INCREASING CEMENT GRINDING CAPACITY WITH VERTICALROLLER MILL TECHNOLOGY

Gary R RoyGeneral Manager – Product Management

F.L.Smidth, Inc.

ABSTRACT

With the many large-scale plant expansions made in the US in recent years, there has been a need touse new and complex burning and emission control technologies to comply with modern emissionregulations. However, in many cases these same plants have chosen to use very traditional technologyfor finish grinding – typically a ball mill and separator. With today’s power price issues in the US, and thelarge experience base that the rest of the world has with the use of new technology in finish grinding, theUS market is considering the use of modern Vertical Roller Mill technology for cement grinding. Thispaper discusses the different options available for upgrading the capacity of an existing plant, providescomparisons of operating costs, simplicity of operation, product quality considerations, etc., based on theOK mill project at Phoenix Cement Company (PCC) as the first vertical mill for cement grinding in the US.

INTRODUCTION

When a new cement production facility is to be built, it is relatively simple to make a decision on what typeof technology should be incorporated. The decision will be made based on cost of the installation,constructability, projected operating and maintenance costs, product quality issues, and customerpreference. For a new installation, all of these variables are fairly well defined and comparisons of onetechnology vs another is fairly straightforward.

When an existing facility is investigating how to significantly increase the cement grinding capacity, thereare many different solutions that can be considered. However, it is important to realize that although thegoal of increasing production seems to be very straightforward from a technology standpoint, each retrofitis a unique situation. The details of the existing facilities can often become the critical factor in making thejustification for one type of system vs another. The justification may then be additionally influenced by thelayout, re-use of existing equipment or other constraints as opposed to technology issues only. What is agood solution for one installation may be a terrible solution for another.

INCREASING CAPACITY: PREGRINDING OR NEW MILL CIRCUITS

In order to significantly boost the grinding capacity of an existing facility, it requires that additional grindingpower be added. Depending on the situation, this additional grinding power can either take the form of apregrinding system attached to the existing grinding circuit, or a completely new grinding installation tomake up the desired additional capacity.

For cement grinding circuits, there are several options available for pregrinding. Depending on the typeof pregrinder and system selected, a 10 to >100% production increase can be expected. Pregrindingsolutions include vertical shaft impactors, hydraulic roller presses, and vertical roller mill pregrinders. Notonly do pregrinders add grinding power to the grinding circuit, but they also provide an opportunity toincrease the overall grinding efficiency of the circuit – so that not only is the capacity increased, but the

overall specific power consumption is reduced compared to the original ball mill circuit. Although many ofpublished materials are available in the literature regarding these technologies, a quick review should beincluded as background for the case study related below.

Vertical Shaft Impactors (VSI’s) have been employed in the cement industry for pregrinding clinker inorder to boost the capacity of existing mill circuits. The VSI reduces the size of the feed by acceleratingthe material and projecting it against a stationary surface. The amount of work that is done on the clinkeris directly related to the speed of the rotor, or the impact velocity, and is typically in the range of 2-3kWh/mt fresh feed. The power consumed by the VSI is roughly equivalent to the power saved in the ballmill in crushing, therefore little savings in specific grinding power consumption is realized unless thereduced ball mill feed size eliminates other problems inherent in the original mill. It is typical to have theVSI in closed circuit with a screening system to prevent any uncrushed particles from being fed directly tothe ball mill.

Literature shows that production increases of 20-30% are possible without changing the separator circuit.However, each case must be analyzed for the inherent problems with the existing circuit. A more realisticprediction is a 10-20% production increase for a well operated ball mill. Although the cost of the VSIitself is relatively attractive, the cost of installing the auxiliary equipment (screens, elevators, etc.) can besignificant, as can the maintenance and down time attributed to wear of the VSI and screens.

Vertical Roller Mill (VRM) Pregrinders have also been used as pregrinders to boost existing millproduction. The vertical pregrinder differs from the vertical roller mill in that only the grinding portion of thevertical roller mill is employed. There is no airflow or separator attached directly to the machine, as itspurpose is to perform the coarse grinding only. A partial recirculation of the product is made both tobalance the mill operationally and to crush any clinker that passes through the mill uncrushed on the firstpass. With a recirculation rate of 50-70%, the vertical roller mill pregrinder can take up 6-8 kWh/mt feed.This means that an additional 20-25% more grinding power can be added to an existing ball mill circuit.In combination with adjustments to the ball charge in the mill, a 30-40% capacity increase can beattained. For a well operated system, an overall specific grinding power savings of about 10% can berealized.

With a capacity increase of this magnitude, normally the auxiliary systems in the mill circuit, including theseparator, will need to be evaluated to insure that it can accommodate the increased throughput, else alarger capital investment will be required. Down time of the original system may be an issue during theerection period if the separator needs to be replaced. The overall cost of the vertical roller mill pregrinderis much higher than the VSI due to the much heavier machinery and foundation works, however greaterincreases in the capacity can be realized. Also, the wear of the rollers and table is very low, andmaintenance downtime and associated costs are quite low.

Hydraulic Roller Press (HRP) have been used in the cement industry for years, and today there arehundreds operating worldwide. The major attractiveness to using an HRP is that high pressure grindingis much more efficient in grinding than a ball mill, therefore a significant power savings can be achievedwhich can offset part of the investment and maintenance costs.

Maximizing the work done by the HRP results in a more efficient overall grinding process. In order toincrease the amount of work done by a HRP, it is necessary to recycle a portion of the pressed materialback to the press for further grinding. However, the HRP suffers from operational instability if the amountof fines recirculated becomes excessive. For these reasons, there are two main arrangements using anHRP: Pregrinding and Semi-finish Grinding.

In a pregrinding arrangement, the HRP can be erected separately from the ball mill and separator circuit.This can allow the HRP to be erected while the ball mill is still in operation, reducing the downtimeassociated with the conversion. There is no separation of the recycled material back to the press;therefore the circulation is limited to prevent operational instability. For this reason, the amount of workthat the HRP inputs to the material is only about 4.5 kWh/mt of fresh feed. As the HRP is more effectivein grinding than a ball mill, the result is a decrease in the specific power consumption of the entire mill

circuit by about 10%, depending on product Blaine. Changes can be made to the mill ball charge to takeadvantage of the smaller feed size as well. In total, a capacity increase of 25-35% can be achieved byadding an HRP as a pregrinder.

To maximize the capacity of an existing ball mill, the work done by the press must also be maximized.This will require a large size press with a high material recirculation rate in order to maximize the specificpower input. As the press operation will become unstable due to the large amount of fines that isgenerated, the press must then be put in closed circuit with a grit separator so that the fines in thematerial recirculated to the press can be extracted. In this arrangement, the press can do most of thegrinding work, and the ball mill only “finish” grinding the material and flatten the product particle sizedistribution. A capacity increase of greater than 100% can be realized in this “semi-finish” grindingarrangement. The fines separator, elevators, and transport systems will all need to beupgraded/replaced due to the significantly higher capacity, and therefor the investment cost anddowntime associated with the modification can be quite high. As the wear of the roller press isproportional to the amount of clinker pressed, there can be some significant maintenance costsassociated with the system.

Milling circuits with a VSI or vertical roller mill pregrinder do not provide a significantly lower grindingpower consumption than a ball mill in closed circuit with high efficiency separator. As a circuit with apregrinder has more mechanical equipment, higher maintenance and lower run factors can be expected.Only the HRP in a semi-finish arrangement significantly lowers the power consumption of the finishmilling circuit. However, this option is more expensive to install and maintain due to the additionalequipment. It is only when the power costs are significant that the HRP can give a positive ROI to offsetthe initial capital investment. TABLE 1 gives a short summary of the benefits of each of thesepregrinding options as compared to a standard ball mill system.

TABLE 1: General Comparison of Pregrinding Options

Ball Mill(Base)

VSIPregrinder

VRMPregrinder

Roll PressPregrinder

Roll PressSemi-Finish

System CapacityPotential 100% 110-120% 130-140% 125-135% 135->200%

Specific GrindingPower 100% 90-100% 85-92% 75-90% 60-80%

MaintenanceLow100

High130

Moderate115

Moderate125

High130

Availability 95-99% 80-90% 90-95% 88-95% 80-90%

Ease ofOperation 1 2 3 2 4

New Grinding Circuits can be a more economic solution when considering the options available forincreasing the grinding capacity significantly. Depending on the inefficiencies and physical condition ofthe older, existing equipment, actual layout of the plant and requirement for new storage’s, transport andstructures, it may be preferable to install a completely new grinding circuit rather than upgrade theexisting mills. This is particularly attractive when there are several small mills that all need to bemodernized to achieve the desired final capacity. This would have a high cost not only for the additionalequipment, but also in terms of down time and constructability. If some of the existing mills are very smalland inefficient or suffer with high maintenance costs, the incremental cost of increasing the new mill sizecan be offset by the operating advantage of shutting down the smallest mills.

CASE STUDY – INCREASING CEMENT GRINDING CAPACITY AT PHOENIX CEMENT COMPANY

Phoenix Cement Company (PCC), located in Clarksdale, AZ, currently operates (2) two-stage preheaterkilns and one long dry kiln, built between the late 1950’s and early 1960’s. The annual output of the plantis in the range of 600,000 short tons of clinker. In 1997, Phoenix Cement Company began to discussplans for increasing the plant capacity to one million short tons per year.

In the initial planning for the expansion, it was considered that the additional clinker capacity would begained by adding a small, new kiln line, and maintaining production on the older lines. In order to supportthe increased clinker production, it was determined that the overall plant cement grinding capacity shouldbe increased by about 30%. On site there are four 1500 HP ball mills, in which all of the raw material andfinish grinding take place. As the capacity of both the raw and finish grinding needed to be addressed, itwas originally thought that some synergy could be gained by converting the raw mill to a finish mill, andinstall a completely new vertical raw mill. Although this would save the cost of the mill and somebuilding/civil works, it would involve a lot of transport of materials both to the mill and back out to the silos.

As the project started to develop, the scope was changed to that where a new precalciner kiln wouldprovide total clinker production, and the older, inefficient kiln lines be shut down. At that time PCCabandoned the idea of converting the used raw ball mills to finish grinding, and began to pursue thepurchase of an existing 4400 HP Nordberg ball mill for making up the required finish grinding capacity.There were very brief discussions held at the time to consider the addition of hydraulic roll presses, but itwas determined that the initial capital investments did not warrant its use, and PCC would have enoughgrinding capacity with the existing ball mills.

The next challenge came about when the used Nordberg mill ball mill was sold and no longer available onthe market. Naturally, the project then came to consider a completely new ball mill to supplement thefinish grinding capacity. Although more expensive than the used mill, this would be a fairlystraightforward installation comprised of all new equipment.

PCC has a history of installing progressive process solutions in the U.S. cement industry. In 1988, thefirst SEPAX flashdryer was installed in the US, operating as not just a classifier, but also a flash dryer forthe raw material. Additionally, Gamma Metrics installed the first on-line analyzer for raw materialproportioning back in 1985. PCC was also one of the first cement companies to explore the use of avertical shaft impact (VSI) crusher to boost the cement mill output. However, this VSI was nevercommissioned for various reasons.

At the time when PCC was considering the new ball mill circuit, they were introduced to vertical roller milltechnology for cement grinding. For decades, the vertical roller mill (VRM) has been used for grinding ofcement raw materials and coal, and the VRM has become the dominant mill for these applications. TheVRM for cement grinding has been used in other parts of the world for many years; however; it had notbeen installed in the North American market to that time. In considering the VRM for cement grinding,PCC had to not only consider the financial implications of this technology in terms of installed andoperating costs, but also the product quality and comfort of using this technology as a supplement to theexisting finish grinding ball mills.

It is a well known fact that the energy consumption of a vertical roller mill system is significantly lower, 65-75%, of that required for a comparable ball mill system. See TABLE 2 for a comparison of the proposedinstalled power for a new ball mill system and new vertical mill system. As the capacity of the vertical millis higher than that proposed for the ball mill option, the table normalizes the power consumption by thecapacity. The operating power saving is expected to be in the range of 10 kWh/mt as compared to a newball mill installation.

TABLE 2: Comparison of Installed Power for Ball Mill and Vertical Mill

Ball Mill Vertical Mill

Mill Size UMS 4.6M x 13.0M OK 33-4

Mill Capacity (@ 3900 Blaine) 90.7 mtph 118 mtph

Installed Power

Mill Drive 41.1 kWh/mt 25.3 kWh/mt

Separator Drive 1.6 kWh/mt 1.3 kWh/mt

Fan(s) 4.0 kWh/mt 11.5 kWh/mt

Total 46.7 kWh/mt 38.1 kWh/mt

The vertical mill provides the grinding, transport and separation processes in a single, compact unit thatcan perform with the same reliability as seen with roller mills in raw grinding. As a result of the reducedequipment requirements, the layout of the finish grinding solution is simple and foundation and buildingcosts are low, especially in temperate environments when an enclosure is not required. The costdifference between the ball mill and vertical mill system, on an installed basis, can be 10% or less –depending on the final building requirements for the vertical mill. At Phoenix, the mill is arranged withonly a minimal weather covering. A light, open structure is used, with access for a mobile crane to beused for maintenance and repair work. FIGURE 1 provides a sketch of the compact arrangement of thePhoenix installation.

FIGURE 1: Phoenix Ok Mill Arrangement

The quality of vertical mill cement can be different from that produced by a ball mill – and the mainreasons for that is the steeper particle size distribution, and the low retention time which can impact thedehydration of the gypsum. The vertical mill is quite flexible in it’s operation, and the particle sizedistribution can be adjusted though changes in the dam ring, grinding pressure, and airflow rates. Ahigher dam ring results in a thicker grinding bed, and a longer retention time on the table. The material

will thus receive more overrollings before leaving the table and therefore a lower circulating load to theseparator. The result is a flatter particle size distribution. A lower grinding force provides a lower energyinput per overrolling, resulting in a higher circulation to the separator and a steeper particle sizedistribution. See FIGURE 2 for a graphical depiction of these relationships.

FIGURE 2: Relationship of Product PSD and Operating Parameters

The dehydration of the gypsum can be adjusted by changing the mill exit temperature, which is simplydone by recirculation of the mill exit gases and adding additional heat from an auxiliary furnace ifnecessary. Both the setting times and water to cement ratio can be adjusted by controlling the amount ofgypsum dehydration. As gypsum can come from different sources and have different characteristics, thesystem is initially set up to operate with a mill exit temperature of 110ºC (230ºF) to insure that the gypsumdehydration is sufficient, however it can be operated up to 125ºC (257ºF) if required.

The biggest concern regarding the use of the vertical roller mill was the impact on the product quality. Asthe market in which PCC operates in is accustomed to receiving cement produced in ball mills, there wasnaturally some concern that the product quality would be different than that of the existing ball millcircuits. The impact of the changing product quality is a very serious marketing concern for any cementproducer. Before PCC would decide on a vertical mill solution, these questions had to be answered.

FLSmidth was able to supply information from many of its vertical mill installations that also operated withball mills in order to answer these questions. PCC also went to Cementos Progresso in Guatemala, aplant that has two OK vertical mills and two ball mills in closed circuit with roll presses, to furtherinvestigate these quality issues. TABLE 3 provides a comparison of the main cement quality databetween the ball mill and OK mill cement at that plant. As can be seen in Table 3, the cement from theOK mill has a somewhat lower Blaine value and lower sieve residue on the 45 ìm sieve, i.e. a morenarrow particle size distribution. Nevertheless, all other quality figures, including water to normalconsistency, setting times, and compressive strengths are almost identical.

High dam ring

Medium dam ring

Low dam ring

1.1

1.2

1.3

1.4

78 83 89 100

% Compressive Force

N-value

TABLE 3: Comparison f Cement Quality BetweenOk Mill and Ball Mills

SettingTime Compressive Strength (ASTM)MPaMill

Blainecm2/gm

Residue%>45µ W/C

%Initialmin

Finalmin

1Days

3Days

7Days

28Days

OK 3700 6.4 28.6 130 310 12.3 22.3 26.8 34.2BM 1 4000 7.5 27.7 130 310 12.8 21.3 25.4 32.0BM 2 4000 7.6 28.0 130 310 12.7 21.6 25.8 32.9

* PCC cement: 75% clinker, 15% Pozzolan, 5% limestone, 5% gypsum

At the end of all of the evaluations, PCC was confident in the vertical roller mill technology for cementfinish grinding and selected a new, OK mill system to supplement its’ existing cement ball mills.

FINAL FINISH GRINDING SYSTEM DESCRIPTION

The new finish grinding area consists of an OK 33-4 vertical roller cement mill with a 4000 HP (2983 kW)motor and an OKS-70 classifier with a 200 HP (149 kW) motor that is capable of producing 130 STPH(118 MTPH) of cement at 3900 Blaine. As mentioned previously, the mill is arranged with only a minimalweather covering. A light, open structure is used for the mill itself, with access for a mobile crane formaintenance and repair work. The dust collector is outside the weather protection. The mill exit gas isvented directly to a baghouse dust collector. The gas then goes through the dust collector fan, afterwhich part of the gas is recycled back to the mill inlet, while the remainder is vented to atmosphere. Thereis little energy wasted on heat generation in the mill as compared to a ball mill, so an auxiliary air heater issupplied to provide additional heat if required for drying the gypsum. The product collected in thebaghouse is then conveyed to cement product coolers. Two identical cement product coolers (10’0” x20’8") are provided to cool the cement temperature from 110°C (230°F) to 63°C (145°F). The cool cementis then conveyed to the existing cement storage silos.

CONCLUSION

Many different options were considered before the final decision to use the OK vertical roller mill forcement grinding was made by Phoenix Cement Company. With the changing scope of the project, theoptions including the use of existing mills, pregrinders, as well as new mill circuits were evaluated, and anew vertical finish mill was finally selected. Even though there are no running references for verticalcement mills in the U.S., PCC decided to go forward with this cutting edge technology. After thoroughlyinvestigating all of the aspects of vertical mill performance, PCC has shown confidence that this solutionwill provide them with the most economic operation while producing the high quality product that theircustomers demand.