Mining a domestic source of chromite: a mineral deposit in Oregon is being developed to be a significant source of chromite sand for U.S. metalcasting facilities, offering a cost-effective alternative to the current South African chromite choices.

For many years, the metalcasting industry has been using chromite as a specialty sand to produce high quality castings poured with various high temperature steel alloys. Currently, the only source of chromite used in the U.S. metalcasting industry is supplied from underground mining operations in South Africa. However, Oregon Resouces Corp. (ORC), Portland, Ore., has commenced the development of a mining operation in southwestern Oregon that will supply a domestic source of chromite sand for U.S. metalcasters.

The Oregon deposit is extensive and consists of an unconsolidated mixture of chromite, zircon, garnet, ilmenite, magnetite and other commercially valuable minerals. Through the use of the latest generation of mineral separation equipment, including spiral separators, high tension rolls, electrostatic separators and rare earth magnets, these minerals can be separated into high purity products for various markets.

The improved heat transfer characteristics of the ORC chromite can modify and improve the surface characteristics of specialty steel or iron alloys, and the initial market focus for the mining operation is on the chromite portion of the deposit for use in the metalcasting industry.

Initial development will include a 10 ton/hr, pilot plant to be in operation by spring 2006. This plant is anticipated to produce more than 2.5 ton/hr, of HMC (heavy minerals concentrate) and 1.25 ton/hr, of chromite sand. The full mineral sand operation is scheduled for completion in early 2007 and will be capable of processing 400,000 ton/yr, of mineral sand ore. This includes an estimated 40,000 ton/ yr. of chromite sand and an estimated 12,000 ton/yr, of garnet.

The balance of the minerals in the ORC project will be stockpiled for future processing. This mineral sand operation will be a great opportunity for U.S. metalcasters. The chromite sand has had extensive testing to prove its usefulness in casting applications.

Highlighting the Differences

The chromite sand from the Oregon deposit is significantly different than that supplied by South African producers. The grain structure of the South African chromite has a rhombohedral grain shape, which is formed when the mineral is crushed during the mining operation. This crushing is necessary to produce the particle sizes for the various markets they supply, such as the steel industry (ferrochrome), the chemical chrome market, refractories and the metalcasting chromite market. The coarsest material is provided to the metalcasting market, typically with a four-sieve distribution and a 50-55 AFS GFN.

The Oregon chromite does not require crushing and occurs as a two-sieve distribution with an 80 to 85 AFS GFN product. The grain structure of the Oregon chromite is sub-rounded with a smooth and highly polished surface, allowing the metalcaster to use less binder to achieve the desired core and mold handling properties.

The sub-rounded grain structure allows the grains to pack more tightly, reducing the opportunity for the molten metal to penetrate into the formed cores and molds, even at high ferrostatic pressures. This packing characteristic may also reduce or eliminate the need for refractory coatings.

The increased grain-to-grain contact of the Oregon sand has shown a higher potential for the extraction of the metal's temperature, affording a higher heat transfer rate and chilling capacity than other chromites or specialty sands.

[FIGURE 1 OMITTED]

Beginning the Study

To provide a credible comparison among the aggregates currently used in the metalcasting industry, ORC funded an extensive study comparing the ORC chromite sand with an Australian zircon sand, a ceramic sand, a silica sand and the two current South African chromite sands for core and casting properties (Fig. 1).

These comparisons included complete chemical analyses of the base aggregates, their particle size distribution, AFS GFN, pH, ADV (acid demand value) base permeability, surface area, coefficient of angularity, bulk density and pyrometric cone equivalent values, as well as photomicrographs of the individual size fractions for all of the aggregates.

Core tensile strengths, scratch hardness determinations, core permeability, core density and other core sand analyses were compared. Test castings were poured using ASTM A216 WCB steel at 2900F (1593C), and the steel's nominal composition was 0.28% C, 0.55% Si and 0.80 Mn. Gertsman and step cone test castings were poured to evaluate the sand's resistance to metal penetration, potential for expansion related defects and the casting's surface finish.

In addition to the comparison among the six aggregates, blends of the ORC chromite with the two South African chromites, the Australian Zircon and the ceramic sand were investigated.

ORC Chromite Stands Out

In every case where the ORC chromite was blended at a 75% test aggregate to 25% ORC chromite ratio, the core and casting properties were improved. Although the ratios of addition in this study only included 25% of the ORC product, the test results indicate that the product can function at any level of addition or as a stand alone refractory. Due to the narrow particle size distribution of the ORC chromite, mathematical blends at 10% increments indicate that blending the ORC at any ratio, with any of the five other specialty sands investigated, will not produce a bimodal or double peak sand distribution.

Comparisons of the ultimate tensile strengths for both the phenolic urethane nobake and phenolic urethane coldbox binder systems are summarized in Figures 2 and 3.

The test castings were evaluated on the basis of metal penetration and casting finish. The rating system to determine the level of penetration used the following scale:

* "0"--No Penetration;

* "1"--Slight Penetration;

* "2"--Moderate Penetration;

* "3"--Severe Penetration;

* "5"--Fused Metal.

The core sections in the step cone casting ranged from a 4-in. (10.16-cm) diameter to a 1-in. (2.54-cm) diameter in 0.5-in. (1.27-cm) increments, with the lowest sand to metal ratio (1-in. [2.54-cm] core section) at the bottom of the test casting. The cores were placed in a mold cavity with a 5-in. (12.7-cm) cross section, providing each core section a different sand to metal ratio. The 1-in. (2.54-cm) diameter bottom ring of the core was surrounded by a 2-in. (5.08cm) metal section. Each level of the step cone casting was evaluated for penetration and multiplied by a factor based on the thermal input into the core section, with the 1-in. (2.54-cm) core section receiving the highest factor.

The product of these calculations resulted in the determination of an overall "P" (penetration value) index and ranking. Each section of the same step cone casting also was evaluated for surface finish by comparing the casting's surface finish with a standard "Casting Surface Comparator" and assigning a "V" index (casting finish rating value).

The products of the "P" and "V" ratings were summarized to provide the overall ranking for the casting quality produced with the various aggregates and blends (Table 1). All of the step cone cores were made with a 1% coldbox binder addition, with the exception of the "C" ceramic aggregate, which was bonded with 1.7% total binder based on the sand's weight.

Samples of the ORC chromite, a South African zircon and various blends of these two aggregates were coated at a major shell coating operation and evaluated for their melt points and hot and cold tensile strengths. All of the test mixtures contained 2.5% resin and included:

* Test Mix #1: 100% of South African Zircon, this zircon sample was provided by the resin coater from one of their bulk shipments;

* Test Mix #2: 100% of the same South African Zircon product, provided by a foundry distributor;

* Test MIX #3: 75% South African zircon with 25% ORC chromite;

* Test Mix #4: 50% South African zircon with 50% ORC chromite;

* Test Mix #5: 25% South African zircon with 75% ORC chromite;

* Test Mix #6: 100% ORC chromite. Table 2 reveals the data generated in the coating operation's quality control laboratory.

Future Work

Casting evaluations similar to those discussed in this article currently are being completed at the University of Northern Iowa. Data related to the casting's quality for each of the individual aggregates and the blends will be reported when the testing is completed and summarized.

Since the Oregon deposit contains many high quality commercial minerals, the marketing of these minerals will aid in controlling the mining and production costs associated with this special and unique chromite product, making it competitive with current specialty sands.

The ORC chromite product is capable of providing an excellent casting surface finish, with or without the application of mold and core coatings and provides a high refractory value with excellent heat transfer and chilling capabilities.

Once the mining operation is in full swing, this chromite product could become the aggregate of choice for facilities casting hydraulic valve assemblies or producing castings that require intricate core assemblies and are subjected to extreme casting conditions.

Daryl Hoyt is the president of Foundry Sand Technologies, Marseilles, Ill.

For More Information

"Quality Issues in the Selection of Chromite Sand for Steel," W.D. Scott, E.M. Thomas, L.L. Strohmayer, 2003 AFS Transactions (03-099).

Table 1. Ranking of Casting Quality of Aggregates Investigated

                              "P" Rating/   "V" Rating/   Overall
   Sand Type and Source         Rank #         Rank        Rank

    O.R.C. Chromite "0"        5.5 / #2        840 / #3      #2
South African Chromite "A"      11 / #6       1065 / #6      #6
South African Chromites "B"      8 / #4       1080 / #4      #5
   Australian Zircon "Z"        20 / #8     1662.5 / #7      #7
  Ceramic Sand "C" (1.7%)      9.5 / $5      657.5 / #2      #3
      Silica Sand "S"         17.5 / #7     1722.5 / #8      #7
      75% "A"/25% "0"          5.5 / #2       1345 / #5      #3
      75% "Z"/25% "0"          4.5 / #1        645 / #1      #1

Table 2. Properties of Test Mix Samples

              Melt Point   Hot Tensile   Cold Tensile
                 (F)          (psi)         (psi)

Test Mix #1      217           610           575
Test Mix #2      214           610           660
Test Mix #3      212           690           660
Test Mix #4      218           740           680
Test Mix #5      216           760           795
Test Mix #6      218           740           740