The production methods, mentioned in this block are examples of the powder method. Indeed, the glass is ground to a powder, a foaming agent is included and the powder mixture becomes sintered later on at about 650°C. At this point, we have glass with a built in foaming agent.
Grinding of the glass is an important part of the production process. For the grinding of glass and other brittle materials, we have a lot of different possibilities but the old fashioned ball mill is the most popular one. Engineers will study and choose between the different equipment to find the most efficient ones. Scientists will more try to change the glass surface in the direction of more efficient grinding. This blog is written by a scientist and for that reason, the concept grinding aid is mentioned.
The readers, hitting the “grinding aid” link, have download a nice (but old) paper about grinding additives. The paper learns that
- Only about 1% of the energy in a ball mill goes to actual surface creating.
- 20 kWh/ton is a typical energy consumption.
- Less than 1% carbon black improves the grinding of cement with 30%.
Carbon black is already mentioned in this blog. It is used as a foaming agent for glass because it is very fine carbon, which can be well distributed over the glass surface. Because it is also a grinding aid, it makes sense to add it already in the ball mill.
In the gravel methodology, it is the common practice to grind the glass without any additive and to mix the foaming agent with the glass powder afterwards with special equipment like for example Eirich mixers.
Some people doubt that a ball mill can mix with the same quality as an efficient mixer but others mix in the ball mill and do not invest in these mixers. I am sure about one thing: this knowledge is the key to a superior cell structure like for example the Neoporm ware shows to us. Mixing and grinding is still 90% empirical knowledge, which means that the solution is hard laboratory work.
Foaming agents which are also effective grinding aids are favorites in this business. That is probably the reason why carbon black was hard to replace.