- THE MAGAZINE
While long known as excellent glass formers, natural sources of alkaline earth silicates such as wollastonite (CaSiO3) and diopside tend to be contaminated with both heavy metal and refractory minerals that have precluded their widespread use in glassmaking. In addition, the logistics of delivery from the remote and limited deposits available have created an economic barrier to success. In 1995, researchers began attempting to synthetically produce minerals required by the glass industry in optimal locations, near clusters of glass plants or within a glass plant, thereby overcoming the inconsistent quality and logistic barriers associated with natural deposits. Recently, these efforts have resulted in a great deal of success.
The synthetic diopside uses two primary raw materials, sand and lime, and a special binder and catalyst package to economically synthesize a chemically consistent, high purity diopside. Lime is the product of decarboxylation of dolomite (CaMg(CO3)2 or calcite (CaCO3). Since sources of calcium oxide (CaO) and magnesium oxide (MgO) are mixed together, the CaO to MgO ratio of the resultant product can be controlled to a consistency far tighter than that afforded by natural, mined products. While natural diopside deposits are yellowish green, green or almost black, depending upon impurities, the synthetic diopside is white.
Laboratory WorkEarly in the development program, it appeared that significant acceleration in melting and refining could be realized when diopside was used as a glassmaking raw material instead of dolomite. This effect was confirmed by independent studies conducted at Alfred University, which were presented at the 1999 Glass Problems Conference. The following conclusions were drawn from these studies.
The rate of raw materials dissolution and bubble removal in soda-lime-silicate melts can be increased through the use of diopside as a batch component. Batch-free and fining times are decreased by 25 to 35% when all of the MgO and most of the CaO is supplied by diopside. The decreases in batch-free and fining times are achieved because this mineral does not produce carbon dioxide (CO2) gas and supplies silica in a more easily dissolved form, thus reducing the amount of sand needed in the batch. The replacement of carbonates by diopside in soda-lime-silicate batches should be an effective way to decrease fining times or to allow use of lower temperatures during the fining portion of glass production.1
The Alfred University study was accomplished using 25 g melts. With the laboratory benefits quantified, researchers looked for a glass manufacturer who was willing to partner in a “small” 130,000 to 1 scale up trial in a glass manufacturing facility. The L. E. Smith Glass Co. of Mount Pleasant, Pa., agreed to several production trials. The furnace to be used was a 3.5-ton, oxygen and natural gas fired day tank.
Production TrialsTwo production runs were compared. Both runs were L. E. Smith “Crystal” compositions that contained no colorant additions. One run used dolomite, and the second used the synthetic diopside. The glass sand level was maintained at 4000 pounds per furnace run. The furnace run that contained the synthetic diopside produced 11% more glass than did the regular dolomite run. For each furnace run, data were recorded and a glass sample was taken from the furnace every hour. The recorded data included temperature, fuel flows, oxygen flows and furnace pressure. L. E. Smith personnel were responsible for the operation of the furnace, including the determination of when the glass was melted and refined to the company’s manufacturing standards. The synthetic diopside batches yielded 11% more glass, and melting and refining were completed in 19 hours. Control trials required between 23 and 23-1/2 hours to complete melting and refining.
Availability of Synthetic DiopsideThe two synthetic diopside products currently available are chemically identical but differ in particle distributions. One features a particle distribution between -30 and +140 mesh, and the other has a particle distribution of -140 mesh.
The first customer sampling facility capable of sustaining full-scale melting trials is now operational in Woodville, Ohio, some 15 miles southeast of Toledo, Ohio. The Woodville facility was chosen based on its proximity to sources of lime and sand. It is also near a significant number of glass melting facilities. Initial production at the Woodville facility is limited to 36,000 tons of synthetic diopside per year. However, the facility can be expanded to 200,000 tons per year if needed. Trials are scheduled across a number of magnesium oxide containing glass segments, including specialty, lighting, fiber glass and specialty fiber glass.
Synthetic diopside serves as a combined raw material and melting aid. It flows just like current silica raw materials. If a glass plant is melt limited, the synthetic diopside can easily be introduced into the glass melting process without significant capital investment. As evidenced by both the lab and plant trials, the use of synthetic diopside could contribute significantly to profitability with increased throughput and lower energy and/or oxygen costs.