TENORM: Gold, Silver, Zircon and Titanium Mining Wastes
While few studies have been done on these ores, some western mines produced uranium as a secondary product when extracting precious metals. Pitchblende (a naturally occurring material containing low concentrations of uranium) has been found in the same ores as gold and silver. Waste rock from some of these mines may be radioactive.
The mineral zircon (zirconium silicate) occurs in nature and is a coproduct of the mining and processing of heavy-mineral sands for the titanium minerals ilmenite and rutile. There are only a couple of domestic producers of zircon. Zirconia (zirconia dioxide) is produced directly from zircon by plasma fusion or electric-arc. Zirconium, in the form of zircon, is widely used in ceramics, foundry sands, refractory paints, various refractory materials and some electronics. Zirconium metal is used in corrosive environments, nuclear fuel cladding and various specialty alloys. Zircon is also used as a natural gemstone and may be processed to produce cubic zirconia, a synthetic gemstone and diamond simulant (USGS 2016). NORM and TENORM can be found at zircon processing sites.
As a result of the high chemical inertness of zircon, most of the uranium and thorium found within it was present during the crystallization of the mineral from the molten host rock (although some may also occur within other minerals present as inclusions in the zircon sand grains, for example monazite). The uranium and thorium atoms and their decay products are bound within the zircon crystal structure, substituting for a small number of zirconium atoms. In most other uranium-containing minerals, including uranium ores, the uranium atoms are not bound within the crystalline matrix but form part of the cementing material between the grains. The nature of the zircon crystal is such that the removal of uranium and thorium is not easily accomplished without destruction of the crystal lattice.1
Zircon, along with other minerals of value, for example ilmenite, rutile and monazite, is separated from the complex mineral mix by magnetic and electrostatic separation processes. These processes can be associated with high concentrations of radionuclides due to the presence of minerals other than zircon, notably monazite, leading to a need for significant radiation protection measures. Once the zircon is separated from these minerals, the absence of these high radionuclide concentrations leads to a much lower radiological risk.
The activity concentrations of 238U and 232Th series radionuclides in commercial zircon fall mostly in the range of 55 – 107 pCi/g and 1 – 27 pCi/g respectively (2 – 4 and 0.4 –1 becquerel per gram (Bq/g)). The radon emanation coefficient for zircon is very low due to the crystal structure (0.0016 to 0.014). Gamma exposure rates near bulk supplies of zircon average ~ 190 microroentgens per hour (1.9 microsieverts per hour).2
Note: Zircon is one of several so-called heavy minerals — these are normally defined as minerals with densities exceeding 3000 kg/m3. The heavy minerals of major commercial importance are, in addition to zircon, the titanium bearing minerals ilmenite, leucoxene and rutile and the rare earth bearing materials monazite and xenotime.
2 Radiation protection and NORM residue management in the zircon and zirconia industries. International Atomic Energy Agency (IAEA), Vienna, 2007.
Titanium is the ninth most abundant element in the earth’s crust and can be found in nearly all rocks and sediments. It is not found as a pure metal in nature. Mining of titanium-bearing minerals is usually performed using dredging and dry surface mining techniques for the recovery of heavy minerals. More than 90% of titanium mineral production is in the form of ilmenite. Roughly three quarters of this is ilmenite sand, a component of heavy mineral sand, while the remainder is ilmenite rock.
Uranium, thorium, and radium commonly occur in titanium ore, and monazite is found in sands from which the titanium is extracted. As a result, the mineral sludges, dusts and sands from the extraction process may be radioactive. The natural thorium content of ore can range from 10 – 500 parts per million (ppm)(0.04 – 2 becquerels per gram (Bq/g)). The uranium content can range from 2 – 30 ppm (~1 pCi/g to ~12 pCi/g, or 0.03 - .4 Bq/g). The radionuclide activity concentrations are moderately elevated above those in normal rocks and soil. During processing, the radionuclides may become mobilized and migrate to dusts, scales and other process residues, leading to the possibility of radionuclide activity concentrations higher than those in the relevant feedstock mineral. Isotopes of radium in particular may become concentrated in scales. Titanium dioxide (TiO2) and other titanium-containing products are essentially free of radioactivity.1
The annual worldwide production of TiO2 pigments is close to 6 million tons. About 95% of titanium mineral concentrates are consumed by domestic TiO2 pigment producers. The remaining 5% is used in welding-rod coatings and for manufacturing carbides, chemicals and metal. The US imports over 90% of titanium mineral concentrates and almost 60% of titanium sponge (a name for metallic titanium metal). The US is also a net exporter of TiO2 pigment and wrought titanium products.
There are a variety of chemical processes used to make TiO2, and as such concentrations in residues can vary across those methods and facilities. Residues from the production of titanium slag and upgraded slag (essentially dust from the smelting operation and solid metal oxides from the upgraded slag process) are generally found to have radionuclide activity concentrations well below 27 pCi/g (1 Bq/g). For processes using ilmenite sand, the activity concentrations in the furnace dust may occasionally exceed 27 pCi/g (1 Bq/g), depending on the characteristics of the feedstock. In some situations that are not considered representative of normal production, the furnace dust has been reported to contain 210Pb and 210Po at concentrations of ~60 pCi/g to ~105 pCi/g (2.1– 4.0 Bq/g) in the coarsest of the dust fractions sampled and 230 pCi/g to 365 pCi/g (8.5–13.5 Bq/g) in the finer fractions.1
Other Titanium processing methods however, can yield higher concentrations of residuals.1 Radionuclide activity concentrations measured in samples of digester residue, scale and discarded filter cloths are found to be highly variable and, in the case of scale and filter cloths, can sometimes reach values of the order of 28,000 pCi/g (1000 Bq/g). The radionuclide concentrations are generally higher for the 232Th decay series than for the 238U series, and can be summarized as follows:
a) Radionuclide activity concentrations in digester residue derived from ilmenite feedstocks are in the range of ~1 pCi/g to 68 pCi/g (<0.04–2.6 Bq/g). The concentrations in material derived from feedstocks containing 90% or more slag are less than 27 pCi/g (1 Bq/g). Regardless of the type of feedstock used, there is evidence of enhanced concentrations of 228Ra (or 228Ac) and 226Ra.
b) Radionuclide activity concentrations in scale deposited in various steps of the process vary over a very wide range (3 pCi/g to 42,000 pCi/g) (< 0.1 to 1644 Bq/g), with the highest values being found in scale from the pre-hydrolysis heat exchanger.
c) Radionuclide activity concentrations in discarded filter cloths from Moore filters vary over a very wide range (1 pCi/g to ~26,000 pCi/g) (0.04–968 Bq/g). The concentrations in a cloth from a gypsum filter were found to be less than 27 pCi/g (1 Bq/g).
Information about annual production values can be found at the USGS Titanium web page.
1 Safety Report Series No. 76: Radiation Protection and NORM Residue Management in the Titanium Dioxide and Related Industries: International Atomic Energy Agency, 2012 (PDF) (124 pp, 1.56 MB, About PDF)