Trendsetting: Reclamation of used sand secures the future of the site

Heinrich Wagner Sinto Maschinenfabrik GmbH (HWS) is one of the leading international suppliers of machinery and equipment for the foundry industry and part of the Sintokogio group of companies with its headquarter in Japan. The company employs more than 300 skilled workers at its site at Bad Laasphe. The focus is on the manufacture and sale of moulding machines and moulding plants as well as pouring units for sand and die casting. The portfolio is supplemented by machines and plants for the mechanical reclamation of used sands in green sand foundries.

The disposal and landfilling of foundry sand are causing steadily rising costs. Reducing the quantities of used sand and reusing it in the material cycle are more important today than ever before. The reuse of raw materials has always been a core competence of the foundry industry. In this context, the use of used sand as a recyclable material is the logical extension of internal recycling management. It offers opportunities for improving economic efficiency and conserving natural resources. The reclamation of used sand contribute to the long-term security of the site, ensures greater sustainability by increasing the recycling rate, reducing the volume of transport and lowering the CO₂ emissions. The question is not if we are able and willing to reclaim, but when we start. There is a consensus that the disposal of one of the world's most used and valuable resources, sand, is not a sustainable model.

Introduction sand reclamation

The reclamation of moulding material serves to break down and remove the binder shells and other non-sand constituents in order to obtain a reusable basic moulding material [1]. The aim is to use this basic moulding material (reclaim) as a substitute of new sand, mostly in core production. In a green sand foundry, it can be assumed that the used sand to be reclaimed is in most cases a mixture of bentonite-bonded moulding sand and organically bonded core sand (cold box). Various processes exist for their reclamation, which can be divided primarily into main groups (variants/combinations not taken into account).

More diverse than the number of processes are the properties of the specific used sand to be reclaimed, which usually varies from foundry to foundry. The appropriate solution should ideally be sought in close coordination with the machine and binder suppliers. An optimum design of the overall process can only be made after determining the initial situation and the planned reuse of the reclaim. Further consideration here will concentrate on mechanical reclamation. Unless explicitly stated otherwise, the term "reclamation" in the following always refers to mechanical reclamation.

Mechanical sand reclamation

The mechanical reclamation processes can be divided into various subgroups. The reclamation of used sand must always be regarded as an interlinked overall process. This consists of several stations for sand pre-treatment and sand post-treatment as well as the cleaning process on the sand grains themselves. Depending on the choice of the reclamation process and the available starting material (used sand), appropriate conditions for efficient cleaning, such as residual moisture or grain separation, must always be ensured. These conditions are set by sand pretreatment and ensure that the required reclaim properties are achieved. They are also largely determined by the reclamation unit selected for the cleaning process and the process parameters set. The correct ratio of the degree of reclamation (reduction of the signal components compared to the used sand) and the reclaim output (ratio of reclaim to used sand quantity) should be observed (cf. [2]). For the economic operation of a sand reclamation plant, the following applies in a simplified way: "Reclaim as clean as necessary and output as high as possible!". Depending on the reuse case, an associated reclaim aftertreatment ensures compliance with further target parameters, such as the target temperature, which are not directly set by the reclamation unit.

For grain cleaning, a ceramic-lined rotary drum is rotated by an electric motor via V-belt. The used foundry sand is continuously fed to the drum by means of a vibratory feeder (see Fig. 2). Pneumatic press cylinders apply pressure on the sand layer between the rolls and the drum via the press rolls. The resulting relative movement of the sand grains among themselves ensures that the binder shells are rubbed off, which are then present as dust in the reclaim. The use of ceramic components ensures a long service life of the components that come into contact with the rubbing sand. Uniformly flowing material displaces the sand already reclaimed and the binder residues from the rotary drum.

The generated sand-dust mixture falls onto the fluid bed in the lower part of the machine and is transported by it to the integrated classifier (cf. Fig. 2). The connected dedusting system separates the sand from the fine particles of the sand-dust mixture whirled up by the fluid bed. The sand grains are transported via the fluid bed as reclaim to the machine outlet.

In a green sand reclamation process, this process or pass is typically repeated two to four times, depending on the target properties, in order to achieve the required reclaim quality. With the right process chain, this continuous flow process can successfully reuse green sand-core sand mixtures in distributions of up to 100% in each case.

In addition, Fig. 4 shows that the grain-size distribution curve changed only slightly over the individual passes in relation to the reference value of the new sand.

The loss on ignition decreases in percentage terms with increasing number of passes and increasing degree of cleaning. As less and less foreign material is present on the surface of the sand grain, the amount of removed foreign material on the sand grains decreases and the efficiency increases. In addition to the degree of cleaning, the fines content in the reclaim is a significant value for determining the usability of the reclaim, which means that the dedusting or screening of the material produced plays a decisive role. More important criteria (sand properties) and their effects on the use in the cold-box process can be taken from Fig. 6, so that they are not further addressed here.

On the basis of the values achieved, further investigations involving the binder supplier showed further optimization possibilities for reducing the binder content while maintaining the required core strengths.

Example 2 – Green sand foundry with <10-20% cold-box core sand:

Use of the reclaim: Use in the core shop as a proportional substitute of new sand (cold box process)

Figure 8 shows as a sequence of pictures with sand grains (light microscope 200x magnification) of the different passes. Here, the increasing cleaning can be seen. After three reclamation passes already, the target values for the application case of the reclaim could be achieved.

Empirical values show a trend towards reusability at a determined loss on ignition in the range of less than or equal to 0.5 %. However, this value alone is only an initial indicator. It must be supported by corresponding laboratory tests on further criteria from Fig. 6. The final reusability must be verified in each individual case on the basis of the target specifications.

Example 3 – Green sand foundry with <10% cold-box core sand:

Use of the reclaim: Use in the core shop as proportional substitute of new sand (cold-box process)

Figure 10 shows as a sequence of pictures with sand grains (light microscope 200x magnification) of the different passes. As in example 2, the required degree of cleaning could be achieved after three passes.

The evaluation in Fig. 12 shows the ratio of loss on ignition and efficiency of the individual passes and the screening process (dedusted).