Trendsetting: Reclamation of used sand secures the future of the site
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 . 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. ). 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.
The HWS sand reclamation unit type USR-II (Ultra Sand Reclaimer) with a processing capacity of up to 5t of used sand per hour, forms the heart of a HWS reclamation plant for green sand foundries. It provides efficient cleaning based on a friction process with high flexibility with regard to used sand composition and quantity. The process can be individually adjusted with a view to high reclaim yield (ratio of input quantity to output quantity) for both easy and demanding used sands to be reclaimed. An important feature is the gentle friction from grain to grain, without additional friction tools (see Fig. 1). This largely avoids grain breakage and keeps wear on the sand grain low.
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 its in-house test plant, HWS offers interested foundries the opportunity to convince themselves of the efficiency of used sand reclamation. With the USR-II machine in a near-series design, reclamation tests can be carried out under conditions that are similar to production. The focus here is on determining the suitability for reclamation and the process parameters for the conceptual design of an overall plant.
Joint reclamation tests
The following explanations show representative excerpts from reclamation trials already carried out with European foundries and different objectives for the use of the reclaim. In addition to the use in the core shop, where large quantities of new sand are processed, the reclaim can also serve as a substitute of new sand in the green sand system of sand preparation. Three examples with different compositions of the used sand with regard to core sand and green sand content are shown below, together with a brief classification of the test results.
Example 1 – Aluminium green sand foundry with >95% cold-box core sand:
Use of the reclaim: Use in the core shop as a proportional substitute of new sand. Target reclaim rate 50% to 75% (optional, temporarily up to 100%) in cold-box process.
Fig. 3 shows a sequence of images of the sand grains (optical microscope 200x magnification) of the various passes (revolutions). Significant cleaning of the surface was achieved, with only very slight edge rounding, without causing significant grain breakage.
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.
Fig. 7 shows the evaluation of the flexural strengths of test cores with the reclaimed material at different proportions of new sand. The values of the flexural strengths could be maintained or exceeded. The test series was carried out in a cold-box system with the same resin and activator content in all five series. The higher strengths with increasing reclaim content can be explained by the proportional filling of deeper structures on the sand grain surface, as well as the slight edge rounding. With the same binder content, more binder is available for the bonds of the sand grains than with a complete recladding of new sand.
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.
Eliminating the fourth pass, with only a slightly improved degree of cleaning, results in higher efficiency (cf. Fig. 9) as well as in a simplified plant concept, reduced dust quantity for disposal and operating costs of the overall plant. It is important to reclaim only as much as necessary, not as much as possible. In this way, economical operation of the reclamation plant is 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.
In the present application, the reclaimed material was additionally screened to further reduce the fines content (material <0.090mm). The requirement for the reclaim quality was to achieve the same strength values of the sand cores with the same binder content as far as possible. Fig. 11 shows the corresponding grading curves of the passes. The additional screening (result called dedusted) caused a significant decrease in the fines content after the third pass.
Table 1 shows the strengths of test cores from the series application with fresh sand and a small amount of grain-separated, non-poured core breakage as well as the test cores from a mixture of reclaim and fresh sand in the ratio shown. The test series was carried out in the cold-box process and the necessary strength was finally achieved by a slight increase of the binder content by 0.05 %.
The mentioned examples with their extracts from test series clearly show that the moulding materials of the respective foundries are to be considered individually. In particular, the suitability for reclamation of the material has to be examined depending on the defined target specifications. The evaluation and interpretation of all data between process suppliers and foundry forms the basis of a customized overall concept.
Reclamation of used sand instead of landfilling
Is the reclamation of used sand a real alternative to landfilling? Many foundries have to ask themselves this question. In case of suitability for reclamation, individual concept planning in cooperation with the process supplier and an economic analysis for the overall investment, reclamation can be a real alternative. For this, a sustainable cost reduction must be guaranteed. Other effects, such as the conservation of natural resources and the improved CO2 balance of the plant, also contribute to increased sustainability and should be taken into account in the decision-making process for investment approval. Reclamation must already today be included in the strategy for securing the long-term future of our foundries in Europe. With its own reclamation process and technical center, HWS offers a full service to successfully address this issue.
 Oberschelp, P.: Reclamation of moulding material. In: Fundamentals and practice of sand preparation and control of clay-bound moulding materials. VDG Qualification course, Düsseldorf, 28./29.02.2008
 Polzin, H.: Condition of the sand grain surface before and after reclamation. 1. BDG Symposion Used Sand – 03./04.04.2017, Hannover, April 2017
 Verein Deutscher Giessereifachleute e.V. - DATA SHEET R 093, RECLAIMED SANDS AS MOULDING BASE MATERIAL Requirements, assessment criteria, February 1992