In 1974, Southdown's Clinchfield, Ga. plant underwent its initial conversion from a wet-process plant to a preheater plant. However, the raw material being processed retained extreme high moisture levels of 18% to 22%. The plant had been through two other expansions and modifications since 1974, yet the problem with the materials remained, contributing to the decision to move forward with what was deemed the "Clinchfield 2000 Project."

Because the previous mill was only able to grind at 6% to 8% moisture, the raw materials had to be sent through a rotary dryer to remove excess moisture prior to being ground. The major challenge faced by Southdown was to install a raw grinding/drying system that could handle the wet raw materials, and eliminate the dryer in order to reduce energy and maintenance costs, while increasing production. The solution was found in the LM 46.4 vertical roller-grinding mill, manufactured by Loesche.

Raw materials Understanding what type of raw materials were being used at the Clinchfield plant was essential to the success of the project. The mixture being ground is composed of 68% limestone, 26% fuller's earth, 5% alumina clay, and 1% iron slag. Fuller's earth is a highly absorbent hydrated compound of silica and aluminum, which contains 45% silica, 40% lime, 11% alumina, 2% magnesia, and 2% iron oxide.

This raw material mixture is extremely sticky due to its hydroscopic properties. The limestone and fuller's earth were mined and crushed together prior to the project. However, with the modifications made, the two materials are being mined separately. The fuller's earth is now crushed in a primary crusher and stockpiled under a 6,000-ton storage building located in the quarry to keep it dry after it's been mined.

Equipment Much of the feasibility of the project depended on being able to modify and upgrade the existing equipment while adding a new grinding mill that would eliminate the separate rotary dryer, thus reducing the energy consumption by utilizing more waste heat from the process. This presented the difficult task of finding a roller mill that would be able to both grind materials at a high moisture level of 22% and achieve a higher degree of fineness.

The existing roller mill was only capable of grinding the raw materials at a maximum level of 8% moisture, while grinding the raw mix to a fineness of only 72% passing on a 200 mesh, and 95% on a 50 mesh. A separate coal mill operating at a rate of 3 tph combined with supplemental gas heat also was required to operate the existing rotary dryer.

The goal was to install a mill capable of grinding and drying such a moist material using a minimum amount of auxiliary heat. The existing kiln, cooler, and preheater tower would have to be modified to supply a maximum volume of hot gases to the roller mill. The elimination of this rotary dyer alone would reduce the energy consumption by 0.7 MMBtu per ton of clinker, or about 18% of the total heat consumption.

It was critical to ensure that the gases drawn from the preheater tower, the clinker cooler, and the heat generated from the furnace were enough to prepare the material to enter the kiln. Aiding in this process, special raw material feeder bins also would need to be installed in order for it to handle the material being fed. The existing rotation of the kiln was 140 rph, and it was to be increased to 210 rph. Modifications to the bag house also were to be performed to increase its capacity and reduce its pressure drop. In addition, a calciner burner needed to be added to increase the production capability of the pyroprocess.

Selection process Prior to even considering a new mill, several critical requirements were established by the selection committee. The new mill had to be able to handle the moisture of the material, which led to the conclusion that the mill had to have a central inlet feeding system. These specifications limited the selection process to two major mill producers with seven cement plants where mills that met the requirements were in operation.

Selecting the Loesche system was done relative to mill life and maintenance costs and its ability to operate the closest to the parameters required. The airflow and temperature levels available to dry and grind the materials also were considered. In addition, Krupp Polysius was contracted to perform the modifications to the preheater tower and clinker cooler, and Penta Engineering was contracted to perform the plant modification.

The Loesche mill is a modular-designed, four-roller mill with conical rollers that operate on a horizontal grinding track. The grinding rollers are hydro-pneumatically actuated against the material bed on the grinding table. The installed counter-pressure hydraulic spring system provides a vibration-free and energy-efficient grinding effect, according the Loesche.

In the case of the Clinchfield project, the piston accumulators for the spring system were mounted on the pedestal wall. This system provides for a counter pressure generated on the piston side of the hydraulic cylinder, allowing the roller force to be adapted to the material to be ground and to keep the grinding bed thickness constant within close tolerance.

The ground material flows to the periphery of the grinding table, where the grinding forces are exerted by the rollers. Material then is swept toward the dynamic classifier by the airflow of hot gases.

Coarse material is separated and returned either along the exterior wall of the classifier or through the grit funnel back to the table. The fine material goes through the classifier and is transported by air to the four cyclones, where 95% of the dedusting occurs. The speed of the classifier is variable to adjust the fineness distribution.

The starting torque of this mill is kept low by lifting the four rollers at start up, which also enables the mill feed to be stopped and restarted without the necessity of stopping the mill. A feature of the mill that was particularly appealing to Southdown was the hydraulically operated swinging-out device, which makes roller maintenance easier. The high-efficiency separator type LKS, incorporated in the housing of the vertical roller mill, allows for a larger spectrum of particle sizes to be ground.

Modifications The modifications to the quarry began in the winter of 1998, and the construction of the raw material handling equipment and erection of the mill began in mid-1998, with the modification to the kiln, clinker cooler, and preheater tower occurring in April 1999.

Quarry-The first of the modifications had to occur in the quarry where the materials are mined. The two main components of the raw material mixture had to be separated at the working face in the mine. Prior to this project, the two were combined during blasting. The separating of fuller's earth from the limestone required the installation of a separate system for metering and feeding fuller's earth to the plant's 2.5-mile-long conveying system.

Fuller's earth is mined first and sent from the primary crusher to the new storage building. Limestone is fed directly into the primary crusher and then combined with the fuller's earth prior to a new Stamler secondary single-roll crusher. They are then sent via a conveyor to the new GammaMetrics cross-belt analyzer, where the chemical composition is analyzed, and the feed rate is automatically adjusted to maintain the optimum targeted chemical composition. The crushing and conveying system was designed to operate at 1,400 tph.

Raw Materials Handling-The raw material handling system was designed to minimize material handling problems. A stacker places the pre-blend limestone/fuller's earth from the quarry into a covered storage building capable of storing 36,000 tons. A reclaimer with active chain-driven harrows was installed to ensure that the sticky pre-blended material could be reclaimed.

The clay additive bins and roller mill feed bins were constructed with divergent hoppers. These hoppers are specially designed with a wider opening at the bottom than the top, to ensure that raw materials would not hang up on the walls of the bins. The limestone bin was designed with a stainless-steel bottom to prevent build up. All of the materials bins were designed with apron feeders. Each apron feeder is equipped with a spill drag for clean up.

In order to increase the clinker capacity of the kiln system, the two existing first-stage (top) cyclones were replaced with two low-pressure drop cyclones. A new 3,000-hp preheater ID fan and gas-conditioning tower also were installed.

Prior to this project, the sole source of fuel on the feed end of the kiln was the burning of 15% scrap tires that went through the riser duct into the back of the kiln. The project added a new oil-firing system to the calciner allowing for an additional 15% fuel to be burned, thus making the entire preheater work more productively.

Kiln-The kiln was operating at a maximum rotation of 140 rph. A new reducer and drive motor was installed to increase the maximum rotation capability to 210 rph. The kiln and roller mill dust collector inlet duct were relocated to introduce the dust-laden gases to the end opposite to the exit of the cleaned gases. The dust collector pressure drop was reduced as a result.

The dust collector was refurbished with a stainless-steel interior, due to corrosion problems produced by the high moisture levels in the raw materials. In addition, a new higher capacity, higher efficiency fan was installed to increase system gas flow capability. Due to the addition of the calciner firing, NOx emissions are expected to be reduced.

Clinker Cooler-It was essential to the project to keep all the existing equipment. However, the cooler needed numerous modifications. The cooler was operating at a 5% slope, which was removed and a static inlet grate section added to aid in heat recuperation and reduce maintenance costs.

The cooler fans were modified to accommodate the increase in production from 83 to 102.5 tph. A new duct and cooler booster fan were installed to withdraw hot gases from the mid-point of the cooler to supply the maximum amount of drying heat and combustion air to both the calciner and the vertical roller mill. The vast majority of the second drive cooler grates were replaced due to wear.

Another modification was enclosing the clinker storage hall and adding a conveyor system that takes the clinker from the hall to the kiln feed bins directly to the finish mill feed bins. By enclosing this storage hall on two sides, the fugitive dust is significantly reduced.

Other general modifications made to the plant included upgrading the electrical-distribution system to handle the increased power requirements, and upgrading the water-distribution system.

Installation and production rates Installation of the Loesche vertical roller-grinding mill began in April 1998 and started for the first time in April 1999, at which time the kiln was also restarted for the first time after its modifications were completed. The guaranteed rate of production for the completion was expected to be 240 tph. However, at the end of the initial performance test, the figures were more favorable. The mill has not yet been run to its full capacity and is already producing 250 tph, with some limitations set forth by the auxiliary equipment.

Due to the tight construction schedule, there was not enough time to perform a dry run on the system prior to commencing production. During commissioning, the initial high reject level was optimized to virtually no rejects.

There also was some difficulty getting the hydraulic system properly programmed. Initially there was some vibration at the base, and the mill motor was running slightly higher amps than anticipated. These problems were due to the high moisture levels of the raw materials and prior to operating could not have been detected. However, this initial setback was corrected with minor internal modifications.

There have been problems with the triple gate feeder, which were expected due to the sticky nature of the raw material, which was getting stuck to the flaps despite a heated gate. Several modifications have been made to minimize this problem. Despite this, however, the mill has met all of its performance requirements.

Another advantage is that the mill uses all preheater gases for drying, which has significantly reduced plant SO2 emissions via the scrubbing effect of the circuit. When the mill is running, the SO2 level is 30 ppm, and when the mill is not in operation, the level increases to 130 ppm.

One of the challenges will be learning to operate the mill at lower production tonnage rate without supplemental heat. This is one of the difficulties that will be addressed in the future.

Results The operation of the Loesche LM 46.4 vertical roller-grinding mill at Southdown's Clinchfield plant has exceeded all guaranteed figures and handled the grinding of the extremely wet material far better than the previous mill and eliminated the need of a rotary dryer. The vast increase in production and fineness has enabled the modified kiln to operate at planned capacity. The mill feed can be restarted without the necessity of stopping and emptying the mill adding to the efficiency of the plant. The mill is operating under low vibration levels.

The new storage and handling facilities have reduced the moisture levels that presented the project team one of its most significant challenges. The modifications have been mostly successful, and the capacity increase is the single most significant outcome of the expansion.