By H. Robert Goltz and Matthew Rodgers
Mining entails removal of rock from below the surface of the earth to recover metals of value. Regardless of the method used, undesirable elements and compounds may also be brought to the surface and transferred into process streams that can contaminate a final product—or they may not be noticed until there is waste water, tailings ponds or acid mine drainage to deal with.
There is a need to remove trace contaminates from process and waste water streams to keep product quality high, and to meet waste disposal limits on hazardous materials. Some of the more well-known contaminates include uranium; perchlorate; nitrate; copper, nickel, zinc and cobalt; platinum group metals; radium; arsenic; and mercury. This will provide a quick overview of options for selective contaminant removal, using selective media, from mining process and waste waters. Please note that, because every process/waste stream is different, validation of the suitability of these media and separation strategies must always be done.
Selective Uranium Removal—Ion exchange operations are governed by a phenomenon called “selectivity.” This is a natural preference the ion exchange media functional group has for different ions. An example is uranium capture with a Type 1 strong base anion exchange resin. The selectivity of this functional group for U(CO3)3 is so strong that, when the uranium is present at sub-ppm levels, it still effectively loads onto the resin in the presence of ppm levels of sulphates and chlorides. In mining applications where U is present at 100 ppm levels, loadings of 6 to 12 lb of U/ft3 are common. When U is present at ppb levels, loadings of 1/4 to 1/3 lb of U/ft3 of resin occur.
Selective Removal of Perchlorate over Nitrate—The amine functional groups of anion exchange resins have long been modified to adjust the performance of the resin. By increasing the chain length of groups attached to the nitrogen, the distance is extended thus decreasing the charge strength on the nitrogen. In part, the amine group also takes on a more hydrophobic or “greasy” character that can have an impact on the affinity for the target ion. The following example shows how increasing selectivity for perchlorate over nitrate can be achieved by increasing the chain length on the amine groups.
Selective Nitrate Removal—While selective removal can be done, it is not always the best option. A good example is nitrate capture. When nitrate is present along with lower affinity anions like chloride and carbonate, the best option is a non-selective Trimethylamine (Type 1) or Dimethyl ethanolamine (Type 2) strong base anion exchanger. These products have high capacities and are generally widely produced so their costs are lower.
The more exotic triethyl amine functional groups are offered as “Nitrate Selective Resins.” This is not a completely accurate description because what really occurs is the larger, bulkier alkyl group increase the distance between the amine groups and the high affinity sulphate ion thus de-selecting for sulphate rather than selecting for nitrate. The outcome is that the relative selectivity for nitrate against sulphate is improved.
Generally speaking, when the sulphate concentration is 30 to 50 ppm or higher, or when the sulphate concentration is three times the nitrate concentration, a nitrate-selective resin is the best option. Having said this, microeconomics can also come into play in the guise of regenerant and disposal costs, so it is always a good idea to account for these costs when making a media choice.
Copper Selective Media—Some unusual functional groups have been developed over the years that are very selective for Cu, Ni, Zn and Co under acidic conditions. These chemistries are designated as hydroxypropyl picolylamine (HPPA) and bispicolylamine (BPA).
BPA resins are particularly useful for the valuable separation of cobalt and nickel in refinery operations or in primary mining circuits.
Platinum Group Metals Recovery—Another unusual functional group is the Thiouronium group. Some plating operations produce sludge and waste that contain low levels of platinum group metals. When they are dissolved under acid conditions, valuable metals such as Pt, Pd, and Rh can be recovered from these strongly acidic solutions.
The data indicates almost complete removal of platinum, palladium and rhodium in the presence of significant concentrations of copper and zinc with negligible loading of many other metals. In other systems we have seen Pt loading of 10-12 oz/ft3 of resin. This high loading makes recovery of the metal by ashing practical and cost effective.
Selective Radium Removal—Some ions can be removed using the low solubility of that ion. Here is an example of selective radium removal using a radium-selective complexer.
A convenient way to use solubility properties is to precipitate the salts inside an ion exchange bead. The bead provides good flow properties and ease of handling. As soluble radium ions enter the bead, they encounter the precipitated salts and co-precipitate inside of the bead. Since this is a precipitation mechanism, the ultimate loading capacity of the media is very high.
Selective Arsenic Removal—Arsenic can be selectively removed by complexing with iron- or titanium-based media. Both have been shown to exhibit strong complex formation so that removal can be achieved down to a sub-ppb levels.
An interesting facet of titanium-based media is their stability when confronted with pH upsets. Shown below is arsenic removal data for iron- and titanium-based media showing complete arsenic removal. At about 45,000 bed volumes of feed, the pH fluctuated from 6.5 to 7.2, resulting in arsenic release from the iron-based media while the titanium-based media shows only a slight increase in arsenic release. This shows the benefit of stronger titanium binding in the face of pH fluctuation.
Selective Mercury Removal—Mercury is challenging because it can exist in multiple forms: as a zero valence metal, as a cation and as an organic complex. Because of the active redox nature of mercury, it is suspected that it can move between these different forms, depending on the solution conditions, making it very difficult to remove.
The approach that we are taking with mercury removal is to use a family of different media that have different functional groups and properties. Recent demonstrations show that these media are capable of reducing mercury levels to <5 ppt (parts-per-trillion).
Dr. H. Robert Goltz ([email protected]) is an application development leader serving the Americas at Dow Water and Process Solutions. Matthew Rodgers ([email protected]) is a technical service specialist for mining and chemical processing for North America, Dow Chemical Co.