GRINDING/AIR CLASSIFICATION

Grinding is widely used in the silica industry to produce ground silica for the textile fiberglass industry. For the textile fiberglass industry, consistency of chemical components along with size distribution is as important as with other glass products. A typical grinding mill such as Outokumpu Technology’s 375kW is For the size distribution, it is important that the ground silica is void of any oversize grains and that the extreme fines be as low as possible. Typically, the textile fiberglass industry uses a product that is either 95 percent passing –75 micron or 95 percent passing 45 micron. For these sizes, the oversize is defined as grains coarser than 250 microns and extreme fines a less than 5 microns. Both of these can cause unmelted silica stones and result in breakages of the fiber during the drawing process. Although it is obvious why the over size grains are difficult to melt, the possibility of the fines not melting does not seem to be valid. In this case, when there are excess amounts of fines, the fines tend to agglomerate and not mix with the fluxing agents. Therefore, the agglomerates do not melt. To maintain a consistent particle size distribution, void of oversize grains and low percentages of extreme fines, the grinding mills are used with high efficiency air classifiers such as the one produced by Progressive Industries Inc., as shown in the Figure 12 below. The unique patented seal prevents oversize from entering the product and the highspeed rotor, combined with the secondary air washing, prevents product size particles from returning to the mill for more grinding and production of the extreme fines.

Since consistent chemistry is important, the use of natural flint grinding media and liners has been on the decline during the past 10 years. These natural products were used for many decades but due to a combination of declining product quality and the demands for better silica products, they have been replaced with high alumina ceramic materials.

These engineered products are significantly more costly per kg than the natural materials but due to their better wear characteristics offer lower cost per ton of silica. The ceramic liners are approximately 20 percent as thick as the natural flint liners but last 5 times longer. As important to the wear characteristics, is that since the liners are not as thick, they allow greater mill volume to be filled with grinding media and therefore allow for higher production rates.

SLILICA SAND PROCESSES

The previous portion of this paper has discussed the unit operations that are used in a silica plant. These unit operations can be used in various combinations to achieve the desired end product based on the starting ore quality.

Below, the flow sheets for two recent projects by Outokumpu Technology Inc. will be discussed.

Plant A

In the first process, shown in Figures 13 and 14 below, the objective of the plant was to produce two different glass sands with premium glass sand having low iron content and a higher iron standard glass sand that would be sold damp. In addition, the company also wanted to produce a ground silica product for use in the textile fiberglass market.

Figure 1 Wet Processing Diagram of Glass Sand Plant A

In this plant, attrition scrubbing is used to break down some loosely consolidated sand grains and to liberate any clay from the silica grain surfaces. In the next step, the FLOATEX ® Density Separator is used to make a 200 micron separation. For this deposit, this separation stage was needed since the coarser silica grains tend to be more pure and the minor inclusions have less effect on the iron content compared to the finer grains. The FLOATEX ® Density Separator underflow product reports to two stages of Outokumpu Technology’s CARPCO ® Spirals to remove the free iron bearing minerals. The iron content of this product will be less than 0.015 percent Fe2O3 prior to further treatment in the dry process (see Figure 14).

The overflow of the first Density Separator also reports to two stages of Outokumpu Technology’s CARPCO ® Spirals to remove any free iron bearing particles. The flowsheet could be simplified by conducting the spiral separation prior to the Density Separator. However, the process produces highergrade products by first classifying the sand into two more narrowly sized products. The product from the spirals then report to another Density Separator to remove the –100-micron fines generated by the attrition scrubbing stage. The underflow of the product is then stockpiled, allowed to drain and then shipped as damp glass sand with a Fe2O3 content of

Figure 14 shows the remainder of the process, which incorporates the dry processing section of the plant.

Figure 14 Dry Processing Diagram of Glass Sand Plant A In this section of the plant, the premium glass sand from the wet processing section is dried and then reports to a 2 stage INPROSYS ® Magnet for final cleaning. The spiral removed the free iron bearing minerals and some rutile that was present in the sand. Rutile has a high specific gravity but is nonmagnetic. The rare earth roll magnetic separator removes the iron bearing minerals that were not removed by the spiral. For the most part, these minerals are present as inclusions in the silica grains. After magnetic separation, the iron content of the glass sand is reduced from 0.015 to

In addition to the magnetic separators, the plant will also produce ground silica for the textile fiberglass market. The mill, grinding media, and air classifier have been chosen to assure the rigid product quality needed for this industry.

Plant B

In the second plant design, the ore quality was considerably lower than that of the first plant design. This deposit was substantially closer to the market place than the competition. Therefore, although the processing is very extensive, and operating cost higher than average, the delivered price to the customer was still substantially lower than the competitors.

Figure 15 shows the processing diagram for Plant B. In this plant, a screen was used to not only remove the oversize, but also break down the loosely agglomerated sand particles. Due to the high percentage of clay, a FLOATEX ® Density Separator is employed as a desliming process after the initial washing stage. The pumping action helps liberate the sand grains from a majority of the clay. In the plant process, approximately 15wt percent of the feed is lost as –100-micron material in the desliming stage. After the initial desliming, three stages of attrition scrubbing with desliming by cyclones after the first and second stage are used to liberate and remove the remaining clay and silt. A FLOATEX ® Density Separator is used for the final desliming stage, instead of a cyclone, to assure that the –100 micron material is removed.

Figure 15 Processing Diagram for Plant B

This deposit contained higher amounts of feldspar than desired by the glass customer. The feldspar resulted in alumina contents too high to be used for float glass. After the sand is dried, it is activated using fumed HF in a rotary mixer. Once activated, it reports to the T-Stat separator where the majority of the feldspar is removed as a waste product. The middling fraction is recycled back to the activation process to allow for a second pass through the unit.

The sand fraction from the T-Stat then reports to the INPROSYS ® rare earth roll magnetic separators to reduce the iron content. The non-magnetic fraction is then stored and shipped to the customer. The process results in a product with an iron content of

Although the iron content is much higher than in Flowsheet A, the close proximity to the market place makes this an acceptable product.

The previous flowsheets show two examples of glass sand processing. Each process is unique to the ore body and the glass sand customer’s needs. This is true for all sand deposits. However, there is always a common need for any sand process. The sand process must be the most economical process possible since the selling price of sand is very low. At the same time, the process must be capable of producing a product that has a consistent chemical composition with iron contents generally lower than 0.035 percent Fe2O3 and at times lower alumina levels. In addition to the chemical composition, the process must also be capable of eliminating the oversize and fines so that the sand particles are within the –500 +100 microns size range.