Alkaline phenolic ester cured sand binders are widely used across the globe for the manufacture of ferrous and non-ferrous sand castings. Sand from knocked out moulds is predominantly reclaimed by single and double attrition mechanical attrition with a much smaller number being thermally reclaimed. This article, by Andrew Tagg, technical manager of John Winter & Co Ltd, will focus on mechanical reclamation and how the reclaimed sand detrimentally affects sand properties and the methods by which the process can easily be controlled.
THE PROCESS
It is a two-part binder process: either no-bake or vapour/gas hardened, consisting of:
- A phenol formaldehyde resin dissolved in an aqueous solution of potassium or sodium hydroxide, or a mixture of both.
- Hardening is achieved using an acidic agent to form a reaction, either:
- A liquid aliphatic ester.
- A vapour of aliphatic ester.
- The passage of CO2 gas.
- In the case of the no-bake process, both the ester and resin are mixed with the sand aggregate, usually in a high speed continuous mixer which is compacted around the pattern and allowed to harden.
- Alternatively, the binder is mixed with the sand, blown into a core box and hardened by the passage of aliphatic ester vapour or CO2. gas.
The resin hardens by the neutralisation of the alkaline solution which allows for the formation of potassium or sodium carbonate, or salts of carboxylic acids. In the case of aliphatic esters, the materials undergo base-catalysed hydrolysis (saponification); the carboxylic acid reacts with the alkaline to form the salt and the alcohol is liberated. It is the build-up of these potassium and sodium salts that creates the detrimental effect on the performance of the binder process when using reclaimed sand. It can be seen from fig.4 that at levels of seventy per cent reclaimed sand, from a single attrition mechanical reclamation plant, strength development is significantly impaired, while bench life (work time) is shortened, and strip time is extended. Therefore, it is imperative that we control the level of salt contamination in the new/reclaimed sand mix, either by improving the efficiency of the reclamation process or by dilution with new sand. The presence of soluble salts will affect the electrical conductivity of deionised water. This is measured in Siemens/cm2. The concentration of the salts are low such that the results will be in micro-Siemens/cm2 (μS/cm2).
The conductivity test is a low cost, quick and very simple shopfloor test to determine the level of salt contamination in the sand blend.
METHOD
1. Weigh 20g of sand into a beaker.
2. Add 100ml of deionised water and mix well.
3. Leave for twenty minutes.
4. Measure conductivity with a similar meter to that in fig.1. The result will be in Micro Siemens (μS /cm2).
The salt concentration is directly proportionate to the electrical conductivity, as seen in fig.2. The foundry can determine their own optimum parameters to ensure consistency with the moulding operation.
The proportionality enables a strong relationship between conductivity, bench life and strip time, and can be expressed in relation to such, see figs.3 and 5.
Mould output from no-bake foundries is governed by the strip time and this is the primary controlling factor. The extension of the strip time can be somewhat offset by utilising a faster hardener. However, this is limited by the fact that such a change will further shorten an already shortening bench life and could result in difficulties ramming the mould.
As aforementioned, the effect of the build-up of potassium or sodium salts adversely affects strength development, requiring an increase in binder to off-set the strength loss.
In most cases the actual binder required to achieve no loss of strength is rarely employed. In this case at seventy per cent reclaimed sand, double the binder level used for new sand mix is required. This can result in other issues such as gas defects, poor mould consolidation, increased salt contamination and cost. It is this loss of strength development that will result in an increase in strip time, see fig.6. Ideally with such a reclaimed sand, levels of no higher than fifty per cent should be employed to minimise adverse effects. It should be taken into consideration that new sand and waste disposal costs can be prohibitive, which is why foundries tend to operate at higher levels and manage the loss of performance.
To minimise salt contamination and improve levels of reclamation, binder levels must be maintained to minimal requirements.
Previously, we discussed that the presence of salts significantly impairs performance and makes the process more troublesome to control, especially when using simple single attrition reclaimed sand. Additionally, these salts are also hygroscopic which can result in moisture absorption from the atmosphere into the mould face. This may cause hydrogen pin hole defects with susceptible alloys, for example, steel, aluminium and bronze castings. Foundries which suffer such will face the moulds with new sand and use reclaimed for backing. In more humid climates where this is even more pronounced, moulds may require drying processes to be added as part of the process. In addition to these potential issues, moisture absorption can cause issues with clagging in sand hoppers. These effects have been somewhat reduced by utilising potassium hydroxide based binders and carbonic acid based hardeners.
The foundry can offset these effects by running nominal blends of reclaimed sand and by utilising the conductivity test to control the quality of the reclaimed sand. To achieve higher levels of sand reclamation, double mechanical attrition and or thermal reclamation should be employed.
CONCLUSION
In summary, using mechanical reclamation with the alkaline phenolic binder process requires a high level of control in the quality of the sand blend. Conductivity testing is a quick, low cost method of determining that conditions are being maintained. This test can identify if the new sand addition needs adjustment, or if attention should be given to the reclamation plant to maintain optimum properties. It should also be used in conjunction with sieve analysis to ensure correct grain distribution is maintained, and fines levels are kept to a minimum.
An increase in conductivity will quickly indicate if there is drift in the process which needs to be addressed.
Contact: John Winter & Co Ltd, Washer Lane, Halifax, West Yorkshire HX2 7DP UK. Tel: +44 (0) 1422 364213, email: [email protected] web: www.johnwinter.co.uk
For a copy of figures and supporting images refer to the full printed version in the April/May 2025 issue of Foundry Trade Journal.