By Chris Wilding, Omega Foundry Machinery Ltd.
Today’s foundry using one of the many chemically bonded sand systems is under increasing pressure to reduce costs, reduce it’s impact on the environment but at the same time improve and maintain it’s casting quality. One of the ways of meeting these requirements is to invest in sand reclamation.
Cost reductions after installing sand reclamation are made by re-using the sand after casting, buying less new sand and with some systems actually reducing the binder content at the mixer.
Due to less sand being dumped there is an obvious reduction to the impact on the environment.
Casting quality is improved by using less resin at the mixer and improving the sand grain’s characteristics so that it becomes more rounded. A more rounded sand grain leads to better compaction after mixing, greater strength and better surface finish.
In order to reclaim the sand, the equipment used must first of all reduce the lumps back to grain size, remove all coarse grains, agglomerated sand grains as well as dust and fine particles. The sand must be cooled before re-use but more importantly, as much binder should be removed as possible.
Characteristics of the sand
After reclamation, the shape of the grain changes due to the attrition part of the process where sand grains rub against other sand grains. The effect is that all sharp edges are removed and extracted from the sand as dust. It is also important that the AFS or average grain size does not change dramatically from that of the new sand first used. Therefore it is important that the type of reclamation is not too harsh or damaging to the sand grain. The silica content of the sand should be as high as possible and the Acid Demand Value (ADV) should be as low as possible, particularly if the furan resin system is to be used.
There are four main grain shapes: rounded, sub-angular, angular and compounded. The best type of grain shape is rounded and so long as the correct type of equipment is used this should be the shape of the sand grain after processing. It is also useful to note that the cavities of the natural sand grain become filled with resin, again leading to the more rounded shape, therefore requiring less resin absorption at the mixer.
There are three main options for sand reclamation: primary attrition, secondary attrition and thermal.
Primary attrition can be further broken down into three main types: primary attrition – low-level, primary attrition – high-level and combined shake-out/attrition.
High-level refers to the loading height of the attrition unit. This type would typically be positioned in a pit and fed via a separate shake out and vibratory feeder. It would not have it’s own shakeout deck and would be used where heavy castings or high throughputs would be processed.
Low-level units are floor mounted requiring no special foundations and can be used as a shake out as well as attrition unit. This type of plant is usually more compact, easier to maintain and has a low investment cost. The main limitation is the maximum load capacity of three tons and a maximum throughput of 15 tons per hour.
Combined shakeout attrition units are usually mounted in a pit but can take much higher load capacities.
All three types of primary attrition unit will consist of a heavy duty shake out grid, a secondary perforated plate screen made from mild steel with 6mm diameter apertures, a third screen usually of stainless steel with 1.6mm apertures with a wedge cross section to enable a certain amount of self cleaning and finally a 1.6mm square aperture mesh screen for final sand classification.
The actual attrition process takes place between the shake out grid and the area prior to the final mesh screen. Here there should be sufficient retention to enable grain scrubbing and binder removal. With any type of attrition unit, there should always be the facility to remove flash metal, chills, reinforcing bars and other non-sand contamination. Therefore a clean-out door should be provided to enable quick and easy access to the screen areas.
Secondary attrition can be employed after the primary attrition unit to enable further binder removal. These units are especially suitable for the alkaline phenolic process and give furan levels of reclaim sand re-use at the mixer. Secondary attrition is also not as costly to purchase or operate as thermal so can be considered a cost effective alternative to thermal.
The basic principle of secondary attrition is to use a spinning drum at high speed to propel sand against sand at the right force so as to remove binder but not harm the sand grain. RPM can be adjusted to suit different types of sand grain and binder removal levels.
There are two types of secondary attrition, hard and soft. The soft system uses the centrifugal method only and is more suitable for the Furan process where not so much binder removal is required. The hard system uses the centrifugal method as well as a pair of squeeze rollers that force the sand grains together to give greater attrition and binder removal. This type of unit is more suitable for Alkaline Phenolic’s, Silicate and Green Sand back to core shop.
It is worth noting that in the case of the hard system, the spinning drum and the squeeze rollers have ceramic outer linings to give a much longer lifetime. Also the pressure of the squeeze rollers can be varied to suit different sand grain structures and different levels of binder removal. The sand can be passed through the unit up to three times (three separate cells) to further reduce the binder on the sand.
The ultimate in sand reclamation has to be thermal, whereby 100% of all binder and other organic material is removed. The sand is generally better quality than when it was first bought.
Typically a thermal unit will run on gas or electric and operate at temperatures of between 600C and 750C depending on the type of binder used. The sizes range from 250kg/hour up to 12 tons per hour.
For the alkaline phenolic system, a special inhibitor must be pre-mixed with the sand to prevent the alkaline salts causing low temperatures and the fusing of sand grains.
Thermal reclamation with the addition of pre- and post-mechanical scrubbing can also be used for reclaiming green sand back to the core shop.
Sand cooling and classification
The final section is regarding the cooling of the sand and the removal of dust and fines. Often overlooked, the sand cooler/classifier is equally as important as the attrition unit because unless we remove all of the dust and fine particles from the sand we will not see a reduction in the loss on ignition (LOI) at the mixer.
Typically a cooler/classifier would be a fluidised bed type with a copper tube heat exchanger for sand cooling. The fluidising air and the extracted air must be finely balanced so as to provide a negative pressure inside the fluidising chamber. This pressure can be adjusted according to the amount of fines in the sand.
Fluidised sand is not abrasive therefore copper tubes can be employed as the heat transfer medium. Using copper, the cooling system can usually cool the sand from 300C to within +6C of the water supply temperature.
Whilst no two foundries are the same and all have different sand systems and requirements, there are many variations of sand reclamation plants available. All that has to be determined is the amount of binder removal required and the level of investment.
Modern sand reclamation plants have evolved in such a way that even the smallest foundry can be accommodated in terms of floor space required by the plant but also in terms of investment and running costs. Essentially, there is a sand reclamation plant to suit every foundry requirement and budget.
For further information contact Roy Dias of Endeco Omega on TEL: 011 907 1785 or email firstname.lastname@example.org, or Peter Petersen of Mondeco on 079 448 1277 or email email@example.com or visit www.endeco-omega.co.za