Grinding, an empirical science

logo_smallThe best cellular glass is produced with the powder method. A fine glass powder is produced and a foaming agent is mixed with the glass, the resulting powder is allowed to sinter with incorporation of the foaming agent and further foamed by increasing the temperature. It is not a surprise that there could be a relation between the obtained cell structure and the way the glass was ground and the foaming agent mixed.

In that perspective, we advice to start with an introduction on Wikipedia, a fantastic site. The software, which drives Wikipedia is open source and a very good tool to create your own internal company documentation system under Linux.

But the more interested reader will have fun with a paper, which demonstrates a lot of practical issues with as for example Steatite grinding media. The paper is concise and is also a good introduction to the way the glass is ground in ball mills and other instruments.

The more academic reader will find this text book very enjoyable. It gives  already a good trial to describe the kinetics of the grinding and gives a calculation of the absorbed power of a ball mill.

The reader, who can find the time and energy to absorb the above is however not ready to select the right grinding tool for his foaming process. Indeed, much more equipment than ball mills were developed during the last years and some of them have to be incorporated in the glass foaming plants.


Waste glass conversion to cellular glass was already hot in 1977

logo_smallAlmost 37 years ago, a report about the production of foam glass from waste glass was published. This report describes three foaming systems:

  • After water absorption by the waste glass powder
  • After mixing the waste glass powder with carbon (black)
  • After mixing the waste glass with milled limestone

The report learns that up to 6% water can be absorbed in waste glass by using an autoclave and the addition of NaOH in the water. This glass can be foamed but only small pellets are possible. The glass with 6% water has a low viscosity and during reheating, we have steam as the driving gas for the foaming. When the water has left the glass, the viscosity increases and the foam freezes.

But also carbon black can be mixed to the waste glass. In this case, also bentonite is added to able to form green pieces. This clay also seems to reduce the water absorption of the foam. Between the carbon blacks we find exotic ones and the ASTM defined ones, produced mainly in China. The paper assumes that the absorbed gas in the carbon is inducing the foaming, while we know that the sulfate in the glass is also important (Demidovich p12). Last but not least, the importance of a reducing atmosphere is mentioned.

CaCO3 was also mixed with the glass together with again bentonite. It was found that milled limestone gives the best results and larger dimensions are possible. A thermal conductivity as low as 0.052 W/mK is reported with this recipe at 160 kg/m³. Controlling temperature and foaming agent allows to foam a closed cell structure.

The report is in favor for the carbon black method but gives only thermal conductivity measurements for the CaCO3 system. Personally, I think that this last system deserves more attention.

Preparation of the foaming powder

logo_smallThe 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.

Composition, synthesis and properties of foam glass obtained from waste container glass

logo_smallThis paper, about a year ago published is another proof that the race for the best cellular glass, based on waste glass without remelting is really going on. The research is done at the university in Sofia in Bulgaria. Indeed, East Europe is doing a lot of effort for cellular glass today.

The paper reports on the use of glycerin and water glass for the foaming of the container glass. What do we learn?

  • They start from 6000 cm²/g glass powder while another source mentions 8000cm²/g.
  • Glycerin is used as foaming agent.
  • Also water glass is needed although the role is not clear.

But they also report a thermal conductivity of 0.02 W/mK. I doubt that this is possible and for that reason, I have my doubts about the measuring system C-Therm TCI. These dynamic cheaper systems are popular these days but BELGLAS advises to work with the stationary systems for thermal conductivity measurements. In a future blog, we will report again on that subject.

I am looking forward to measure a real 0.042 W/mK on a cellular glass plate, foamed from not remelted waste glass. In my opinion, that is the way to go.

A book to read: Cellular Ceramics, Manufacturing, Properties and Applications: ISBN: 3-527-31320-6

logo_smallIf foamed glass is your passion, you should study the above book. Looking to glass foams as an example of a ceramic cellular structure is broadening your mind. The table of contents looks as follows, where you can also download a part of the glass foam chapter as a seducer to buy the book.
1 Introduction
1.1 Cellular Solids – Scaling of Properties
1.2 Liquid Foams – Precursors for Solid Foams
2 Manufacturing
2.1 Ceramic Foams
2.2 Honeycombs
2.3 3D Periodic Strutures
2.4 Connected Fibers: Fiber Felts and Mats
2.5 Microcellular Ceramics from Wood
2.6 Carbon Foams
2.7 Glass Foams
2.8 Hollow Spheres
2.9 Cellular Concrete
3 Structure
3.1 Characterization of Structure and Morphology
3.2 Modelling Structure-Property Relationships in Random Cellular Material
4 Properties
4.1 Mechanical Properties
4.2 Permeability
4.3 Thermal Properties
4.4 Electrical Properties
4.5 Acoustic Properties
5 Applications
5.1 Liquid Metal Filtration
5.2 Gas (Particulate) Filtration
5.3 Kiln Furnitures
5.4 Heterogeneously Catalysed Processes with Porous Cellular Ceramic Monoliths
5.5 Porous Burners
5.6 Acoustic Transfer in Ceramic Surfac Burners
5.6 Solar Radiation Conversion
5.7 Biomedical Applications: Tissue Engineering
5.9 Interpenetrating Composites
5.10 Porous Media in Internal Combustion Engines
5.11 Other Developments and Special Applications
Concluding Remarks


logo_smallThis patent application US20110302961 describes a continuous foaming method without cutting or sawing. Also in this case, continuous foaming on an endless belt is used but the glass/foaming agent powder is put in segments on the belt. In this way, the resulting foam is segmented. The individual blocks are lift up to a vertical position and introduced in a lehr. This lehr is essentially a hollow glass lehr. In this way, the total setup can be also much shorter than in the case of a horizontal lehr.

But the patent also gives a nice summary of the current state of the art. In my opinion, the foamed segments will not be flat and this process will generate a lot of waste due to extra cutting.

The idea to anneal vertically remains an important point due to the huge saving which can be generated by reducing the length of the lehr. But on the other hand, the ribbon width will be limited in that way because forced convection has also its practical limits.

If a suitable land can be found (length, flatness, ….), BELGLASCZ advises to work with radiation / natural convection cooling of a wide ribbon like developed by CNUD EFCO. All methods to work with a shorter lehr end up with more waste. It was suggested to use this waste for foamed glass gravel, but the added value of that product will always be  much smaller than boards.


logo_smallFor every cellular glass plant, we have to choose between mold and continuous foaming on an endless belt. In case continuous foaming is chosen, we must decide where to saw the cellular glass ribbon. For float glass, cutting always happens on the cold end and this seems the obvious choice for cellular glass. Indeed, sawing the cellular glass after annealing seems to be the logic choice.

I was surprised to find a patent application US20130145796 where the ribbon is cut between the annealing (550°C)  and the end of the structural relaxation point (450°C). The cutting can be transverse and longitudinal on the cellular glass ribbon. A saw, cutting wheel and knife are suggested.

But why cutting hot? Float glass people are familiar with the famous longitudinal flaw in the ribbon. Regular cutting will stop this flaw in time. Longitudinal cutting reduces the width of the ribbon and in that way, the area stress on the ribbon. In fact, larger temperature deviations are allowed in the lehr without inducing breakage. On the other hand, skilled lehr builder like CNUD-EFCO are able to work within very small temperature deviations.

But there is also an important other reason to work in this way. It allows to anneal the ribbon vertically, reducing enormously the length of the lehr. In this case, an hollow glass lehr is the obvious choice.

Patent study: RU2004000415, a STES patent

logo_smallThis STES patent looks very interesting because they make a comparison with the best cellular glass today on the market at that time. Like we already mentioned in another post about NEOPORM,  they claim the best quality without remelting the glass, which should induce a huge cost reduction.

The abstract is written in English and French, but the content was only Russian. I used GOOGLE TRANSLATE which gives a poor understandable English. (Russian patent).

I understood that it is possible to start from waste glass with different compositions. Water glass is added in a rather high percentage and the mixture is stirred. After some time, a gasifier, containing carbon (I gamble on glycerin) is added and the stirred mixture is heated to 530°C until all water has disappeared. The mixture is further ground to a fine powder with a maximum grain size of 15 micron. This powder is put in molds and heated to about 800°C for 90 minutes.

While for the highest quality cellular glass, it is needed to melt the recycled glass with additional raw materials, this patent discloses a method to obtain this quality without remelting at high temperatures. It is well known that a melting furnace is a huge investment and has only a short life between 5 and 15 years (depending on the oxidizing components in the glass)  while the extra heating is about 1250-550°C=700°C. But we also know that the measured thermal conductivity depends strongly on the used methodology. I am not sure we are comparing the “same thermal conductivity”.

I would be surprised that Andrei’s patent is fake and therefore I am hopeful that cheaper high quality cellular glass can be put on the market while the investor will be pleased. On top of that, the problem of the unsorted piles of waste glass in Russia will be solved. In the next days, I will have a good translation of the patent.

The cold war has given us not only two very different cultures but also two different very competitive processes for cellular glass. And free competition is all what a fair market needs, although free is relative in this case, the patent is valid for another 10 years.

The dimensions of cellular glass

logo_smallRecently, I was in a small glass company. These people make small but old fashioned hand made glass plates as window and for other applications. Since 5 years, I regularly visit float glass plants for CNUD EFCO, where glass plates up to several meters width and height can be produced.

And suddenly a question came up? Why did this not happen with cellular glass? Cellular glass started (in the metric system) from about 45 cm by 30 cm, scaled up to 45 cm by 60 cm and recently 120 cm by 80 cm became available. I can imagine that the thickness is limited because annealing time goes linear with inverse of the thermal conductivity and with the square of the thickness. But I do not see a reason why several meters length and width are not available if I watch the cold end of a float glass line as installed for example by Grenzebach or Lisec.

But this week I visited a plant and the owner was proud to show me what he could produce. The actual dimensions are still secret but they are huge. And suddenly, I had a very nice application in mind for the world of tomorrow.

2014-04-17 09.20.48

Foamed glass gravel

logo_smallSince already many years, foamed glass gravel is on the market as a loose fill thermal insulation. The thermal conductivity is roughly the double of the best cellular glass for the same compressive strength. This thermal conductivity is quite high but when there is enough space for a large thickness, we have an interesting material. The material is foamed from recycled glass with a minimum of treatment. Indeed, in principle a mixture of different glasses (different compositions) can be used without too much cleaning. Two foaming agents are well known in this perspective: glycerin and silicon carbide.


The business seems already well organized because the Czech company Refaglas is delivering also the glass powder with the requested particle size. Indeed, it makes sense to transport the ground glass to a foaming plant as close as possible to the customer to reduce transport cost. The grinding equipment will also be more efficiently used, reducing CAPEX of the different plants.

For the grinding equipment, I learned that today Hosakawa-Alpine is the major grinding equipment supplier, delivering a ball mill with efficient classifier.

It must be hard for a sales man to sell boards for floor insulation if gravel is still possible to install. But on the other hand, the difference between a gravel production line and a board line is only an extension with a new recipe, an annealing furnace and some finishing equipment. It must be very seductive for investors to make that relative small extension to increase the value of the output. And at the end, the invisible hand of the market competition will bring the price of high quality cellular glass down to about 200€ /m³, the price GLAPOR is handling since July 2015.

About this post, I got a reaction from Arjen Steiner:

There is another supplier/maufacturer with two factories in Germany. The name is Veriso ( The sales office is in Berlin. The production sites are close to Hannover ( and close to Würzburg ( The total production capacity is 130.000 m^3 per annum. Welcome to competition!