4.1 Hardening by gasification process
4.1.1 Water glass CO2 process
The historical importance of the process
The principles of water glass CO2 process have been known for a long time. The first patent law mentions are found in 1862 by Gossage and in 1892 by Hargreaves and Poulsen, however at that time an industrial application was not yet realized. The wide-scale technical application started in 1947 when Petrzela introduced another patent with details of a “carbonation solidification” process 44. The development work on the procedure was mainly carried out in the former Wittkowitz ironworks. At this time no other form of chemical curing processes were available. One spoke generally of “chemically hardened moulds and cores” and later of the “CO2 process”. The procedure was soon patented in a number of other European countries such as Italy, Switzerland, France and England. The impetus for the development of the procedure was realized through the lost wax casting method by Petrzela through the use of ethyl silicate as a binder system. What was truly revolutionary in the new procedure was that the aqueous silicates (water glass solutions) with the addition of carbon dioxide, cured within minutes!
The water glass-CO2 process is the forerunner of the moulding processes that use alkali silicate or water glass solutions as binder systems. Its introduction allowed a significant increase in productivity and improvements in properties of produced cores and moulds when compared to the previously applied mainstream mouldings of dry sand, fireclay or cement. Curing was carried out by exposing the mould material mixture with carbon dioxide and thus the first, and still used “cold box gasification method” was born. The basics of this chemical curing are comprehensively covered by Petrzela in 45.
Therefore, the basis of the chemical hardening in the formation of hydrated silicon dioxide is through the decomposition of aqueous solutions of alkaline silicates and also possibly hydrolysable solutions from the organic ester of the silicon dioxides. The reaction between the sodium silicate and the carbon dioxide can be shown by the following formula:
R2O * nSiO2 * mH2O + CO2 = R2CO3 + qH2O + n(SiO2 + pH2O)
where R is the alkaline component, that usually corresponds to Na or K.
The state of the SiO2 hydrogel and therefore also the physical and technological properties of the mould mixtures produced depends on the process of polymerization as the basis of solidification. The carbon dioxide goes into solution in alkaline silicate, and the SiO2 hydrogel is generated on the contact border of the water glass excess in a neutral medium. Under these conditions the polymerization achieves its greatest speed, because during the process a grainy precipitate forms which has no strength. By slower curing (or drying), massive silicate gels are formed which also have higher strengths. Optimal strengths would therefore be achieved in low molecular silica binder solutions that solidify by gradual polymerization. This however, is not possible in practice and so the foundries have to make compromises in the use of water glass binders. Petrzela had already noted that hydrated silica disintegrates well in the temperature range between 400 °C and 500 °C which is important for the application of water glass binder in aluminium casting even today. Conversely, the decay can be worsened by the use of water glass of higher alkalinity. The solidification of water glass bonded moulding material theoretically runs in three stages:
- In the first stage only a part of the water glass is broken down. The hydrated silicon dioxide which is excreted dissolves in the alkaline water glass residue and the sodium carbonate. In this way semisolid, rubbery binder shells form on the grains of the moulding material.
- After completion of the neutralization all of the water glass is decomposed.
- Through the neutralization of the water glass solution the formed sodium carbonate reacts further with the carbon dioxide to form sodium bicarbonate.
According to Petrzela, curing begins before the completion of the neutralization. The ultimate strength of the produced moulds and cores will be higher according to the amount of free bases included in the final product. This goal can be achieved by using shorter times of carbon dioxide gassing or by using water glass of low modulus (with high alkalinity). Since using water glass binder with low modulus can be associated with difficulties in processing, using shorter gassing times is more advantageous; this is recommended by Atterton 46 among others. Furthermore, it is found that highly alkaline (low modulus) water glasses show poor decay behaviour with high residual strength. In order to improve this behaviour, Petrzela recommends the addition of organic disintegration aids, a method not currently advisable. In order to keep the gassing with carbon dioxide as short as possible, the use of a carbon dioxide-air mixture is recommended. To quantify the use of the gas curing, Petrzela gives following recommendation: 100 kg mould material (with 4–5 % water glass) with 0.3 kg to 0.4 kg carbon dioxide. Petrzela conducted the first extensive studies with a water glass modulus 3.5 (36–38° Be). He gives an interesting basis for calculating the compressive strength:
compressive strength (kg/cm2) = % water glass -2,5.
The inventor of the process further notes that the bending strengths are lower here than in the production of oil sand cores and can be compared to the strengths achievable when using the dry-casting process. This is sufficient for a whole range of non-complex cores even today. Even so, moisture in the moulding material that is kept unnecessarily long during transport and storage of the cores is to be avoided during post cure. Petrzela determined that the problem area for residual strength or decay behaviour after pouring is related to the prevailing post-casting maximum temperatures in the moulding material. Accordingly, the highest strengths develop in areas which have been heated to temperatures between 800 °C and 1000 °C. Lower strengths are developed in the temperature range between 400 °C and 600 °C. This is explained by the formation of alkaline glasses of sodium carbonate and silica in conjunction with increased sintering phenomena from about 800 °C. On further heating there is an onset of increased sintering between the binder system and the mould base material silica sand. At a temperature of 1400 °C an improved decay behaviour is evident, the exact causes of which will not be explored here. However, it is worth noting that the improved decay probably occurs as a result of sintered material stresses and subsequent cracking. This also explains why today more steel foundries work with water glass CO2 cores than iron foundries. In steel casting the problem of decay behaviour is much less pronounced than in the production of iron castings.
The storage of water glass bonded uncured moulding mixtures requires some special precautions; they must be protected against drying out and also the ingress of carbon dioxide. According to [45] the (normal) carbon dioxide in the air is no problem for the workability of the moulding mixture. In this context, there is an interesting example of a steel foundry that processed 25t of water glass bound material on a saturday. The material was then left open to the atmosphere over the weekend. Only one layer of 10mm mould material had dried on the surface which was removed on monday. However, it is undoubtedly cheaper to cover the mixed resin to prevent such a drying out and this example is surely no longer conventional for weekend storage of materials. The workability of sodium silicate-bonded moulding substances over a period of a few hours (which is common) is possible when using a sealed container at the workplace or the core shooting machine.
The key to Petrzela advantages over the hitherto mainly applied technologies, (e.g. those based on sand moulds which helped in the breakthrough of new methods) should be summarized at this point again briefly:
- Reduction of mould and core production times
- Time, space, and cost savings as a result of no longer needed drying times for moulds and cores
- The possibility of the reclamation of sand waste
- Lower price for materials used
- Good finishes, and reduced cleaning requirements
These benefits can only be realized if certain conditions are met, which is also mentioned in this source. Because this knowledge is unfortunately not available in all foundries, these conditions should be mentioned here again:
- Precise, tightly closing ports with the necessary inputs and outflow provided for the hardening gas core boxes
- Exact observance of technological requirements for the processing of the moulded materials which of course presupposes such requirements exist
- The use of suitable mixing, transportation and storage technology
- reclamation technologies and aggregates suitable for the process (this is currently being subject to various developments)
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