A NEW CHROMITE SAND THAT OFFERS UNIQUE CHARACTERISTICS TO THE METAL CASTING INDUSTRY

Transcription

1 A NEW CHROMITE SAND THAT OFFERS UNIQUE CHARACTERISTICS TO THE METAL CASTING INDUSTRY DARYL F. HOYT FOUNDRY SAND TECHNOLOGY, MARSEILLES, ILLINOIS, USA The mining of a mineral sands resource, containing chromite sand, is under development in Southwestern Oregon, U.S.A by Oregon Resources Corporation ( ORC ). This chromite sand has a sub-rounded grain structure with highly polished surfaces and occurs in an unconsolidated ore that does not require crushing. The chromite s two sieve distribution, and 85 AFS fineness number, allows it to be used for many highly specialized applications in the metal casting industry. This paper will address the mine site, the physical, chemical and performance properties of the sand and how its unique properties necessitated a detailed evaluation of the product s performance and the development of a new marketing approach. Castings produced with the ORC chromite were compared with castings produced using other aggregates as well as blends of the ORC chromite with these other aggregates. Background Information: During the 1930 s and 1940 s, a U.S. geological survey discovered the existence of a heavy minerals deposit in Coos County, Oregon, USA. This deposit is located in the southwestern corner of Oregon and contains several marketable heavy minerals, in a relatively unconsolidated form, that will not require crushing to produce the desired particle size products. During World War II chromite was considered a strategic mineral and was mined by the Krome Corporation Mineral Sand Plant, with much of the chrome used in the war effort and a portion sold to the U.S. government for stockpiling as a strategic resource for future use. Shortly after World War II, the Krome Mineral Sand Plant stopped its mining operations and the deposit was inactive until 1989 to 1990, when Oregon Resources Corporation conducted an extensive exploration, acquisition and development plan for commercially mining the various minerals. The Cape Arago District contains 95% of the government drilled mineral sands resources in this deposit. Based on a 1989 Bureau of Mines estimate, these resources total 8.1 million metric tons of heavy minerals sands and encompass 240 square kilometers and three counties. Approximately 12%, of the 8.1 million metric tons, is chromite sand. These deposits are not contiguous, with the deposits occurring in ancient, elevated beach terraces, ranging in elevation from 35 to 1500 feet. Other desirable heavy minerals, present in the deposit, include Garnet, Kyanite, Zircon, Ilmenite and Magnetite. ORC undertook a major drilling and sampling program, in 1991, to determine the extent and character of the mineral sand deposits. The program entailed exploration and development drilling of 550 drill holes, for a total of 16,000 feet. 3,995 samples were collected with 2,603 assayed, primarily for their chromite and zircon content. In addition to this extensive drilling program, ORC completed a regional reconnaissance and an airborne geophysical survey, designed to identify hidden mineral sand deposits. In 1993 an 8.2 metric ton sample was shipped to Australia for processing and to separate the various minerals into high purity products. This processing included attritioning, slimes removal, fluid spiral separation, drying, high tension rolls, electrostatic plates, low intensity magnetic separation and high intensity induced roll magnetic separations. The individual products were returned to the ORC mining site and the chromite sand evaluated in local steel foundries. Two of the foundries evaluating the chromite sand submitted letters to ORC with the following commentary: Foundry #1 stated The flow slot core (center one of five) weighs ½ pound, with a slot width of 3/16 and a depth of 7/8. This core is surrounded by manganese steel, which pours at less than 2700F. The jaw weighs 2375 lbs. and the slots shook out, leaving a clean surface without any sign of fusion. Foundry #2 stated, We ran a comparative test using your chromite product versus an Australian zircon. The sands were both used in our core process using the same binder types and percentages. The cores were used to manufacture hydraulic control valves that have extremely critical internal passageways that are very 1

2 difficult to clean. Castings using both products were poured from the same heats to minimize temperature and other heat-to-heat variations. The results of the above trials were more than encouraging. The castings produced, using your chromite product, for the cored passage ways, exhibited a far superior surface finish with no metal penetration or veining. The majority of the remaining chromite was consumed in additional casting evaluations, with a small portion retained for future evaluations. Another portion of this initial sample was submitted to a large silica sand producer s laboratory, where their report indicated the material was a very high quality chromite which provided core tensile strengths as high as 417 psi, when mixed with a 1.0% phenolic urethane binder. An additional sample was submitted to the AFS Cast Metals Institute s laboratory where they poured Gertsman test castings with excellent results. In 2004, additional samples of the mined ore were submitted to Outokumpu Corporation, for mineral separations using similar separation procedures with the latest state of the art separation equipment. This second evaluation confirmed the conclusions of the previous analyses and indicated the potential for higher yields and purity of the desired mineral components. The second processing also provided additional chromite for the comparative analyses with the zircon, chromite, ceramic and silica sands currently in use by the metal casting industry. Developing a marketing strategy for the mineral products: Since the major constituent, of the ORC heavy minerals is chromite sand, the initial marketing approach was to determine the most viable application for this product. Currently there are two major distributors of chromite to the U.S. market. Both of these distributors obtain their chromite from the same region in South Africa; therefore both of the products are very similar in their physical and chemical composition. South African chromite is mined and crushed to provide aggregates for four major markets, chemical applications, refractory applications, metallurgical applications and for use as a foundry aggregate. The current foundry market for chromite in the U.S. is approximately 100,000 tons per year. The physical characteristics of the Oregon Resources chromite sand is quite different from the South African chromite currently used in the U.S. foundry market. The ORC chromite is much finer, AFS GFN, as compared with the current South African chromite s AFS GFN. Another unique characteristic is the well-rounded grain structure with highly polished surfaces, reducing the grain s surface area, allowing the foundries to reduce the amount of chemical binder required to provide similar core tensile strengths, while providing an improved casting finish. This paper discusses the results of these comparative tests between the Oregon chromite and the ceramic, chromite, zircon and silica sands currently used in the U.S. foundry market. In addition to these comparative tests, a series of tests were performed using the current foundry aggregates, blended with the Oregon Resource chromite. Based on the initial comparative analyses, it was determined that the most viable market for the ORC chromite would be as a foundry aggregate, which could be used in conjunction with, or as a total replacement for the specialty aggregates currently used by this industry. To ensure that the data presented to the industry was free from any perceived bias ORC contracted the University of Northern Iowa to perform the physical analyses and casting evaluations. The University of Northern Iowa has a complete sand laboratory, with the capability to produce test molds/cores, using the latest binder systems, and can melt and pour test castings to evaluate the performance of these aggregate under foundry conditions. U.N.I. is an FEF, Foundry Educational Foundation, participating university. The FEF program provides scholarship grants to students interested in entering the metal casting industry and therefore the university could provide a student workforce that had the capability and interest in evaluating the latest foundry products. The majority of the foundry testing, performed at the University of Northern Iowa, included core tensile, scratch hardness, density, ph, ADV, permeability, sieve analyses, AFS clay, step cone castings, Gertsman test castings, fluidity spirals, heat transfer data, casting finish evaluations, pyrometric cone equivalents, chemical analyses and photomicrographs. These tests were performed using both phenolic urethane 2

3 nobake and cold box binder systems. A portion of the sand s more sophisticated physical, chemical, and thermal property analyses were performed at additional highly credible laboratories. After developing this comparative data, the information was organized into several technical papers that will be presented to the metal casting industry at their American Foundry Casting Congress, AFS regional conferences, AFS chapter meetings, at the 42 nd Annual Industrial Minerals Forum, The Industrial Minerals International Congress and published in Modern Casting Magazine. Current estimates indicate that ORC will produce approximately 40,000 tons of chromite per year and be in full production in In addition to the production of the ORC chromite, ORC estimates they will produce approximately 12,000 ton per year of garnet. This garnet is also an 85 AFS GFN material and has been evaluated by OMAX, a large waterjet equipment manufacturer. Samples of the ORC garnet were compared with Barton Mines, New York garnet. Assuming a cutting value of 100 for the standard, Barton Mines N.Y. garnet, the ORC garnet provided cutting values of 95.8 in aluminum cutting and 90.9 in steel cutting. Other studies, related to the remaining heavy mineral products, are ongoing. Introduction: The foundry industry is constantly challenged to produce higher quality castings, with closer tolerances and at lower costs. In addition to producing high quality castings, at reasonable costs, the industry has been impacted by environmental pressures, related to the handling and disposal of used foundry sands, and health issues associated with respiratory exposure to the molding and core-making, aggregates, used in the casting operations. Today s foundry market is also impacted by the limited availability of specialty sands, used in casting applications, where silica sand has been found to be ineffective. The steel casting industry has been using specialty sands, chromite, olivine, zircon and ceramic media, to meet these more critical applications for many years. In the past two years these aggregates have become much more expensive and in some cases, the sand producers have limited the amount of sand a foundry can purchase. Some foundries in the northwestern U.S. have been advised that they would only be allowed to purchase 11% of the zircon sand they purchased in the last year, forcing them to seek a replacement sand or to purchase the required zircon sand on the spot market, at elevated prices. Currently the only source for chromite sand, used in the U.S. foundry market, comes from South Africa and, due to unfavorable monetary exchange rates, labor disruptions and high shipping fees, has become much more expensive. These chromite sands are produced in underground mines, with the chromite sand crushed, washed and sized prior to its shipment to the U.S. The Oregon mineral resource consists of unconsolidated sands that can be mined without crushing or fracturing the grains into the desired size ranges. The chromite sand occurs with other saleable minerals that will be separated into several additional high quality products. The deposit contains chromite, zircon, ilmenite (a source of titanium), magnetite (similar to the black oxide used in foundries, to control expansion defects), garnet (used in water jet cutting applications) and other minerals that, when separated can be marketed, absorbing a portion of the processing and separation cost and keeping the chromite cost competitive with other specialty sands. Aggregate Analyses: Attachment 1, Photomicrographs of the six aggregates investigated (100 Sieve, 70X Magnification), illustrates the grain structures of the sands used in the comparative testing performed at U.N.I. Each of these photographs shows the 100 sieve particles (150 um) photographed at 70X. The aggregates, used in this testing program, were provided by various foundry distributors and are typical of those currently being supplied to the foundry industry. A resin manufacturer provided the chemical binders, to ensure the products were fresh and representative of those currently used in foundries. The binder systems, used in the U.N.I. evaluation, included both phenolic urethane nobake and phenolic urethane coldbox binder systems. Core tensile tests were performed at 1.0%, 1.4% and 1.7% total binder levels, based on the sand s 3

4 weight. This paper will only address the 1.0% binder levels, other than the higher binder levels required by the ceramic and ceramic/chromite blended material. To avoid any commercialism, all of the samples were encoded. These codes are shown under the photographs in attachment 1. Since there are currently two chromite suppliers servicing the U.S. foundry markets, these sample sources were designated A and B chromites, the ceramic sand was designated C, the zircon sample designated Z and the silica sand S. Since Oregon Resources Corporation is the only chromite producer in the U.S., and the purpose of this paper was to compare its properties and performance with the other five suppliers, this sample was designated O. Attachment 2, COMPARISON OF THE AGGREGATE S PHYSICAL, CHEMICAL AND THERMAL PROPERTIES, This chart details the results of a series of analyses that were performed to illustrate the sand s physical characteristics, chemical composition and thermal properties (Pyrometric Cone Equivalent Values) PCE. With the exceptions of the Z Zircon Sand, S Silica Sand and the blend of 75% Z Zircon Sand with 25% O Chromite Sand, all of the PCE values were in excess of cone # 36, 3279F/1804C The Z Zircon Sand should have provided a fusion point of 3,700-4,000F, C. The typical fusion temperature for the chromite samples should be between F, C. Due to delays in completing the PCE evaluations, the lower zircon value was not anticipated and test castings were poured prior to receiving this data. The affect of this lower fusion point was very evident in the step cone castings, with both the Z Zircon Sand and the S Silica Sand having severe fusion and penetration defects, when exposed to 2900F molten steel. When the ratio, of the Zircon to Chromite blend, was decreased from 75% Z /25% O to 50% Z / 50% O and 25% Z /75% O ratios, the PCE values increased to the >36 PCE cone value. Although the 75% Z Zircon/25% O Chromite showed a lower PCE value, than the straight Zircon sand, the step cone casting, poured against this blend, showed excellent resistance to fusion or penetration and provided a superior casting finish. Each of the aggregates/aggregate blends were evaluated for their tensile strength, specimen weight, permeability, scratch hardness, the core s solids and, porosity content at 1, 2, 4 and 24 hours after stripping from the pattern. The same testing procedures were used to prepare the coldbox mixtures with these properties determined at 30 sec., 5 min., 1 hr., 4 hr. and 24 hr. after stripping from the pattern. The nobake and coldbox values are shown in Attachment 3, SUMMARY SHEET OF NOBAKE PROPERTIES, Attachment 4, BAR GRAPH OF THE NOBAKE TENSILE PROPERTIES, Attachment 5, SUMMARY SHEET OF THE COLDBOX BINDER PROPERTIES and Attachment 6, BAR GRAPH OF THE COLDBOX TENSILE STRENGTH. Each of the sand mixtures, other than the C Ceramic and the 75% C Ceramic/25% O Chromite blend, were evaluated at 1.0% total binder, based on the sand s weight. The nobake specimens for the C Ceramic did not provide sufficient strength to be handled at the 1.0% binder level. The nobake data indicated that the highest, individual aggregate s, tensile strengths were produced with the 100% O chromite and the 100% Z zircon sands. A blend of 75% Z zircon/25% O chromite sand provided tensile strengths similar to these individual aggregates. In the coldbox series, the blend of 75% Z zircon/25% O chromite provided tensile strengths higher than all of the individual products. To evaluate the aggregates and blends, bonded with the coldbox binder system, 16-step cone castings were poured. The metal used for these castings was an ASTM A216 WCB grade steel, poured at 2900F/1593.3C. None of these step cone cores were coated with refractory coatings. The step cone exposes the test core to various sand to metal ratios, with multiple 90 0 core section changes. The casting formed has a diameter of 5 /12.7 cm with core sections ranging from 1 /2.54 cm to 4 /10.1 4

5 cm, in 1/2 /1.27 cm stages. The changes in the sand to metal ratios, and the 90 0 section changes, allow the foundryman to evaluate various casting conditions with one casting. Attachment7, SHOWS, SHOWS THE CORES/MOLDS, THE ASSEMBLED MOLD COMPONENTS, THE LOCATION OF THE THERMOCOUPLE, AND VIEWS OF A POURED TEST CASTING. Attachment 8, SHOWS A SCHEMATIC OF THE TEST MOLD, THE THERMOCOUPLE PLACEMENT AND A SERIES OF THERMOGRAPHS, ILLUSTRATING THE CHILLING CAPABILITIES FOR THE SILICA, CHROMITE AND ZIRCON SANDS. Attachment 9, COMPARISON OF THE METAL PENETRATION, FOR ALL STEP CONE CASTINGS, depicts the degree of pentration observed on the test castings. Based on a sliding scale, quantifying the amount and location of the penetration, an overall ranking of the data was established. These calculations indicated that the aggregate or aggregate blend (average of both casting poured) with the least penetration was #1, the 75% Z /25% O blend, followed, respectively, by #2, O chromite, tied with #2, 75% A /25% O blend, followed by #4, B chromite, #5, C ceramic, #6, A chromite, #7, S silica sand and #8, the Z, zircon sand. Attachment 10. COMPARISON OF THE CASTING S SURFACE FINISH, provides a numerical reference to the surface finish of the test castings. An ASTM Casting Surface Comparator, pictured below the chart, was used as a standard. Based on the surface finish, the highest quality casting was produced with the 75% Z /25% O blend, followed respectively by the C Ceramic, O Chromite, A chromite, B chromite, 75% A /25% O chromite blend, Z zircon and the S silica sand. Attachment 11, SUMMARY COMPARISON OF THE CASTING FINISH AND PENETRATION EVALUATIONS, provides an overall casting quality evaluation by combining the penetration and surface finish rankings. Based on the combination of the penetration and surface finish rankings, the best overall casting was produced with the 75% Z /25% O blend, followed respectively by O chromite, C ceramic, 75% A /25% O blend, B chromite, A chromite, S silica and the Z zircon sand. Attachment 12, GRAPH OF THE AVERAGE HEAT TRANSFER, A portion of this study was to investigate the metal chilling capability of the finer grain size O chromite sand, as compared with the other aggregates. To determine how quickly the sand s temperature increased, indicating the cooling of the metal, a thermocouple was placed in the center of each step cone core, 7.25 /18.4 cm down from the top of the core. Sixteen step cone castings were poured, using the eight aggregates and aggregate blends. The castings were poured in batches of eight castings per heat with duplicate castings poured for each aggregate. A computer was interfaced with the thermocouples, recording the rate of temperature increase and the ultimate temperatures achieved. The thermographs, produced on each of the two duplicate castings, were almost identical, with the second curve lying on top of the first. Due to the delay in pouring, from one casting to the next, the curves had to be shifted so the graph of each aggregate would start when the core s temperature began to increase. Based on these curves, the O chromite provided the highest chill capacity followed respectively by the B chromite, the 75% A /25% O blend, the A chromite, the C ceramic, the 75% Z /25% O blend, the Z zircon and the S silica. Commentary: The O chromite sand has been shown to produce high quality cores with either the phenolic urethane nobake or coldbox, foundry binders. The properties of these cores meet or exceed the properties produced by each of the other individual specialty sands. Blending this finer chromite sand with other aggregates has shown that, the blending of two different aggregates can produce properties that exceed the properties of either individual aggregate. Since the blending of the Z, zircon sand with the O chromite sand provided an improved casting finish, we are now arranging to coat blends of the Z zircon sand with the O chromite for shell sand applications and will have this data available prior to the presentation of this paper. This project has produced a great deal of information that will be beneficial to the foundry personnel looking for an 5

6 alternative aggregate, to replace or to be blended with their current sand. This information should also help in evaluating this material without consuming an excessive amount of the foundry s time and resources. Acknowledgements: The author would like to acknowledge the technical and financial support provided by Cheryl Wilson, Philip Garratt and Jim Dingman, Oregon Resources Corporation, in the development of this report. We would also like to recognize the efforts of Jerry Thiel, associate director, The Metal Casting Center, University of Northern Iowa, Mike Firgard, graduate student at U.N.I. and Tyler Schneiter, undergraduate at U.N.I. 6

7 ATTACHMENT 1, COMPETITIVE SANDS, 100 SIEVE, 70X MAGNIFICATION CERAMIC SAND C SILICA SAND S AUSTRALIAN ZIRCON Z SOUTH AFRICAN CHROMITE A SOUTH AFRICAN CHROMITE B OREGON CHROMITE O 7

8 8

9 9

10 10

11 11 ATTAC NT 5

12 12

13 ATTACHMENT 7 STEP CONE CORES/MOLDS READY FOR ASSEMBLY ASSEMBLED MOLDS/CORES READY FOR POURING TEST CASTING BEFORE WIRE BRUSHING SECTIONED TEST CASTING, O CHROMITE 13

14 ATTACHMENT 8 THERMOCOUPLE TEST CORE CROSS SECTION OF THE METAL MOLD MOLD CROSS SECTION OF THE MOLD SILICA SAND THERMOGRAPH THERMAL SCALE CHROMITE SAND THERMOGRAPH ZIRCON SAND THERMOGRAPH 14

15 15

16 16

17 17

18 18