BENEFICIATION OF BAUXITE TO REFRACTORY GRADE QUALITY BY HIGH INTENSITY MAGNETIC SEPARATION

J. Iannicelli

President and Technical Director

Aquafine Corporation, 3963 Darien Highway Brunswick, Georgia

ABSTRACT Samples of low silica Arkansas bauxite have been beneficiated to meet

refractory grade standards by use of high intensity, high gradient magnetic separation. Besides reducing the iron content to less than 3%, the TiO2 content of the bauxite was also reduced to less than 2%. The beneficiation can be accomplished by means of a simplified wet process flow sheet with an estimated cost comparable to that of water-washed, filler grade kaolin. The process has been carried out on a small pilot plant and will be scaled up to produce 25 tonnes of calcined refractory grade bauxite meeting National Stockpile Specifications.

INTRODUCTION

At present essentially 100% of refractory grade bauxite used in the U. S. is imported from China and Guyana. These two sources supply all of the raw bauxite or calcined bauxite which meet National Stockpile Specifications. This constricted supply from two overseas sources creates dependence of the United States which could be undesirable in a time of a national emergency.

This is a preliminary report of work under FEMA Contract EMW -83-C-

12S8 which requires production of 25 tonnes of calcined refractory grade bauxite meeting National Stockpile Specifications. The contract stipulates that United States crude bauxite be used and that the iron content be reduced using high intensity, high gradient magnetic separation.

There may be significant reserves of domestic bauxite which could

meet National Stockpile Specification when calcined, except that the iron content is much too high.

In order to meet current Type 1 National Stockpile Specifications

for calcined bauxite, it is necessary to calcine a hydrous bauxite having the analysis shown in Table I.

It is known that there are reserves of bauxite in Arkansas which would

meet the chemical requirements of the hydrous bauxite shown in (I), except that the iron content considerably exceeds 1.6%. It has been demonstrated on a laboratory basis that a variety of Arkansas bauxites, ranging up to 15% Fe2O3 and up to 2% TiO2, can be beneficiated by high intensity wet magnetic separation. Murray and Iannicelli (1982) showed that Arkansas bauxite containing 11.3% Fe2O3 (7.9% Fe) and 1.6% TiO2 could be beneficiated to less than 1.2% Fe2O3and less than 0.7% TiO2.

High intensity wet magnetic separation is an established industrial

operation and is in widespread use throughout the kaolin industry of Georgia. At present, there are 17 large high intensity wet magnetic separators in use in Georgia. These units are capable of processing all of the water-washed kaolin production in Georgia, (approximately 3-4 million tons per year). Large magnetic separators have been in commercial production in Georgia for over 10 years and have established a superb record for cost effective and reliable operation. It has been stated that this development has doubled the useable reserves of kaolin in Georgia because it allows mining of sub-marginal quality kaolin which would have been rejected. A number of kaolin companies have stated that they would not be able to operate today without the use of magnetic separators.

OBJECTIVE

Approximately 50 tonnes of Arkansas bauxite were produced for the purpose of beneficiation using a dry pulverization flow sheet, followed by wet processing on a small commercial high intensity wet magnetic separator, followed by dewatering and calcination to produce high grade calcined bauxite meeting National Stockpile Specifications.

Technology currently available for use in this project includes these

industrial operations: (1) standard kaolin wet process; (2) high intensity, wet magnetic separation on kaolin: and (3) bauxitic kaolin wet process through refractory calcining, The principal knowledge gap is successful merging of key elements

of the above technology to demonstrate tonnage production of calcined

bauxite meeting chemical and physical specifications for National Stockpile purchase.

Justifications for this project are:

1) demonstrate that domestic reserves can be upgraded as an alternative to stockpiling high grade green or calcined bauxite;

2) demonstrate that domestic and foreign bauxites now in the stockpile

can be upgraded to meet specifications needed for refractory grade; 3) encourage exploration of other low -silica high -iron domestic reserves

(t, e., Northwest); 4) provide an incentive to develop practical processes for the removal of

silica from bauxite; 5) provide an economic method to beneficiate domestic and foreign

bauxite and bauxitic clays intended for chemical, less stringent refractory, and abrasive outlets; and

6) demonstrate the application of fine particle processing to

beneficiation of bauxite.

TECHNICAL APPROACH

A typical flow sheet used for the processing of kaolin is shown in Figure 1. Tasks include a dry handling section, followed by wet beneficiation, and finally a dewatering section.

Only four of the unit operations described in Figure 1 are necessary

in the proposed bauxite beneficiation flow sheet. These are blunging, screening, magnetic separation, and classification by centrifuge (dewatering) and are circled in Figure 1. In addition, it will be necessary to use an extrusion and a calcining step.

Figure 2 shows the dry process flow sheet to be used for size

reduction of the bauxite ore to prepare for the wet process flow sheet and calcination shown in Figure 3.

The proposed sequence used in the beneficiation of bauxite is

circled on the kaolin flow sheet, Figure 1.

The sequence in Figure 3 consists of the following:

6) blunge (makedown bauxite in water slurry to approximately 25% solids);

7) screen slurry on 200 mesh (to remove oversize which could plug

matrix in magnet);

8) magnetic separation (to remove most of Fe2O3 and some TiO2);

9) dewater the non -magnetic fraction using a solid bowl centrifuge;

10) extrude and calcine beneficiated bauxite (pressure extrusion to

achieve high density after calcining); and

11) screen calcined bauxite and package for shipping. The preliminary results shown in Table II reveal that the nonmagnetic

fraction contains 2.93% Fe2O3 vs. 2.5% required by the refractory grade specifications (calcined basis). It is confidently expected that grinding of the bauxite on commercial equipment will reduce fineness below 325 mesh and enable the magnetic separation step to reduce the Fe2O3 content below 2.5%. In all other respects, the calcined non -magnetic fraction was significantly improved compared to National Stockpile Specifications. The alumina content was over 5%, higher than the specifications, while the SiO2 and TiO2 contents were only half of that permitted by the National Stockpile Speci-fications. Average density after firing was 3.50 vs. 3.05 required by the Specifications. Previous work (Halaka and Iannicelli, Reference 2) show that a 1% reduction in TiO2 content was equal to a 3% reduction in Fe2O3 content, as far as refractory properties are concerned.

As mentioned, the critical magnetic separation step to reduce Fe2O3

in hydrous bauxite has been demonstrated on a PEM 5" pilot plant high intensity wet magnetic separator on a wide variety of bauxites, including Arkansas bauxite (Reference 1).

In summary, the proposed flow sheet in Figure 3 is a selective

amalgamation of kaolin wet process knowhow with bauxite calcination practice. It requires special attention to avoid introduction of deleterious soluble salts in the bauxite and to achieve the desired reduction in iron

content by magnetic separation. DESCRIPTION OF PROCESSES, EQUIPMENT AND

MATERIALS USED IN PILOT WORK

Approximately 50 tons of pulverized Arkansas bauxite was selected after initial screening and analysis of one kilo samples established suitability of the bauxite to meet the chemical specifications shown in Table I after magnetic separation. Bauxite was made down to 25% solids using a high speed impeller in a 100 gallon tank. Water used for makedown was relatively free of soluble salts (less than 100 parts per million total dissolved solids) and a dispersant which would not leave a residue alkali, phosphate, or other deleterious chemicals in the calcined final product (ammonium hydroxide or ammonium salt or a polyacryate).

The resulting slurry containing about 25% solids was screened on a

200 mesh Sweco vibratory screen. This was necessary to avoid plugging of stainless steel wool matrix used in the high intensity wet magnetic separator.

The magnetic separation was performed on the Aquafine 5" high

intensity wet magnetic separator, Figure 4, which has a canister diameter of 5" and a nominal capacity of 200-400 pounds per hour. The magnetic separator produced a non -magnetic fraction containing less than 3% Fe2O3 and a magnetic fraction containing the concentrated iron which is discarded. This unit operation is a batch mode with a duty cycle of 70% (during which product is produced).

Beneficiated bauxite from the magnetic separator was then dried,

pressed into pellets, calcined, and analyzed by x-ray florescence with results shown in Table II.

PROPOSED SCALEUP OF WORK

Following additional laboratory test work on more finely pulverized

crude bauxite, this work will be scaled up to the tonnage using the basic flow sheet shown in Figure 3.

The magnetic separation step will be performed on the Aquafine 30"

mobile high Intensity wet magnetic separator (shown in Figure 5). This unit has a canister diameter of 30"and a nominal capacity of 5,000 to 10,000 lbs, per hour. As far as is known, this is the only mobile 20 kilogauss magnetic

separator of this size which is available in the United States. The beneficiation of the non -magnetic fraction will be dewatered to

75% solids using a solid bowl centrifuge and the dewatered cake containing about 75% solids will be extruded and calcined at high temperature under contract with an established refractory bauxite producer.

Following calcination, the bauxite will be screened to meet

National Stockpile Specifications and packaged in plastic-lined gal-vanized steel drums as stipulated in the FEMA contract. Testing of the final product will be carried out at a number of commercial laboratories, including end users, refractory producers and independent laboratories.

ACKNOWLEDGMENT

The author gratefully acknowledges the assistance of Alcoa for

supplying the low -silica bauxite used in this study, and of Mulcoa Division of Combustion Engineering for performing the analytical work.

REFERENCES

Murray, H. H., and Iannicelli, J., 1982, "A Survey-Beneficiation of Industrial Minerals, Ores, and Coal by High Intensity Wet Magnetic Separation," NSF /RA-800286, 1982, pages 24-32.

Halaka, N. J., Iannicelli, J., and Negrych, J. A., "Upgrading lone Refractory

Kaolins by. High Extraction Magnetic Filtration, "1977 SME Fall Meeting and Exhibit, St. Louts, Missouri, October 19-21, 1977.

TABLE I

CHEMICAL REQUIREMENTS

Percent by Weight (Dry Basis)

Calcined Bauxite

Hydrous Bauxite Type I Alumina (AI2O3) Min. 56.2 86.5 Silica (SiO2) Max. 4.55 7.00 Iron Oxide (Fe2O3) Max. 1.63 2.5 Titania (TiO2) Max. 2.44 3.75 Potassium Oxide plus Sodium Oxide (K2O+Na2O) Max. 0.13 0.2 Magnesium Oxide plus Calcium Oxide (MgO+CaO) Max. 0.2 0.3 Loss-on-Ignition (H2O) 2 Max. 34.5% 0.5 1 Assumes about 35% water of hydration. 2 Determined by igniting to constant weight at 1050° C.

TABLE II

ANALYSIS OF ARKANSAS BAUXITE FOR FEMA PROJECT

DESCRIPTION AI2O3 SIO2 Fe2O3 TIO2 CaO MgO Na2O K2O

Head Sample – Hydrous* 56.6 2.6 8.65 2.46 0.14 0.10 0.01 0.02

Head Sample – Calcined 3000° F* 82.4 2.98 10.69 3.51 0.20 0.14 0.02 0.03

Nonmagnetics-Calcined 3000° F* (120 Seconds Retention-20KG)

92.1 3.12 2.93 1.61

National Stockpile Refractory Grade Type I

86.5 7.00 2.5 3.75 0.30 0.20

*Average of ten samples

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