Radiological and chemical risks by waste scales generated in the titanium dioxide industry
Introduction
Is a well-known fact that any mining or minerals processing operation has the potential to increase the radiation dose received by individuals, due to the fact that all minerals and raw materials contain radionuclides of natural origin (García-Tenorio et al., 2015). The total radioactivity entering in the processes suffer its distribution among final products, by-products and residues, being the concentration and proportions found in each of these compounds very much dependent on the mineral/material treated and the process applied.
Some of these industrial processes, are characterized for generating small residues amounts along the production processes which present very high enrichment factors in some natural radionuclides and chemical elements.
Between these types of residues, we can remark the followings (García-Tenorio et al., 2015) (Godoy and Petinatti da Cruz, 2003):
- (a)
Scales: layers enriched in radium isotopes, and in some cases in 210Pb, which are formed in the internal walls of pipes and equipment due to the precipitation of elements such as Ba and Pb in the form of sulphate compounds when a change in a physical variable affecting its solubility is produced along the process,
- (b)
Sludges: semi-solid material which is accumulated in deposits of several NORM industrial processes and that due to its very small grain size pass filtration steps accompanying the liquid fractions and experiment later on its decantation in the deposits where the liquids stay during long time intervals,
- (c)
Precipitator dust: very fine or volatilized material released in the processes of smelting and/or combustion of metals and minerals and that are trapped in cyclones and scrubbers before being released to the environment, and
- (d)
Filter-clothes: material used in filtration processes in several NORM industries that needs to be changed periodically due to their colmatation which reduce their filtration efficiency.
Due to their characteristics and their radionuclide content, these residues (scales, sludges, filter-clothes …. ) should be managed in most cases as low-level radioactive waste. Specialized waste management companies should take care of these residues, and equally important, only specialized maintenance workers should proceed to their removal from the production lines periodically due to its interference in the production process (IAEA, 2006). All the residues enriched in radium (226Ra, 228Ra) are, in addition, a source of gaseous radon (222Rn, 220Rn) fact that should be taken also in consideration especially during maintenance and management operations.
This type of residues can be found in a high number of different industrial processes. A very detailed list can be found in (IAEA, 2006). Only as examples, we can indicate that:
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Sludge samples can be found associated to the oil and gas production, the phosphoric acid production, the iron smelting and in water treatment plants
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Precipitator dust samples can be found associated to the thermal phosphorous production, to the fused zirconia production, to the niobium extraction and to all the metal smelting industry (Harvey et al., 1994; Penfold et al., 1999), and
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Filter clothes can be found associated to the phosphoric acid production and to the titanium dioxide production, More in detail, due to its general higher radiological importance, we can indicate that scales heavily enriched in Ra and or Pb isotopes can be found also in a high variety of NORM industries. For example, in the petroleum extraction and production facilities, precipitation scales are generated in specific points of the production lines associated to abrupt changes in pressure and temperature and with typical concentrations for both 226Ra and 228Ra in the range 1–103 Bq/g, although concentrations up to 15.103 Bq/g can be reached (USEPA, 1993).
Another typical NORM industry where scales are generated is the phosphoric acid production where the raw material used (phosphate rock) contains high levels of natural radionuclides from the uranium series (1.0–1.5 Bq/g) (Bolívar et al., 2009) and where scales with an activity concentration of 4–12 Bq/g for 210Pb and 226Ra were reported (Beddow et al., 2006; Guerrero et al., 2020). Also, in the combustion of coal to produce heat and electricity some volatile radionuclides such as 210Po can remain inside the burner kettles, being adhered to the walls and reaching concentrations over 100 Bq/g in the formed scales (Huijbregts et al., 2000).
In this manuscript, the generated scales in a plant devoted to the production of titanium dioxide pigments located in Huelva (south west of Spain) were studied. In addition, also used filter-clothes along the same process have been analysed. In this study, besides a radiometric/radiological evaluation a full mineralogical and elemental characterization were carried out in order to explain their formation process and obtain information about the potential presence of additional chemical contaminants to be taken in consideration during the industrial maintenance (removal) and management processes.
The TiO2 production process presents a peculiarity. The raw material used for the pigment production contains enhanced amounts of radionuclides from both the uranium and thorium series. It can be expected to find radionuclides from both series simultaneously in the formed scales and in the used filters, being the total activity varying over time due to the breakage in these residues of the secular equilibrium in both series as will be discussed in the paper. Additionally, it is well known (IAEA, 2014) that the presence in the wastes from titanium dioxide industry of high concentrations of barium and lead is of concern. According to the World Health Organization (WHO (World Health Organization), 2000) the primary routes of exposure to lead are ingestion of lead-contaminated substances and inhalation of lead particles that come from the maintenance operations inside the factory.
Section snippets
Description of the titanium dioxide pigment production process
A complete description of the titanium dioxide process applied in the Huelva plant can be found in a previous manuscript (Gázquez et al., 2009) and a general evaluation of the flow of radionuclides along the process can be found in (Mantero et al., 2013). Here, only a summary will be included, in order to know in which steps of the process are used the filter-clothes and are formed the scales to be analysed. A complete study about Radiation Protection and NORM Residue Management in the Titanium
Materials and sampling
Highly radioactive scales in the crystallization step were identified and collected during maintenance works. Scales collected in the crystallizers of the Huelva plant in two different collection dates separated between then more than 10 years were analysed for evaluating how the use of titanium slag in addition to the ilmenite as raw material is reflected in changes in mineralogy, elemental and radioactive composition of the scales. Scales collected in 2006 when the raw material processed was
Mineralogical composition
The XRD diagram corresponding to some of the scales analysed are shown in Fig. S1 of supplementary material. In all the samples analysed it was observed that the crystalline fraction content is dominant, higher than 75%, in agreement with its origin and its formation process. This special characteristic was particularly important in the scales formed in the crystallizers as expected.
The scales collected in the crystallizers (codes CS1, CS2 and CS3, Table 1) were composed exclusively of
Conclusions
The scales collected along the TiO2 industrial process are found mostly in crystalline form and are composed mainly by anglesite (lead sulphate), being the dominant element in them the Pb although present also relatively high levels of sulphates of, Ba, Ti and Fe. To note that changes in the composition of the raw material, does not influence the mineralogical composition of the scales and therefore, the formation process of them is independent of the raw material used, only ilmenite or
Credit author statement
M.J. Gázquez: Data curation, writing—original draft preparation, investigation and methodology. J. Mantero:, Validation, formal analysis and methodology. I. Vioque: Metodology, Resources and Data curation. F. Mosqueda.: Resources, Visualization, Investigation. R. García-Tenorio: Formal analysis, Investigation, writing—review, supervision and editing. J.P. Bolívar: writing—review and editing, Funding acquisition, Project administration, supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was partially supported by the Spanish Government Department of Science and Technology (MINECO) through the project “Fluxes of Radionuclides Emitted by the Phosphogypsum Piles Located at Huelva; Assessment of the Dispersion, Radiological Risks and Remediation Proposals” (Ref. CTM 2015-68628-R)), and the project of the Regional Government of Andalusia called “Basic processes regulating the fractionations and enrichments of natural radionuclides under acid mine drainage conditions”
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