HELP IN A BUCKET
MORRIS INC. — USA South Dakota — DISCOVERS THE BENEFITS OF A BUCKET WHEEL
Morris Inc. was faced with the dilemma that the aggregate dug from its South Dakota pits was unusable as mined. It is a problem experienced by many aggregate producers all over North America. Like many others, Morris Inc. has had to be innovative to meet its customer’s needs.
Morris Inc. is a heavy equipment highway contracting company in Fort Pierre, S.D., located along the Missouri River in central South Dakota. The company also has a strong supporting role in the aggregates business. Morris Inc. owns and operates two concrete batch plants, two portable asphalt plants, along with an underground utility division, and is currently operating three portable aggregate-crushing plants and two wash plants. The company produces aggregate for customers both in and out of South Dakota.
Location, Location, Location
Due to the company’s location in central South Dakota, local natural aggregate resources are limited. The lone limestone quarries in the area are located in the western part of South Dakota while all of the granite and quartzite quarries are in the eastern part of the state.
The long distances that are required to travel to acquire these materials have made these products expensive and not cost effective. This challenge resulted in Morris Inc. being forced to produce quality products from the glacier deposits found throughout central South Dakota.
Currently, the company is producing concrete rock and sand in a pit that has high deleterious material, consisting of primarily shale and iron oxide that ranges between 6 to 9 percent on the feed side material. The specification that has been established by the South Dakota Department of Transportation (DOT) for concrete rock and sand is less than 1 percent deleterious material having a specific gravity of less than 1.95. Since 1998, Morris, Inc. has utilized a water pulsating jig to separate the deleterious material and achieve state-specified material from the aggregates. The Jigging process stratifies the feed according to density, and withdraws only high-density product, while allowing the deleterious particles to overflow a weir and report to a reject pile.
Essentially the jig creates an up and down movement of water through the controlled inlet and exhaust of air. As water is pulsing upward it subjects the particles to differential acceleration causing the smaller, lower density material to be lifted higher and faster than larger, more-dense particles.
At the completion of the upward stroke the larger, heavier particles fall faster. By repeating the upward and downward movement, multiple stratified layers of particles are formed. A measuring device then identifies a selected strata to be rejected and the jig automatically controls the withdrawal of high density, in specification material.
By using the jig, Morris Inc. is able to produce hundreds of tons per hour with minimal personnel. Operating costs are also minimal. The rotary air valves, which meter air in and out of the jig, are the only components with any significant movement.
Sand Screw Issues
Once the company resolved the lightweight issue, the next production bottleneck was a single 36-in. sand dewatering screw. The lack of production with the sand screw or the amount of water that remained in the finished aggregate was simply unacceptable. The moisture content in the sand typically would range from 20 to 25 percent and caused long delays as loadout slowed waiting for water to drain.
Another issue with the sand screw was the discovery of small amounts of shale particles pooling at the base of the aggregate stockpile because the excess water would carry the shale to the base of the pile. So, even after shale was reduced by the jigging process, there would be a re-concentration of shale pockets that would form at the base of stockpiles, and ultimately result concrete pop outs in the finished concrete. Also, the excess water not removed by the sand screw caused segregation in the material which resulted in an inconsistent gradation.
Various solutions were researched: replacement of the single 36-in. screw with a twin 36-in. sand screw or possibly a wet screen or a combination of both to handle the high production rate and dewatering the sand in a more manageable manner. None of these options met the company’s long-term economic and technical objectives.
Help in a Bucket Wheel
As a result, other solutions were evaluated, one being the bucket wheel. In the end, Morris Inc. opted to go with a Stichweh Model E-3511 bucket wheel, which has an 11-ft., 6-in., diameter wheel. A 7.5-hp gear motor is the only power supplied. The wheel turns between 1 and 5 rpm. No spare parts are expected to be needed for the next five years.
The bucket wheel was fitted with urethane screens to create a suction effect for maximum dewatering. The dewatering bucket wheel was supplied under lease by MSM LLC – a joint venture between Morris Inc. and Snoby Separation Systems – and came fully assembled.
The initial results from the bucket wheel have proved the validity of the research. Product moisture has been lowered in the fine aggregate to between 12 and 15 percent, while maintaining the higher production rate. It was verified that re-concentration of shale at the base of the stockpile has been eliminated.
Based on the performance of the E-3511, a second machine was leased. However, the use of the second bucket wheel was for a different application and pit.
Through the use of an automatically adjustable blending chute, custom gradations of product can be offered. In addition, a second saleable product can be produced.
The machine allowed for development of different grades of sand, depending on what the customer prefers, simply by adjusting a single setting on the bucket wheel controller.
For example, the sand produced by the bucket wheel is made finer or coarser, depending on the customer’s needs, by changing a set point. Currently, the double bucket wheel is producing concrete sand for various concrete plants as well as making a fine cut green sand and bunker sand for golf courses in the area.
The bucket wheel itself is rated at 150 tph. Two 7.5-hp gear motors are the only power supplied. The machine has a fully automatic speed controller coupled with a frequency converter. The controller adjusts the wheel speeds according to torque load on the machine.
A sand slurry is introduced to the bucket wheel near the first wheel, where the coarse sand settles and is lifted by the rotating wheel. The finer sand is carried farther down the bucket wheel trough before it settles. Once the finer sand settles a set of flights bring the fine sand back to the second bucket wheel. As mentioned earlier, the rotating speed of the wheels are controlled according to torque, thus each bucket is filled to an optimum level. By knowing the torque of both the coarse and fine wheels, custom gradations can be made consistently.
Overall the bucket wheel machines have outperformed all of the company’s expectations.
Like many other aggregate producers, Morris Inc. continually keeps an open mind for innovative solutions to maximize production and lower overall costs, while at the same time producing high quality aggregate products.
By Chad Hicks — the aggregate manager for Morris Inc.- and Andrew Snoby — vice president of operations for Snoby Separation Systems LLC.
ROCKproducts — February 2016