. In this paper we will discuss the changes made to the limestone 

preparation system at Craig Generating Station. The requirement to increase limestone production resulted from the 

need to both increase the SO2 removal percentage and to eliminate the capability to bypass the FGD system. 

Tri-State Generation and Transmission Association, Inc. (Tri-State) implemented a project entitled the Yampa 

Environmental Project at Craig Station Units 1 and 2 in Craig, Colorado. This project included FGD system 

performance improvements based on conversion to 100% scrubbing of the flue gas and FGD absorber upgrades to 

enhance removal efficiency. 

Craig Station Units 1 and 2 consist of two 455 MW coal-fired boilers equipped with four spray tower absorbers per 

unit for sulfur dioxide emission control. As a result of the elimination of scrubber bypass and the increase in 

removal efficiency, the capacity of the limestone slurry preparation system was increased from under 5 tons of 

reagent per hour to 11 tons of reagent per hour with product slurry having a grind of 90% by weight passing 325 

mesh. 

THEORY

Communition is a mechanical process where large particles are acted upon to produce smaller particles. This is the 

fundamental process used by ball mills in FGD systems to produce limestone slurry. For the purpose of this paper 

we will concentrate on two types of ball mills. The most common type of ball mill used in the FGD industry is the 

horizontal ball mill. In this configuration, a horizontal tube or shell containing pre-crushed feed (limestone) and 

media (steel balls) is rotated, resulting in collisions between the feed material, grinding media, and the shell. These 

collisions continually reduce the size of the feed. Water is added to the mill and flushes particles out of the mill into 
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a mill product tank. The mill product pump transfers the slurry from the mill product tank to the classifiers. The 

classifiers separate the limestone slurry based on particle size. When properly set up and operating, the fine material 

flows from the classifier overflow to a limestone storage tank where it is stored until needed by the FGD system. 

The classifier underflow (heavier material) returns to the ball mill to be re-ground to a smaller size. 

The operational parameters which have a major impact on horizontal mill operation and throughput are feed size, 

feed hardness, and output size. The strengths of a horizontal mill lie in its ability to handle a wide range of 

operating parameters to produce a consistent slurry particle size. The weakness of this design is the potential 

reduction in capacity resulting from increases in feed size or feed hardness or decreases in target output particle size. 

The second type of ball mill considered in this paper is the vertical ball mill. Vertical ball mills, as the name 

suggests, have the mill shell oriented in the vertical position. Unlike their horizontal counterparts, the mill shell 

does not turn. However, feed size reduction is accomplished through a similar mechanism as the horizontal mill. 

Attrition grinding occurs with collisions between the grinding media and feed. The movement of the feed and media 

is the result of an internal screw which stirs the material on the inside of the mill and continuously circulates the 

material from the bottom of the mill to the top. This grinding action results in an extremely fine grind of the 

limestone. 

The operational parameters which have a major impact on vertical mill operation and throughput are the same as 

those for the horizontal mill. The strength of this mill design is its ability to produce a consistent finely ground 

limestone when operated in-series with a horizontal ball mill. The weakness of the design is the inability of the 

system to utilize feed greater than 1⁄4 inch when operating as a stand-alone ball mill system. 

As stated above, both mill types are sensitive to feed size and feed hardness. Increasing the feed size in a horizontal 

mill rapidly reduces the mill’s output capacity at a given product size. Increasing the feed size can upset the balance 

in a mill to the point where recycle flows, ball size distribution, and classifier tuning may require adjustment. A 

solution to achieve the same mass flow through the mill with increased feed size would be to let the product size 

float relative to the original design. This would mean that a horizontal mill originally designed for -325 mesh 

product size may be able to produce the same throughput in tons/hr with a larger feed size and a larger product size, 

for instance -200 mesh. 

The same general variations and relationships (although the relationship of throughput and particle size would likely 

be different) could be seen by varying the hardness of the feed (Bond Work Index). The Bond Work Index (BWI) is 

a measure of the amount of work necessary to reduce a representative sample of a material to a specified size. BWI 

is measured in KWH/ton. As the Bond Work Index increases, the capacity of the mill to produce a constant product 

size decreases. A reduction in BWI should also lead to an increase in production, however the amount of the 

increase may be reduced based on other factors. 

Vertical ball mills appear to be somewhat less sensitive to changes in feed size and BWI when operated in series 

with a horizontal ball mill. This is because the horizontal ball mill has done most of the work in normalizing the 

feed size to the vertical mill. 

By configuring the grinding circuit with an existing horizontal ball mill followed by a vertical ball mill, the utility 

can preserve an asset, gain flexibility, and increase production using the strengths of the two ball mill technologies 

while minimizing their weaknesses. 

ORIGINAL SYSTEM DESIGN

The original limestone grinding system supplied with the FGD systems for Craig Station Units 1 and 2 included two 

100% closed circuit ball mill trains. Each train included a KVS 7’x14’ horizontal ball mill rated at a capacity of 10 

tons per hour while producing an 80% passing 200 mesh grind. This provided enough capacity to allow one mill to 
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act as a standby spare even at the original design worst case coal sulfur and SO2 removal levels. Prior to the Yampa 

Environmental Project, the grinding circuits were modified to produce a finer grind, and thus typically operated at 

about 4.7 tons per hour each and at times as low as 3 to 4 tons per hour, but that was still adequate to meet the needs 

of the FGD systems for both units at the coal sulfur and SO2 removal levels while effectively providing a spare mill. 

However, the increase in limestone feed rate required to increase removal efficiency in the scrubbers meant that the 

limestone grinding rate was not adequate and needed to be increased to maintain the original sparing. 

There are two main reasons that the original mill capacity effectively dropped by 50 percent or more. First, the 

original grinding circuit for each mill was designed to produce limestone slurry with a particle size such that 80 

percent by weight would pass a 200-mesh screen. This relatively coarse grind was selected by Peabody Process 

Systems as part of its standard process design at the time. 

The FGD system process design incorporated the use of six Krebs D15B-852 absorber recycle slurry classifiers 

(four operating and two spare) intended to separate the recycle slurry into a low-solids overflow containing 

comparatively little limestone and a high-solids underflow containing a higher limestone content. The classifier 

overflow was sent to the wash slurry tank for washing the interface tray between the slurry spray zone and the mist 

eliminators, while the underflow was returned to the absorber reaction tank. According to Peabody’s process 

description, this feature should have allowed the absorbers to operate at an internal stoichiometry (in the absorber 

spray zone) higher than the external stoichiometry (in the slurry reaching the mist eliminators). When the recycle 

slurry classifier system proved to be unable to function as intended, Tri-State was forced to change the operation of 

the limestone grinding system to produce a finer limestone grind, typically 90 percent or more passing a 325-mesh 

screen. This necessitated a significant reduction in the mill throughput, because more passes through the mill were 

required to achieve the finer grind. 

It is important to understand that conversion to the finer limestone grind was necessary to allow operation of the 

absorber recycle slurry at reagent utilization levels that would be conducive to minimization of scale in the mist 

eliminators. Minimization of the unreacted limestone content of the slurry reaching the mist eliminators is very 

important to scale-free operation, and became even more so as the absorber velocity increased when modifications 

were implemented to scrub 100 percent of the flue gas. 

The secondary reason for reduction in the mill production capacity was the elimination of the original Pennsylvania 

Crushers, model JR7-1, from the flow stream. The original design was for limestone feed up to 1-3/4” size, with 

hammermill crushers provided to reduce the top size to 1⁄4” entering the ball mill. The hammermill crushers at Craig 

were problematic from a maintenance standpoint, and had not been used for many years. The effective increase in 

limestone feed size to the ball mills above that for which they were designed contributed to the decreased production 

rate. One way for Tri-State to reduce this effect would have been to procure stone with a smaller top size. 

A flow diagram of the limestone grinding system as it existed prior to the Yampa Environmental Project is shown in 

Figure 1. 
Figure 1: Original Limestone Grinding System Flow Diagram

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PROCESS AUDIT

The first step in determining the potential of the existing system to support future operating conditions was to have 

the system evaluated by the OEM. This evaluation, called a Process Audit, was used to determine the existing 

operational condition of the mills. Metso Minerals was consulted to assist in the investigation of the ability of the 

mills to increase capacity while operating at the fine grind needed for proper FGD system performance. Two 

separate investigations were conducted. 

Metso began by examining the physical conditions of the ball mills. Metso evaluated the current grinding system 

configuration, the specifics of the feed material and media, mill liners and circuit control. In addition to the 

mechanical audit, Metso determined the ball mill operating performance by taking samples from the circuit along 

with various measurements of inlet and outlet flows to the mills. After collecting the baseline mechanical and 

process data, Metso ran a series of tests to determine the accuracy of water flow and conveyor speeds. 

June 2000 Assessment

In June 2000, Svedala Industries (now called Metso Minerals) conducted a site visit for the purpose of investigating 

the possibility of increasing the capacity of the existing ball mills for the Craig Station Unit 1 and 2 FGD systems. 

The process audit data showed that the Unit 1 mill circuit was achieving a limestone grind of 90.2 percent passing 

325 mesh, whereas the Unit 2 circuit was tested at 84.6 percent passing 325 mesh when operating at a higher than 

normal feed rate. A parametric test was conducted on the Unit 2 mill to determine the effect of continuing to 

increase the feed rate to the mill on fineness of grind. The percent passing 325 mesh decreased as the feed rate 

increased, degrading to 80.2 percent at a feed rate control setting of 55 percent. 
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The report failed to reach a definitive conclusion regarding the capability for increasing the capacity while 

maintaining the current fine grind. Instead it made recommendations for the installation of additional 

instrumentation that would be needed before optimization could be done. However, regarding the effect of the 

oversize feed material (due to retirement of the hammermill pre-crushers) on mill capacity, the report asserted that 

the benefit of returning the hammermills to service would be very small. The possible capacity increase cited due to 

reinstatement of the hammermills was less than two percent when operating at the desired fineness of grind. 

The report also included the results of a Bond Work Index test on the limestone sample taken at that time. The 

reported work index was 10.05 kW-hr/short ton. This parameter would be important in the design basis for any new 

mill to be purchased as part of the Yampa Environmental Project. 

September 2001 Assessment

Metso Minerals was contacted again in July 2001 about performing an assessment of the capability of the Craig Unit 

1 and 2 limestone grinding circuits to operate at increased capacity when producing the desired fine grind. It was 

shortly thereafter determined that the mill circuit control and instrumentation upgrades recommended in their June 

2000 report had not been made, and would likely not be made. Therefore a detailed on-site test program would not 

be possible. Nevertheless, personnel from Metso Minerals visited the site in September to assess the capability of 

the grinding circuit for increased capacity. 

Metso Minerals concluded that the best that could be obtained from the existing mills was to increase the current 

production of each mill from about 4.5 tons per hour to the range of 5.0 – 5.2 tons per hour. A flowsheet illustrating 

this result was included with the Metso documentation, and is shown in Figure 2. This 5.2 tons per hour per mill 

falls just short of the nominal 11 tons per hour production rate target, even if both mills were operated 

simultaneously at full capacity. Metso again confirmed that a reduction in the top size of the feed would not have a 

significant impact on production rate. 
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Figure 2: Process Audit Flowsheet – Original System 

Product

Solids (stph) = 5.2

% solids = 28

Slurry GPM = 60.8

Slurry S.G. = 1.22

Feed

Solids (stph) = 5.2 Cyclone Feed

% moisture = 1 Grind Water Solids (stph) = 31.2

gpm= 38.4 % solids = 56.00

Slurry GPM = 142.2

Slurry S.G. = 1.56

Recycle

Solids (stph) = 26

% solids = 70

Slurry GPM = 81.5

Slurry S.G. = 1.82

Mill Discharge

Solids (stph) = 31.2

% solids = 60

Slurry GPM = 127.4

Slurry S.G. = 1.63

Dilution Water

gpm= 14.8

BALL MILL

WEIGHFEEDER

PUMP

CYC

SUMP

Metso did propose a novel arrangement involving the use of each existing mill in series with their Vertimill® 

vertical grinding mill to greatly increase the overall production rate. A mill circuit flowsheet for Craig Station 

showing the modified system flows for each grinding circuit was included in their documentation and is shown in 

Figure 3. 
Figure 3: Process Audit Flowsheet - Vertimill® System 

EVALUATION

Burns & McDonnell evaluated the results of the grinding circuit process audits conducted by Metso Minerals in June 

2000 and September 2001, and compared options for meeting the increased limestone grinding requirements 

associated with the higher SO2 removal rates. Evaluation of the options for increasing the grinding system capacity 

was based on maintaining the original design and operating philosophy of providing a standby mill at all operating 

conditions. 

Based on the grinding system evaluations conducted by Metso, it was confirmed that an additional ball mill would 

be required. Burns & McDonnell considered two options for increasing the grinding capacity to provide a spare mill 

at all operating conditions. Each option is described and discussed in the following subsections. The evaluation of 

each option includes consideration of feasibility and reliability. Cost was also evaluated at the time of the 

evaluation, but is not included in this paper because of changes in the market. 

Option 1

The scope of Option 1 included the addition of a new weigh feeder, ball mill (7’ diameter  14’ long), mill product 

tank and classification circuit, and reagent slurry tank identical in capacity to the original equipment. 

Feasibility 

Burns & McDonnell’s review identified the possibility for locating the third ball mill in an extension of the existing 

limestone preparation building enclosure. There is extensive underground piping and electrical duct banks that pass 

through the area immediately north of the existing Unit 1 silo. Due to these underground piping considerations, the 

only alternative was to consider extension of the building to the south. Comparison of this possibly available space 

versus the size of the existing mills and their associated mill product tanks and maintenance access platforms 

indicated that the fit would be extremely tight. 

Even if the new horizontal ball mill could be made to fit in this space, the problem of how to get the limestone to the 

mill needed to be solved. A conveyor could be extended southwestward from the bin discharger outlet for the Unit 2 

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limestone silo to a new weigh feeder that would serve the new third mill to be located in the building extension as 

described above. However, this conveyor would need to pass through the concrete silo skirt/support wall. Also, a 

way would need to be found to feed both the new conveyor and the existing feeder for the Unit 2 ball mill, or to feed 

both at the same time. The chutework below the bin discharger would need to be fitted with a diverter gate that 

would accommodate these operating modes. 

There is no room within the available space for the building extension as described above to locate a third limestone 

slurry storage tank. It was presumed that the hydrocyclone classifier assembly for the new third mill would be 

located above the existing slurry storage tanks, so that its overflow could be routed to either tank. 

Based on the space constraints and the feed scenario difficulty, it was clear that there would be several technical 

challenges associated with this option. 

Reliability

If a new third ball mill were sized to match the capacity of that for Units 1 and 2, it would presumably be 

constructed to match the production rate of the original ball mills, which was nominally about five tons per hour 

each. So the sparing situation with a third mill would be that two mills would be required to operate full time at the 

worst case future condition, while one mill was available as the standby spare. If a feasible chutework and conveyor 

scheme could be developed to allow location of a third ball mill in the southward building extension as described 

above, the mill would act as a third 50 percent capacity mill when compared to the total maximum limestone 

demand. One mill would be available as a standby in the event of problems with one of the other two. However, 

the reliability would be affected by the fact that, of necessity, two of the three mills would share the same silo and 

the same bin discharger. If any problem developed in this common equipment, both downstream mills would be put 

out of commission, and the total limestone grinding capability would drop to about five tons per hour. 

Option 2 

Metso Minerals, in its evaluation of ways to increase the capacity of the limestone grinding system at Craig Station 

Units 1 and 2, identified the possibility of adding a vertical ball mill in series with each of the existing horizontal 

ball mills. According to their proposal, the capacity of the grinding train for each unit would be upgraded to 11 tons 

per hour. This would actually exceed the original design capacity of 10 tons per hour per mill, and would provide 

100 percent spare grinding capacity at the target maximum limestone usage rate. 

The Metso proposal was based on the use of two ball mills in series. The existing horizontal ball mill would be 

operated in open circuit (once through) mode, to produce a grind of 100 micron particles, which would then be fed 

to the closed circuit vertical stirred ball mill. The grinding circuit for the vertical mill would include a new 

hydrocyclone classifier. The underflow from the hydrocyclone would be returned to the vertical mill for further 

grinding. The hydrocyclone overflow would be sent to the limestone slurry storage tanks. Metso proposed to 

produce a grind of 90 percent passing 325 mesh. 

The flow scheme is shown in Figure 3 above. According to Metso, the ball charge in the existing mills would be 

changed to optimize the coarse grind. The existing mill product pumps could be used to transfer the coarse-ground 

slurry from the horizontal mill to the inlet of the vertical mill. 

Feasibility

Vertical stirred mills have been used in limestone grinding service for FGD applications for over 20 years. One of 

the largest FGD installations of Metso’s Vertimill® is at Mill Creek Station of Louisville Gas & Electric. Burns & 

McDonnell confirmed that the mills there work extremely well. Although there was no other application of series 

grinding of limestone exactly like Metso proposed for use at Craig, similar applications had proven successful in 

other industries. 
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The most important factor lending support to the use of vertical stirred mills for fine grinding of limestone at Craig 

Station is the consideration of plot space. As described in the previous section, an extension of the existing 

limestone grinding building could be accomplished to the south. The available space described previously, which 

would barely be able to accommodate a 7’x14’ horizontal ball mill, appeared to be able to house the two proposed 

Vertimills®. One notable feature of the vertical mills is that the enclosure would need to be high, with the topmounted motor drive extending above the roof line of the original building. 

Another consideration favoring the use of the vertical mills was the fact that no limestone rock feed was required to 

feed the Vertimills®. The slurry product from the horizontal mills would be pumped to the vertical mill inlets. It 

was assumed that it would be possible to route the new slurry lines from the mill product discharge lines to the 

proposed location of the vertical mills without much difficulty. 

As with the single horizontal mill, the hydrocyclone classifiers for use with the vertical mills would be situated to 

allow overflow to be diverted by gravity into either of the existing limestone slurry storage tanks. The high sections 

of the building extension that would house the Vertimills® would provide areas for location of these hydrocyclones. 

Vertical stirred mills provide fine grinding at significantly lower power consumption than would be needed using 

horizontal mills. The VTM-200 mills utilize 200-horsepower drive motors. It was expected that the combined 

power consumption of the horizontal and vertical mill in each circuit would be less than that which would have been 

needed to operate two horizontal mills producing 11 tons per hour limestone. 

Reliability

Compared to the other option evaluated, the combined use of the horizontal mill and the new vertical mill for 

limestone grinding should provide greater reliability. It would be possible to pipe the mill product discharge to 

provide a backup flow path so that each Vertimill® can receive feed from either existing mill. 

Recommendations

Based upon Burns & McDonnell’s evaluation, the addition of a separate closed circuit vertical stirred ball mill in 

series with each original horizontal mill was selected as the preferred method for increasing the limestone grinding 

system capacity. The new vertical mills would be located in a southward extension of the existing limestone 

preparation building. A cost comparison of the two options found that the price for two vertical ball mill circuits 

was comparable to that of one horizontal ball mill. The power consumption of the horizontal and vertical mills in 

series would be less than the power consumption of two horizontal mills operating in parallel. The series grinding 

configuration would provide the greatest degree of backup at the lowest operating cost. 

CONCEPT

Two main options are available when seeking to expand the capacity of an existing plant. These options are 

installation of additional grinding circuits, and modification to the existing grinding circuits. The installation of a 

Vertimill® to operate in series with an existing grind circuit is a combination of these two alternatives, and provides 

distinct advantages over either individual alternative. 

The Vertimill® has been proven to operate more efficiently than the traditional horizontal ball mill, with 65-70% of 

the power requirement of a horizontal ball mill (see Figure 4). This power savings translates into increased capacity 

for a given installed power. Installing the Vertimill® in series with an existing ball mill allows the plant to take 

advantage of this power savings without experiencing some of the disadvantages of an entirely new grinding circuit 

(additional peripheral equipment, increased floor space requirements, etc.). 
Figure 4: Grinding Power of Vertimill® vs. Ball Mill 

Another distinct advantage of this configuration is the ability for the circuit to handle a coarse feed. Typically, the 

Vertimill® can only be installed with a feed size of –1/4”. However, since the horizontal ball mill is still handling 

the fresh feed, and thus breaking it down before it reaches the Vertimill®, this is no longer a restriction. Not 

requiring a –1/4” feed is an advantage over a stand-alone Vertimill® circuit in that less expensive limestone can be 

purchased without requiring a crushing stage prior to grinding. 

Some considerations that must be made when installing a Vertimill® in series with an existing grind circuit include 

modifications to the existing ball mill and modifications to the grinding circuit logic. These modifications could 

include feed chute design, proper media size, pump speed control, or any number of other design points. These 

issues can vary greatly based on the specifics of the installation, and due diligence should be taken to consider any 

required modifications. 

The concept to increase limestone grinding capacity through a two-stage series grinding circuit using horizontal and 

vertical ball mills was proposed to Tri-State Generation and Transmission as a low cost option to increase limestone 

throughput while reducing particle size. The concept of series grinding allowed Tri-State to leverage their existing 

grinding equipment into a higher capacity system with minimal investment. The concept called for de-tuning the 

existing horizontal mills by converting them to once-through open circuit operation. This conversion required the 

elimination of the classifier system. With these modifications in place, the horizontal mill would be able to produce 

more gross tons of limestone, but at a larger particle size. The mill discharge would then be pumped into a storage 

tank. From the storage tank, the limestone slurry would be processed through a vertical ball mill operating in closed 

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circuit. New Krebs hydrocyclone classifiers were also provided. The vertical ball mill would act as a fine grind mill 

producing 11 tons per hour of 90% passing 325 mesh limestone. 

A summary of the modifications made to the limestone grinding system is as follows: 

• Convert horizontal ball mill to once-through operation 

• Demolish cyclone classifiers for horizontal ball mill 

• Install VTM-200 WB vertical ball mill system 

• New cyclone classifiers 

• Separating tank 

• Recycle tank 

• Install mill product tank and mill product pumps 

The existing feed and storage infrastructure and horizontal ball mill circuit were utilized, offering a lower capital 

cost compared to adding a third ball mill operating parallel to the existing circuits. 

One grinding circuit is able to provide limestone slurry to both units, keeping the second limestone grinding train as 

a backup. Refer to Figure 5 for an overview of the limestone preparation system modifications. 

Figure 5: Series Limestone Grinding Flow Diagram 

IMPLEMENTATION

Construction of the limestone preparation modifications began in May 2003. Initial start-up occurred in November 

2003. Tie in of the vertical ball mill system with the existing limestone preparation equipment was accomplished 

without a unit shutdown. 

One of the challenges faced during initial design of the grinding modifications was the space constraint in the 

existing limestone preparation area. The limestone preparation building layout is shown in Figure 6. The horizontal 

ball mill circuits were located at grade, underneath the limestone storage silos. Column Row 1 located to the West 

of the original grinding equipment is the exterior wall of the FGD Building. A road runs adjacent to the FGD 

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building and limestone storage silo to the South. The vertical ball mill circuits fit into a 31’-9” by 57’-7” space 

between the FGD building, original limestone preparation building, and the road. 

Figure 6: Limestone Preparation Building Layout 

An elevation view of the vertical ball mill grinding equipment is shown in Figure 7. While the footprint is 

significantly smaller than a horizontal grinding system, the height of the building is much taller. The overall height 

of the new vertical ball mill building is 78 feet. A picture of one of the Vertimills® is shown in Figure 8. 

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Figure 7: Vertical Grinding Circuit - Elevation View 

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Figure 8: Vertical Ball Mill 

Start-up of the new grinding system went smoothly, however some problems were encountered with the original 

horizontal ball mills. Metso Minerals evaluated the grinding circuit in the fall of 2003 in order to prepare for their 

performance testing. Their objective was to evaluate the reported lack of capacity of the horizontal ball mills and 

the performance of the vertical ball mills. Metso found that the feed chute of the horizontal ball mills plugged with 

a feed rate greater than 6 tons per hour and concluded that the feed chute needed to be modified. After operating the 

horizontal mill at a feed of approximately 6 tons per hour (which corresponded to 55-58% feeder speed), the ball 

mill output contained coarse particles, which prevented a consistent ball mill operation. 

Metso determined that the feed chutes needed to be modified to avoid plugging and ball size should be increased to 

improve the grinding of coarse particles before the output capacity of the grinding circuit could be verified. The 

recommended modifications were completed and the grinding circuit proved effective at producing 11 tons per hour 

of 90% passing 325 mesh limestone slurry. 

SUMMARY

The installation of Metso Mineral’s Vertimills® was a novel solution to the need for increased limestone slurry 

production at Tri-State’s Craig Station. The existing horizontal ball mill circuit was converted from a closed circuit 

system to a once-through horizontal grinding train feeding a new vertical ball mill. The vertical mills offered a 

comparable capital cost, lower operating cost, and smaller foot print option compared to the addition of a third 

horizontal grinding train. The limestone grinding system at Craig Station Units 1 and 2 consistently produces 11 

tons per hour reagent with product slurry having a grind of 90% by weight passing 325 mesh. 

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