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DuPont Mineral Products – Dredge Mining Process

Heavy mineral mining is made up of several individual stages or sequences of operation. These stages are described below in order of occurrence.

Initially, all marketable timber is harvested from the immediate mining area. Stumps, undergrowth and remaining brush are then collected and piled by crawler tractors. This material is allowed to dry and later burned.

Topsoil is then removed to a depth of 6-8 in and either stockpiled for deposit later or placed directly on an adjacent mining cut. This work is accomplished by a 23-yd3 four-wheel-drive pan scraper.

The ore is mined with a custom-designed Ellicott® brand suction dredges equipped with a cutterhead.  (see picture) The dredge has two spuds located at the stern, which are used as pivots in conjunction with the swing lines to advance or retreat the dredge. A 20-in dredge pump discharges into the 22-in pipeline of 40-ft sections supported on pontoons. These connect the dredge with the floating wet mill.

Wet Mill
The wet mill is constructed on several floating barges. The slurry from the dredge discharges onto vibrating screens with 0.25-in openings. The oversize falls by gravity into a hammer mill where lumps of hardpan are crushed. The discharge from the hammer mill flows by gravity to a screen with 1-in openings. The +1-in oversize is made up primarily of roots, which are discharged directly back into the pond. The undersize is pumped back to the screens.

The wet mill treats raw ore containing about 4% heavy minerals at rates up to 1100 Gt/hr. From this, it produces a concentrate averaging 80% heavy minerals with a TiO2 recovery of 78%. The wet mill uses three stages of spiral concentrators. Spirals are helicoid troughs fixed at a declining slope and arranged in 7-, 5-, or 3-turn configuration. Slurry feed containing sand and water is introduced at the top of each spiral. As the material flows down and around the troughs, the lighter minerals such as quartz and silica sand are carried to the outside of the stream. The heavier minerals, e.g., ilmenite, zircon, and staurolite, tend to concentrate nearer the inside of the spirals and are subsequently drawn off at selected points. The different streams of various concentrations are then routed back for additional retreatment or to successive spirals for further concentrating.

The rougher spirals produce a concentrate averaging 10-15% heavy minerals. The rougher tailings discharge into the pond as backfill. Since 96% of the dredge feed is returned to the pond, the dredge and pond actually move slowly forward in the direction of the mining. About every two weeks, it is necessary to move the floating wet mill to keep up with the advance of the dredge.

The rougher spiral mids are recycled to the head feed for reprocessing over the roughers. The rougher spiral concentrate is pumped to cleaner spirals. The cleaner concentrate flows by gravity to finisher spirals. The finisher spiral tailings flow back to the cleaner feed sump for retreatment. The finisher concentrates are pumped via 5-in pipeline to a land-based stockpile, to be loaded into trucks for shipment to the dry mill.

Dry Mill
Wet mill concentrate is a mixture of titanium minerals, heavy mineral silicates, and quartz. The function of the dry mill is to recover the titanium minerals, and further separate them into an ilmenite product and a leucoxene-rutile product. The process takes advantage of the conductivity of the titanium minerals in high-tension treatment and the higher magnetic The wet mill concentrate is scrubbed using sodium hydroxide before dry mill processing. This removes surface coatings of organic and clays from the particles. The clean grain surfaces improve recovery and allow higher quality mineral products to be produced. The scrubbed ore is dried in rotary dryers. The hot ore is first separated on rougher high-tension separators and rougher concentrates are treated on cleaner high-tension rolls. The cleaner concentrates are passed over high-intensity magnets, each with two banks of five rotors operating in series. The magnetic fraction is collected and shipped as ilmenite. This product contains 98% titanium minerals and averages 64% TiO2. The nonmagnetic fraction is retreated in a final cleaning circuit to recover the leucoxene and rutile for shipment. The final product contains 98.5% titanium minerals and analyzes 80% TiO2. The overall TiO2 recovery in the dry mill averages 97%.

The nonconductive tailings from the high-tension circuit are fed to high-intensity magnets. The magnetic product is commercial staurolite, assaying 45-50% Al2O3 and 13-15% Fe2O3. Different grades are produced to supply special foundry sand (Biasill®) and provide a sandblast abrasive (Starblast®).

Staurolite magnet tailings contain 25-30% zircon, 15-20% aluminum silicate minerals and about 50% quartz. The relatively high specific gravity of the zircon makes gravity separation from the lighter heavy minerals possible using spirals. The zircon spiral concentrate is dried and recleaned by a process patterned after the titanium mineral dry mill.

Large tonnages of zircon are used for molding sands in the foundry industry. Smaller amounts are used in the manufacture of refractories, ceramic opacifiers, zirconium metal, and chemicals. A variety of grades are produced meeting quality requirements for each end-use.

Grass planting experiments began on mine tailings in 1952. Over the years, many varieties of grasses and trees, along with fertilizers, were applied with some success. One grass, with the extraordinary name of “African weeping love grass,” was imported from an area with similar environmental conditions. The love grass accomplished moderate growth and helped stabilize tails so that pine tree seedlings could live.

In early 1969, however, reclamation took a new direction. Instead of planting the new vegetation directly on tails, topsoil is removed prior to mining and used to cover the tailings. Pasture grass is then planted and growth is quickly established. After fertilization and sufficient soil stabilization, pine trees are replanted. This reclamation program has been cited as a model for the mining industry in Florida.

Water Treatment
During the mining operation, humate from the soil is released into a colloidal suspension in the dredge pond water. Humate is partially decomposed organic matter, dark brown in color, and is common throughout Florida. It is especially noticeable in Florida “hardpan” outcroppings.

In the course of moving across the ore body, it becomes necessary to adjust the level of the dredge pond at times to allow the dredge to reach shallow or sometimes deep All pond pumping water and discolored rain run-off are collected and set to a series of treatment facilities before discharge to public streams.

Sulfuric acid (H2SO4) is added to the water to lower the pH to 3.5. This action induces flocculation of the organic matter and leaves a clear supernate, which is decanted from the settling ponds. The clear water is then neutralized to pH 7 with lime and discharged to public streams.

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