Magnetic separation

Magnetic separation

Magnetic separation has changed considerably in the last 10-15 years. Prior to this, electromagnets were used in glass plants to remove the iron bearing minerals. Today, the rare earth roll magnetic separators dominate the industry.

When the rare earth magnets were first introduced, there were problems associated with the magnets including low production rates, low sand temperature requirements, and high costs. However, today, the rare earth magnets have improved greatly. Temperature ratings are above 120 C with better instrumentation to shut down the process if temperatures exceed the maximum rating. Costs have been reduced significantly with capital cost of rare earth magnets being less than 50 percent of the capitals cost/ton of the older electromagnets. Figure 9 shows a close up of the actual separation and 3 pass INPROSYS ® rare earth magnet designed for the silica sand industry.

Production rates have continued to climb per meter width of separating zone due to the increase of the roll diameter. The earlier magnets were 75 mm in diameter, and, until recently, the 100 mm diameter was the standard. Production rates in glass sand were in the order of 3-7 tph/meter depending on purity of feed and the desired end product. Rare earth roll magnetic separators now have diameters of 150 and 300 mm diameters with 2-4 times the production rates of the 100 mm roll.

Typically, the rare earth roll magnetic separators are configured in a two or three pass system with the non-magnetic product from the first pass reporting to the second and third roll for an additional pass. In some cases, four passes are required to achieve the results. However, the vast majority of all minerals are removed in the first two passes and the subsequent passes are only required when the ore quality requires more processing to meet a specification. Several examples of results from magnetic separation tests are shown below in Table 5.

Table 5 Typical Magnetic Separator Performance
Product TPH/Meter Feed     % Fe2Cb Wt % Product      % Fe203
USA 5.0 0.089 97.9 0.039
USA 5.0 0.066 98.1 0.031
USA 5.0 0.053 97.5 0.031
Europe 3.0 0.085 96.5 0.012
Europe 4.2 0.085 97.6 0.013
Europe 5.0 0.085 97.1 0.014

The table shows that with decreasing feed grades for the USA sample, the iron contents continued to decrease. This feed material is the same as the example shown in the spiral data. For the European data, increasing feed rates resulted in higher iron content but even at 5.0 tph, the iron content was within the desired specification of <0.018 percent Fe2O3.Typical capital costs for rare earth roll magnetic separators are $6,000-10,000/ton/hr.

Triboelectric separation

New process or old? Triboelectric separation has been commercially exploited since the mid 1940’s. However, until recently, it has been only used by 1 or 2 companies in the salt industry. Triboelectric separation results when one mineral gains an electron from another mineral. When this occurs, the mineral that has gained an electron becomes negatively charged and the one that gave up the electron becomes positively charged. When the minerals are dropped between two oppositely charged electrodes, the negative mineral is attracted towards the positive electrode and the positive mineral towards the negative electrode and a separation occurs. Figure 10 shows an Outokumpu Technology’s T-Stat Separator.

For the silica industry, the triboelectric separation process is used to separate feldspar from quartz. Traditionally, this separation has been conducted using flotation. In the conditioning stage prior to flotation, hydrofluoric acid is used to activate the feldspar and depress the quartz. The amine is added to float the feldspar from the quartz.

Although the process is widely used, it is not ideal. There are many factors that influence feldspar removal efficiency, such as: water quality, percentage of feldspar in the feed, feed rate, and size distribution. As these variables change, the amount of feldspar remaining in the quartz changes and therefore the Al2O3 content changes. As discussed in the introduction, changes in Al2O3 content in the glass batch results in changes in the glass density and viscosity, which affect the downstream process. In addition, it should be noted that, the hydrofluoric acid is environmentally unfriendly. Since the HF is added to the water, the entire water circuit of the plant is contaminated with HF and water discharge from the plant cannot be tolerated.

With the triboelectric separation process, HF is still required, however, since this is a dry process, the HF can be better controlled. In the triboelectric process, fumed HF is added to dry, hot (100-120C) feed material in a rotary mixer. As the feed material is being mixed in the presence of HF, electrons are transferred from the feldspar to the quartz. The feldspar then becomes net positive and the quartz net negative. When this material is introduced between to highly charged electrodes (+50, -50 KV), the quartz reports to the positive electrode and the feldspar to the negative electrode.

Compared to flotation, the HF is much easier to control. Any excess HF from the rotary mixer reports to a wet scrubber. These commercially available wet scrubbers utilize only minor amounts of water to scrub the air free of HF fumes. They have efficiencies of greater than 99 percent and use only 4 liters/minute of new water. The HF laden water from the scrubber reports to a system that cleans the water to 10-12 ppm F ion and produces a CaF filter cake that can be land filled.

In addition to the improved environmental aspects, the process is more stable since there is no influence from the change of process water quality. In addition, changes in feldspar content or size distribution do not have as significant an influence as flotation since there is no amine (collector) being used that must be adjusted to reflect these changes.

Table 6 Typical Results of T-Stat separation
PRODUCT Wt % % Quartz % Feldspar Qtz % Dist Feld % Dist
Feed 100.0 48.7 51.4 100.0 100.0
Feldspar Prod 41.7 11.9 88.1 18.2 71.5
Middling 17.3 57.3 42.7 20.4 14.4
Quartz Product 41.0 82.4 17.6 69.4 14.1

Table 6 shows the result of a typical quartz-feldspar separation that has been conducted using Outokumpu’s T-Stat. Recycling of the middling product will improve the recovery. Although these products met the specification of the customer, an additional cleaning stage would increase the grade of the products.

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