Espirales

ADV ANCES IN THE APPL ICA TION OF SPIRAL CONCENTRATORS FOR PRODUCTION OF GLASS SAND

Steve Hearn1 and Jim Sadowski2

ABSTRACT

Around the world, spiral concentrators have been successfully applied to glass sand production. Spiral application is both cost effective and environmentally friendly when compared to other techniques for iron bearing and refractory heavy mineral rejection such as flotation and magnetic separation. The coupling of spiral concentrators with hydraulic density separators for damp sand production often results in a process that meets both particle size and mineral/chemistry specifications. Interestingly, glass sand size specifications usually correlate directly to the optimum response particle size for spiral concentrator operation. This paper presents flowsheet alternatives and resulting process performance for glass sand operations representative of commercial operations around the world. The paper also suggests the use of spiral concentrators for rejection of aluminum silicates and mica from quartz sand destined for glass making markets.

INTRODUCTION

The Role of Spiral Concentrators in Glass Sand Production

Glass sand production is dependent on the mineral occurrence characteristics in the deposit, and most importantly, market requirements. Spiral concentrators have long played an important role in the production of saleable glass sand at sites around the world. The traditional purpose of the spiral units is removal of liberated heavy iron-bearing minerals from the sand. Of course, those iron-bearing minerals that are fully liberated will easily be rejected in the spiral, whereas silica sand grains with minor inclusions of the contaminant iron mineral will not reject. Liberation to a degree is imperative for successful application of the spiral technology. With proper feed preparation, i.e., density separator sizing, improved performance capabilities have been realized.

The unwanted minerals in glass sand for the most part are iron-bearing minerals. These minerals have significantly higher specific gravities than quartz. Contaminant minerals such as magnetite and ilmenite, for example, have specific gravities of 4.0 or higher compared to quartz at 2.65. Typically, spirals can separate minerals with a specific gravity differential greater than 0.5units with high efficiency, which makes this separation relatively easy.

In addition to the iron-bearing minerals, aluminum-bearing minerals, such as refractory aluminum silicates and mica, are also likely candidates for rejection in a spiral. The separation of these minerals from quartz is more difficult and requires a slightly different approach than removal of the iron minerals. Testwork has shown promise for removal of mica in a wash water assisted spiral.

Spiral concentrators offer a relatively simple unit operation that translates to low capital and operating cost. This, coupled with reagent free processing, provides the necessary low cost and environmentally desirable process. The actual spiral plant flowsheet most suitable for a particular application will depend on the feed characteristics, especially particle size distribution and

1 OUTOKUMPU TECHNOLOGY INC. - Physical Separation Division, Jacksonville, Florida USA 2 OUTOKUMPU TECHNOLOGY INC. - Physical Separation Division, Jacksonville, Florida USA

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mineralogical characteristics of the resource. However, certain generalizations apply. For instance, in the production of a marketable quality sand is most always the primary driver, with weight recovery of secondary importance. Therefore, two-stage separation is often advisable to ensure final sand product quality.

Spiral Concentrator Design and Operating Basics

Spiral manufactures now offer a variety of models to the industry, each with specific helix trough profile designs, pitches, and other performance improvement nuances. These lightweight models are made of urethane-lined fiberglass and can be expected to last in excess of 10 years even under heavy service conditions. Compared to other sand beneficiation process such as flotation and magnetic separation, spirals present a relatively low capital cost, have no moving parts and consequently have low maintenance costs. Compared to dry magnetic separation, the spiral process is conducted on wet material and therefore the product does not require drying. In those parts of the world that can sell damp glass sand there is no need to expend the capital and energy cost to dry the sand. Even in places that require dry sand, the removal of contaminates prior to drying saves energy cost. If necessary, additional separation efficiency can be achieved by hydraulic classification of the feed and/or re-treatment of the first pass sand in a second pass through a spiral unit.

Spiral Feed Presentation. Spiral concentrators are flowing film separators that work in a similar principle to shaking tables. The design of the feed box is important to assure proper presentation of the feed slurry to the spiral trough resulting in desirable flow characteristics down the helix trough. With proper feed presentation, the separation process begins immediately at the top of the spiral helix. If the box design is problematic, i.e., presenting the spiral with an uneven or unbalanced feed, the pulp will have to stabilize in the trough before separation initiates and that can require up to one complete turn (revolution) of the helix, thus losing separation potential within the length of the spiral. In addition, if heavy minerals targeted for rejection via product cutters at the inner edge of the spiral trough, somehow through unnecessary turbulence, reach the high water (outer) portion of the trough, their ability to re-enter the flowing pulp and migrate to the center portion of the spiral is improbable and therefore, these particles will report to the glass sand product.

Heavy Mineral Entrapment. Another area of concern is entrapment. As the feed pulp flows down the spiral, heavy minerals can be trapped below the bed of sand in the middle portion of the spiral. Like a temperature inversion, these particles become trapped under the blanket of sand and are unlikely to migrate out and into the region of the heavy minerals at the inner area of the trough. To counteract this problem, spirals are often equipped with surface bumps or repulper designs that help free these trapped minerals and allow them to migrate and report to the proper location within the spiral.

Benefit of Centrifugal Force. As slurry flows down the spiral helix, there is a centrifugal force acting to push the lighter minerals up the trough profile to the outside region. The force

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