Moving away from silica sand.

Inside This Story:

* The health and safety implications of the continued exposure to silica sand and the fact that its casting properties are not always ideal create a need for alternatives.

* Detailed within is new process technology that can make silica-free minerals available to metalcasting facilities as cost-effective replacements for silica sand.

* The article discusses the role that sand reclamation systems play in enabling metalcasting facilities to make a cost-efficient transition to silica-free sand.

Silica sand has been a mainstay in the sand casting industry. While there are a variety of silica-free alternatives available that are considered by some to be technically superior, these account for less than 5% of metalcasting sand volumes. The biggest reason for the near total dominance of silica sand can be attributed to one factor--price.

The low cost of silica sand often overshadows the fact that it can cause a number of problems in the metalcasting process, including health concerns associated with prolonged exposure to silica. While many of the problems have been addressed and solutions have been offered, the health and safety issue remains. That leaves metalcasting facilities with three options: do nothing and hope the problem will go away; increase ventilation in tune with the tightening health and safety regulations; or move away from silica sand altogether.

Silica-Free Sand

Technology is now available that allows waste products (tailings) from aggregate manufactures (particularly those used in asphalt) to be converted to metalcasting-grade products. Aggregates used in making asphalt are dried and need to be hard-wearing, crush-resistant and wettable by an organic binder (bitumen in the case of asphalt aggregates). These are all desirable characteristics for metalcasting sand. Most aggregates are made by crushing and sieving quarried stone, and the number of plants performing these operations outnumbers those that can produce silica sand.

There is a catch though. The production of various sizes of aggregate used by the construction industry leads to a surplus of the finest fraction, much of which is returned to the quarry as landfill, giving it a negative value. A typical fines fraction from a plant producing aggregates for asphalt consists of particles less than 0.078 in. (2 mm), of which 10-15% can be less than 0.0029 in. (75 microns). Also, being a crushed product, the particle shape is angular and irregular, making it unsuitable for metalcasting.

In order to convert these materials to metalcasting sand, the fines must be removed and the particle shape improved. Both of these issues have been solved in new patented technology that treats aggregate sand with a combination of a special type of attrition unit and a new type of crossflow air classifier. The technology has been tested and used to reclaim aggregate sand at yields near 90% to a quality that enables it to be used as new sand. These results have been shown in a number of sand/binder systems, including alkaline phenolic-bonded silica, silicate-bonded olivine and furan-bonded silica.

The attrition unit, or cubifier, (Fig. 1) is run under conditions that reduce grain angularity as measured by changes in bulk density. It combines impact and grain rounding forces and controls them independently of one another in a cubifying action. The unit maximizes the frequency of glancing collisions between particles, which take place at high energy input levels so that binder residues are stripped away and surface irregularities are broken off without crushing the grains.

[FIGURE 1 OMITTED]

After processing in the attrition unit, aggregate sand is sent to the crossflow classifier (Fig. 2), which allows particle size distribution to be selected and controlled. The classifier causes an eddy-free air stream to differentiate particles according to their drag per unit of mass. This removes dust, binder flakes and sand fragments from useful sand and allows plants to select the upper and lower cutoff points for an optimal size distribution of the reclaimed aggregate sand. The unit also allows for the separation of mold sand and core sand.

[FIGURE 2 OMITTED]

Silica-Free Sand at Work

One plant currently using the technology is Scana Steel Stavanger, Jorpeland, Norway, a steel casting facility that specializes in wear parts, castings for oil rigs and propeller blades. The firm was investigating ways to reduce its specific sand consumption by increasing the proportion of reclaimed sand it used. To do this, Scana established an on-site facility for aggregate sand consisting of an attrition unit and classifier.

Before installing the new equipment, Scana separated sand from the castings via mechanical shakeout, after which the lumps of sand were crushed and sieved. The resulting product was allowed to cool, and fines were removed in a fluid bed cooler/classifier.

Now, with the new onsite facility (Fig. 3), the sand recovered from mechanical shakeout is taken by way of an elevator to one of two silos where it is mixed with new sand from a silo below to compensate for a loss later in the process. A screw conveyor then takes the material to the attrition unit and flows it directly into the classifier below. After leaving the classifier, the aggregate sand is taken to a pneumatic conveyor, which sends the material to holding silos. From the holding silos, the aggregate sand is pneumatically transported to mixer silos.

[FIGURE 3 OMITTED]

Scana's largest area of cost savings has been in new sand consumption. Since implementing the aggregate sand technology, the firm has used an 85:15 used:new sand ratio, reducing its net aggregate sand consumption by 80%. Hoping to see even better results, Scana initiated a program that uses a 92:8 ratio.

The firm also has seen promising results in reductions in binder consumption and scrap molds. Scana initially set a goal of reducing binder consumption by 10%. Actual results show that Scana doubled that number, reducing binder consumption by 20%.

Another big gain comes from the low rate of scrap molds it produces with the new system. The firm has not produced a single scrap mold since switching to the new technology. This produced an estimated savings of $30,000/yr.

All of these factors helped Scana realize a payback on its investment in just nine months.

New Metalcasting Sands

The use of this technology has been assessed with two minerals, anorthosite (a calcium alumino-silicate) and jotunite (an iron-rich basalt). A pilot plant now is using jotunite to produce metalcasting sand, blasting sand and a wide range of materials for investment casting. The aggregate sand has low expansion, a high chill factor, is easily wetted by organic and inorganic binders (though not by metals) and contains no free quartz. It is suitable for casting aluminum and bronze, where its low expansion and high chill factor improves surface finish and breakdown while reducing cycle times and dendrite arm spacing. It also is useful for producing intricate cores that are prone to finning.

Anorthosite has excellent properties as a low-expansion, silica-free sand for iron casting and may be commercialized in the near future. Deposits of anorthosite-type minerals are less common than those of the jotunite family, but they are plentiful enough to be a valid alternative to silica sand.

Cost Factors

Facilities that replace silica sand with a silica-free product derived from a crushed aggregate will need to ensure the sand packs well without becoming dusty (i.e. has or acquires a round grain shape). This entails an investment in an attrition-based mechanical reclamation plant for spent sand. Losses will be slightly higher than for silica sand the first few times that the aggregate sand is used after reclamation but will then settle down to a level of 90% or more. The effect of the attrition unit is shown in Fig. 4.

[FIGURE 4 OMITTED]

The reclamation rates attainable with the best attrition-based processes are considerably higher than those achieved with vibrating sieve systems and may exceed thermal reclamation. Plants that have a suitable attrition/classification-based sand reclamation process will need to locate a nearby aggregate supplier that has or can produce a dried, pre-classified fine fraction within the proper specification range.

Metalcasting firms then can have either a quarry supply classified, cubified aggregate sand in the necessary size range and take back the fines or buy the pre-classified aggregate and return unwanted fines and oversized particles, reducing the amount of waste that needs to be disposed.

The average cost of silica sand is about $25-$40/ton. The cost for a metalcasting plant to buy classified, cubified aggregate sand is near that range. If a metalcasting facility installs its own attrition unit and air classifier, it can incorporate the aggregate sand as part of its sand recycle operation and pay somewhat less for an aggregate that is only pre-classified.

The savings to metalcasting facilities can be considerable and often will allow firms to see a return on investment for the attrition unit and classifier in less than one year.

Information on Scana Steel's experience with silica-free sand was provided by Foseco Europe.

For More Information

"Silica-Free Foundry Sand at Silica Sand Prices," K. Harris, proceedings from Shape Casting: The John Campbell Symposium held during the 2005 Minerals, Metals and Materials Society Annual Meeting, San Francisco, Calif., Feb. 13-17, 2005.

Kenneth P. Harris is a director of Noram Technology Ltd., London, and managing director of Noramtek AS, Grimstad, Norway, a partly owned licensee of Noram Technology Ltd.