Silica Sand—the most common ingredient in glass. Whether the glass is container glass, window glass, boro-silicate glass, fiberglass or water glass (sodium silicate), silica sand makes up approximately 60-70 percent of the glass batch and therefore has a significant affect on the glass quality.
For all glasses, the consistency of the chemical components and size characteristics are important to producing high quality glass. The three main chemical contaminates in silica sand are usually the iron content expressed as Fe2O3, the alumina content expressed as Al2O3, and Titania expressed as TiO2. If the alumina content is high due to a presence of feldspar, then the other alkalis such as CaO, NaO2 and K2O may also be significant contaminants.
Each of the contaminates mentioned above will impact the end product. The impact of the contaminates may be either positive or negative depending on the percentage of the component in the silica sand and the level of the same component in the glass. For example, generally, lower iron content is better however, to produce amber containers, iron is often added to the glass batch. Therefore, higher iron content could be beneficial as long as the iron content was consistent. Higher alumina due to a Nafeldspar
mineral can help lower the batch cost since the lower priced silica sand is substituted for a portion of the more expensive feldspar or soda ash. However, too much alumina, generally greater than 1.8 –2.0 percent, cannot be used for container glass without blending with lower alumina sand. In addition, Al2O3 levels greater than 0.3 percent are generally unacceptable for window glass.
Regardless of the contaminate concentration, the consistency is extremely critical. Consider a glass container company producing amber bottles with inconsistent levels of iron and alumina in their sand. The results would be a bottle with too high or too low iron content. Too little will affect the color, too much will result in very brittle glass. As alumina levels change, the viscosity and density of the glass change making it impossible in today’s high speed bottling machines to make a consistent product.
In addition to the chemical components, the other key component of glass sand is the lack of refractory heavy minerals. These minerals are generally aluminum silicate minerals but there can be others such as chromite. Whereas the chemical components are measured in tenths or hundreds of a percent, the refractory minerals are measured in the number of grains in a particular size sample. As few as 1-2 grains of chromite in 500 grams of silica sand can render the sand unfit for glass production. If a silica product has 1 grain of refractory mineral per 500 grams, a typical glass plant using 350 tpd of sand would have in excess of 700,000 defects per day; far too many to make the sand a desirable component.
Since silica sand is a low priced commodity, high shipping cost would result in an uneconomical deposit with freight cost being more valuable than that of the silica sand. Thus, it may be necessary to process a lower grade deposit that is closer to the market place than process a higher-grade deposit that is much farther away. Processing can often be achieved at a fraction of the freight cost.