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 is not sufficient to move the heavier minerals to this region therefore they slide down the profile to the inside region of the spiral.

Since this is a flowing film separator, the centrifugal forces are not equal along the depth of the slurry. At the very surface of the spiral, the centrifugal forces are very small. Therefore, the smaller particles have little forces acting on them. At higher depths of the slurry, the forces become greater and therefore, the coarser particles have higher forces acting on to help them report to the outer regions of the spiral.

For glass sands, this variation in forces between the finer and coarser particles generally is a benefit since the majority of the heavy minerals in the deposits tend to be finer than the glass sand. Therefore, the lower forces on the heavy mineral particle help to allow them their reporting to the center of the spiral for rejection. The higher forces on the coarser glass sand particles push them toward the outside and allow them to report to the glass sand product. In many cases with glass sand, the separation between the heavy iron bearing minerals and the sand portion is so extreme

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that the urethane surface of the spiral can actually be seen between the two particle streams flowing in the trough.

USING SPIRALS TO REMOVE IRON-BEARING HEAVY MINERALS

Throughout the world, there is considerable experience in the use of spirals to remove the iron bearing heavy minerals in glass sand deposits. There are currently more than 25 glass sand spiral installations around the world.

In these installations, spirals having either five or seven turn helixes are employed; the number of turns required depends on the amount of heavy iron-bearing minerals that need to be removed, i.e., with higher amounts of particles needing removal then the longer (7-turn) spirals are desirable. At times, there is also a benefit of passing rougher stage (1st pass) spiral sand product through a second stage of spirals (cleaner pass) to remove the remaining heavy minerals. For example, if the first stage is 70% effective then the second or cleaning stage will be nearly 70% effective on the remaining 70% heavy minerals. Thus, the two-stage spiral process can be said to be 91% effective in heavy mineral rejection.

The cleaner stage also fits well from a plant layout and operational standpoint. In the spiral process, the first stage removes a heavy mineral product at a high pulp density, from the inner area of the spiral trough helix. The lighter glass sand product along with most of the feed pulp water reports to the outer area of the trough helix where it exits the spiral. The pulp density of the first pass sand product is at a reasonable density for directly feeding the second stage. Therefore, the first and second stage spirals can be stacked one above the other with the product from the first stage reporting directly to the second stage. Using this practice eliminates the cost of pumping pulp from the first to the second stage. However, where height restrictions prevent stacking, traditional pumping between spiral stages can be employed.

Spiral

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