Replacing Olivine Sand
This article appeared in the December 2012 issue of MODERN CASTING
Olivine sand is no longer mined in the U.S. and is in limited supply for North American metalcasters. Sand casters who have been using the aggregate for decades now are forced to find an alternative. But the change is not doom and gloom, according to Vic LaFay, research and technical development manager for S&B Industrial Minerals, Cincinnati. Nonferrous olivine sand casters currently have three major alternatives to their traditional olivine sand, all of which have been proven in real metalcasting facilities in the last couple of years.
“The foundry industry has been resilient,” LaFay said. “We have a traditional mineral that is no longer available. Now we have substituted it with readily available, reasonably costing aggregates that give us a casting that meets or exceeds the customer’s requirement.”
Although some metalcasters have opted to import Norwegian olivine sand, the shipping costs of the foreign mineral make it an expensive option. At this time, three major cost-effective options are available when transitioning away from olivine sand: silica sand, Green Diamond aggregate or Biasill aggregate. According to LaFay, all options have proven to work as a replacement within traditional olivine shops and can be incorporated into the sand system slowly, without dumping all of the currently used olivine sand first. But choosing which option would work best in an individual metalcasting facility is not as clear cut. Tables 1 and 2 provide side-by-side comparisons of silica, Green Diamond and Biasill.
“The choice is customer specific,” said Jay Morrison, technical sales rep for Carpenter Brothers Inc., Norton Shores, Mich. “Every customer wants different things, so it comes down to what matches up the closest to what they want.”
Green Diamond as Replacement
Metalcasters use olivine sand for a combination of reasons, including its fine surface finish characteristics, thermal expansion properties and elimination of health risks associated with silica sand. When Pride Cast Metals, Cincinnati, found out it would need an alternative to olivine at its aluminum and brass casting facility, it looked for an alternative that would meet certain specifications it had set out to meet two decades ago, when it originally switched to olivine.
“We were running olivine for a purpose,” said Gregg Hamm, vice president of manufacturing for Pride Cast Metals. “First, for health reasons. With olivine you don’t have a concern with silicosis. But we wanted it for its surface finish qualities, too. We do a lot of parts that require a fairly fine finish.”
Pride Cast Metals selected Green Diamond as its replacement aggregate (Fig. 1). Green Diamond is a manufactured, silica-free specialty sand out of the U.S. west coast with similar thermal expansion properties to olivine sand. According to LaFay, metalcasters can put Green Diamond into their existing molding sand, slowly replacing olivine until the system is completely based on the new type of aggregate, which was the experience at Pride Cast Metals.
“We’ve been using Green Diamond for a year and a half,” Hamm said. “We have experienced no problems.”
But Hamm points out the sand might not be right for other shops, with different requirements from its sand and molds.
“It works here, because I like a fine sand for a good surface finish,” he said. “But Green Diamond does not have a wide range of sands. You are limited. It matched the type of sand I was using. I also liked that my supplier offered a pre-mull sand, because I’m running a special bond.”
Hamm was using 120 olivine sand from North Carolina and LA 100 from the west coast. According to Hamm, replacing that with Green Diamond, which he did while also converting to a proprietary organic free preblend, was seamless. During the transition, Pride Cast Metals ran various tests to keep track of how the changes affected mold quality. The company’s experiences will be presented in a technical paper at the AFS Metalcasting Congress and CastExpo ’13 in April. Hamm noted the only two green sand molding properties that changed when Green Diamond was added to the system were lower permeability and wet tensile strength, but neither change resulted in negative casting-related issues.
Silica as Replacement
In some cases, silica sand makes a suitable alternative to olivine, and several metalcasters have already made the switch, according to LaFay.
“The primary base aggregate substitute is silica sand, which is commonly used in molding operations for green sand,” he said. “Many foundries have successfully gone in that direction and are very close to completing a complete conversion from olivine.”
Because core processes use silica sand in production, switching to silica sand for molding gives metalcasting facilities the added benefit of using the same aggregate for both core and mold making.
According to a 2012 research paper authored by LaFay, Stephen Neltner, S&B Minerals, Mark Ziegler, Unimin Corp., and Jerry Thiel, Univ. of Northern Iowa, two silica sands mined and processed in Oregon, Ill., and Roff, Okla., exhibited similar casting characteristics to olivine sand in casting trials (Fig. 2). In the tests, castings produced with 100% new olivine sand exhibited a smooth, clean surface finish with a small proportion of shallow pinhole surface imperfections. Test castings with 75% new olivine sand and 25% new silica sand had a slightly dull appearance with a few small sand particles adhering to the surface but a reduced number of surface pinholes. Test castings with a 50/50 mixture of olivine and silica sand had a smooth but dull finish, with a small amount of adhering sand but no pinholes. Test castings made with 100% new silica sand exhibited a clean surface with some adhering sand (Figs. 3-6). According to the authors, maturity and continued use in the green sand system would bring improved properties and better casting results. A test casting from a mature operating sand system using 100% silica sand exhibited a surface superior to that of a mature olivine sand system (Fig. 7).
For some metalcasting facilities, such as Pride Cast Metals, the use of silica sand brings with it certain health concerns. Although silica sand is widely and safely used in metalcasting facilities throughout North America, transitioning from olivine to silica does add risk of exposure.
“The hazard in a nonferrous shop is less than it would be for a ferrous shop,” said Gary Mosher, senior managing scientist for Exponent, Menlo Park, Calif. “But there still can be a hazard there.”
Mosher said those facilities shifting from olivine to silica will have to institute a silica monitoring program along with some added administrative work to keep monitoring records. Some operation procedures and equipment—such as the type of respirators being used—may need to be changed. And switching to silica could change how workers’ compensation insurance carriers view the operation.
“There’s a lot of nonferrous shops using silica sand. Switching from olivine can be done,” Mosher said. “But you still have to use caution. It’s not a huge risk, but they will now have a risk where they didn’t before.”
Biasill as Replacement
Biasill, which is a slightly finer grade of staurolite sand, has a low coefficient of thermal expansion and high thermal conductivity with an AFS grain fineness number of between 66 and 75 (Fig. 8).
“Staurolite is a naturally occurring mineral from Florida,” LaFay said. “Its unique characteristic is its high density, which gives excellent brass and aluminum casting characteristics.”
In a recent test of Biasill, reported in the paper, “Alternative Molding Material Used in Nonferrous Metalcasting,” by LaFay, Morrison and Charles Rowe, Carpenter Brothers Inc., molding properties and resulting castings produced with olivine and Biasill were compared. Casting inspections during the trials showed little to no difference between castings produced with olivine molds vs. Biasill molds. The round grain shape and higher density of the replacement aggregate appeared to provide comparable surface finish and casting quality to olivine.
Molding sand properties between the two sands were fairly similar with the exception of permeability and density, which did not affect casting quality. However, the higher density of Biasill results in heavier molds.
“Biasill is a viable option, but it needs further testing,” Morrison said. “Most of the metalcasting facilities we have tried so far were squeezer shops, who were worried about the molds being heavier. But we have customers who have found success with it.”
As with Green Diamond and silica sand, Biasill has shown that it can be incorporated into a company’s sand system with the existing olivine sand.
No clear patterns have emerged for types of metalcasting facilities gravitating toward one new aggregate over another. LaFay stresses that each option has proven to work seamlessly with existing olivine sand systems and encourages metalcasters to investigate their options—all of which have recently been the subject of technical research—with their sand supplier.
“The selection of an aggregate is unique,” he said. “Many times it is based on the quantity of aluminum and brass poured in a combination facility, or whether it is aluminum-only or brass-only. And casting configuration is everything. Its level of intricacy will have a bearing. It also has to do with the green sand molding techniques of the foundry and the measurement of those characteristics.”
While some metalcasters may be sitting on a pile of olivine sand they hoarded when word got out of limited availability, they are likely only delaying an inevitable transition to an alternative aggregate.
“Olivine sand is a forgiving sand, so metalcasters will have to ramp up their sand testing for a little bit during the transition to understand how the changes go,” Morrison said. “It will be a little bit more upfront work, but once they get there, they are going to be okay.”