Although, silica sand occurs in abundance in varying degrees of purity, only a small percentage is suitable for glassmaking and other high-grade industrial applications, including foundry, ceramics, chemicals, fillers and the construction industry. In general, mineral processing is an essential step in upgrading the raw sand to meet industrial specifications.
Gravity concentration using spirals has been the preferred process for the production of high-grade glass sand. Spirals are low cost and easy-to-operate unit operations that have no moving parts (Sivamohan and Forssberg 1985). These advantages are sometimes offset by a spiral’s inability to achieve the desired final product grade in a simple and efficient circuit.
Outokumpu Technology, Inc. Carpco Division has conducted extensive test work on various sand deposits. Test results indicate that the Floatex Density Separator, a hindered-bed classifier, in conjunction with spiral, a flowing film separator, provides an efficient and costeffective processing scheme (McKnight, 1996). In this circuit, Floatex was found to be highly effective in removing coarser sized silica and heavy minerals. More importantly, the narrow size distribution combined with the concentration of heavy minerals in the finer size fraction makes the Floatex overflow an ideal spiral feed. The preclassified and preconcentrated feed improved the efficiency of the spiral to remove finer sized heavy minerals.
This paper presents the benefits of a Floatex/Spiral circuit for the separation of sillimanite and ilmenite in a silica sand feed material. Also shown is the overall versatility of this processing scheme for the production of high purity glass sand and different co-products.
Floatex Density Separator
The Floatex Density Separator is a hindered-bed classifier that consists of an upper square tank and a lower conical section (Figure 1). The Floatex can be divided into three main zones,
- the upper zone (zone A) above the feed inlet,
- the intermediate zone (zone B) between the feed inlet, and teeter water addition point, and
- the lower section (zone C) below the teeter water addition point.
Feed slurry is introduced to the Floatex tangentially through a centralized feedwell that extends to approximately one third of the length of the main tank. Fluidizing (teeter water) is introduced over the entire cross-sectional area at the base of the teeter chamber through evenly spaced water distribution pipes. As the feed enters the main separation zone it expands into a teetered or fluidized bed as a result of the rising current of water. The teeter water flow rate is dependent upon a) feed particle size distribution, b) density and c) the desired cut-point for the separation. The separation takes place in zone B and the separated lighter/finer particles and the coarser/heavier particles leave the separator through zone A and zone C respectively. This separator is equipped with a pressure sensor mounted in zone B above the teeter water pipes and an underflow discharge control valve. The pressure, sensed by a level sensor, is transmitted to the underflow control valve using a specific-gravity set-point controller. The instrumentation helps in maintaining a constant height of the teeter bed and a steady discharge of the underflow.
Humphreys Glass Sand Spiral – Model LC-3700
The Humphreys Spiral is a flowing film separator generally comprised of a feed box, 7-spiral turns and a product collection box. The feed slurry is introduced on the top turn of the spiral surface. As the slurry flows downward, the coarser and less dense minerals present in the feed migrate radially outward from the central column, while the heavier and smaller minerals stay closer to the central column. As the slurry flows down, the spiral the segregation of heavier from the lighter minerals become more pronounced. The segregated materials are collected from the bottom turn of the spiral using splitters that channel the product, middlings and gangue into different ports. In this test work, a 6-way product box was used to minimize the operators’ variability; and also to evaluate more exit streams.