Arsenic removal procedure for the electrolyte from a hydro-pyrometallurgical complex

Commercial copper (Cu) is obtained by a hydro-pyrometallurgical process, where the Cu anodes obtained in the furnaces (Cu > 99.5%) are enriched up to 99.99% in "cathodes" by electrorefining at an electrolysis plant. During this process, some impurities accumulate in the electrolyte, mainly arsenic (As), which decrease the quality of the Cu cathode. For this reason, the electrolyte is sent to an electrolyte cleaning plant (ECP) for its purification. Electrolyte sludge (ES) is produced in the last stage of purification and is recirculated back to the furnace due to the high Cu content. This recirculation involves a severe problem of As accumulation in the industrial process. The objective of this work was to develop a procedure to fully dissolve the ES, removing the As and recovering its Cu content. The ES dissolution process was optimised (dissolution efficiency > 99%) in H₂SO₄ (1.4 M)/HNO₃ (1.8 M) medium using a 1:20 g mL^-1 solid-to-liquid ratio. As was removed from the ES solution by its precipitation as iron (III) arsenate, with high efficiency (more than 70%). After As removal, the Cu can be precipitated as copper sulphate, which is used in several applications.

Behaviour of heavy metals and natural radionuclides in the mixing of phosphogypsum leachates with seawater

Phosphogypsum (PG) is disposed worldwide in large stacks usually placed in coastal zones, as in the case of Huelva (SW of Spain), where around 100 Mt of PG are stored on the salt marshes of the Tinto River estuary covering a surface of about 1000 ha. This management generates the weathering of PG, and due to its high acidity (pH ≈ 2) and pollutant load can provoke significant emissions into their surroundings. In this work were evaluated by laboratory experiments the effects of pH increase in the behaviour of heavy metals and natural radionuclides during the mixing of phosphogypsum leachates with seawater. The acidic phosphogypsum leachates showed concentrations of heavy metals from two to three orders of magnitude higher than natural continental waters, and natural radionuclides (U-isotopes and ²¹⁰Po) from four to five orders of magnitude higher than unperturbed aquatic systems. Major elements and some heavy metals as Mn, Ni, Cd, As, Sb and Co showed a conservative behaviour during the neutralisation of the leachates with seawater, remaining in the liquid phase, while other ones as Al, Fe, Cr, Zn, Cu, Pb precipitated and/or were adsorbed onto the solid phase. The U-isotopes and ²¹⁰Po showed a clear non-conservative behaviour probably due to coprecipitation/adsorption processes onto the formed precipitates, but while ²¹⁰Po reached a total removal at pH ≈ 7, U- isotopes after a total removal at pH ≈ 5 returned into the liquid phase due to redissolution/desorption processes at near neutral pH. The formed precipitates, mainly composed by iron phosphates particles, showed heavy metal and natural radionuclide concentrations from one to three orders of magnitude higher than unperturbed soils. All these facts demonstrate the serious environmental impact produced by the PG stacks into their surroundings and the urgency of effective restoration measures.

Radiological and physico-chemical characterization of materials from phosphoric acid production plant to assess the workers radiological risks

The industry devoted to the production of phosphoric acid by using as raw material sedimentary phosphate rock (PR) is considered as a NORM activity (Naturally Occurring Radioactive Materials), due to the high levels of U-series radionuclides contained in this ore, which are 1-2 orders of magnitude higher than those in unperturbed soils. This fact allowed us to develop a deep characterization of the raw materials, wastes, main intermediate materials, and final products obtained at a typical phosphoric acid factory. The elemental composition (major, minor and trace elements), radionuclide concentrations, grain size distribution, mineralogy and micro-structural composition were analyzed. The aim of this characterization was to obtain information for operators and maintenance personnel involved in clean-up and waste management operations. The highest concentrations of some heavy metals and radionuclide activity concentrations were found in the "scales" (or internal incrustations)from the pipes that carry either phosphoric acid (PA) or the phosphogypsum waste (PG). The highest concentrations where found for ²²⁶Ra and ⁴⁰K,with values up to 9 and 5 Bq g^-1, respectively. In addition, high concentrations of many toxic heavy metals and trace elements, such as Cd, Cr, Ni, Sr, Y, V, Zn, Th, and U, were found in some sludge samples. The shielding effect of the containers/vessels/pipes has an essential role in the measured external dose in the intermediate products. The radiological implications of natural radionuclides with higher activity showed that if the maximum particulate matter concentration established in the Spanish regulation is verified, and taking into account the most conservative scenario, the annual limit of 1 mSv y^-1 is not exceeded.

An improved method for radium-isotopes quartet determination by alpha-particle spectrometry by using 225Ra (229Th) as isotopic tracer

A new method for the determination of radium quartet (^{223,224,226,228}Ra) in environmental samples by alpha-particle spectrometry with PIPS detectors is described. This uses ²²⁵Ra as yield tracer, in equilibrium at the beginning with ²²⁹Th. Thorium is removed from the sample by using AG1X8 anion-resin, and then radium isotopes are isolated and purified with a cation-exchange column Biorad AG50X8, verifying that Ac has been fully removed from the sample to ensure the good evaluation of the Ra yield (average decontamination factor > 92%). Finally, the counting source of radium is obtained by micro-precipitation with BaSO₄. This method produces high spectral resolution (<35 keV), and quantitative Ra recoveries (>70%). In this new method, NH₄Ac in 0.1 M HNO₃ is used to remove the ²²⁵Ac contained in the sample in order to avoid its future spectral interference in the yield calculation. The method has been validated by using certified reference samples with known concentrations of radium isotopes.

Assessment of natural radionuclides mobility in a phosphogypsum disposal area

The phosphogypsum (PG) stacks located at Huelva (SW Spain) store about 100 Mt of PG, and covers a surface of 1000 ha. It has been very well established in many studies that this waste contains significant U-series radionuclides concentrations, with average activity concentrations rounding the 650, 600, 400 and 100 Bq kg^-1 for ²²⁶Ra, ²¹⁰Po, ²³⁰Th and ²³⁸U, respectively. However, the radionuclide transfer from this repository into the environment by the aquatic pathway will depend on the mobility of each radionuclide. The mobility of the natural radionuclides (U-isotopes, Th-isotopes, 226Ra, and 210Po) contained in the PG piles were evaluated by using the optimized BCR sequential extraction procedure (BCR "Community Bureau of Reference"). The radionuclides were measured in the liquid fractions by alpha-particle spectrometry with semiconductor PIPS detectors. In addition, to validate the obtained results, waters from different locations of the PG piles (pore-water, perimeter channel and edge outflow leachates) were taken and the alpha emitter radionuclides determined. Uranium presents the highest mobility, being its total mobile fraction in the PG around 70%, while 210Po and 226Ra present an intermediate mobility of (around 50% and 30%, respectively). And finally, the Th-isotopes have very low mobility (mobile fraction < 5%), being fixed to the residual fraction. It is noteworthy that this behaviour has been also found in the water samples taken from the stacks, demonstrating that this sequential leaching operational methodology is a useful tool for assessing the release capacity of radionuclides by inorganic wastes.

Validation of the BCR sequential extraction procedure for natural radionuclides

Determining the availability of natural radionuclides in environmental conditions is increasingly important in order to evaluate their toxicity. A validated procedure is necessary to ensure the comparability and accuracy of the results obtained by different laboratories. For that, an optimised BCR sequential extraction procedure has been applied to the certified reference material (CRM), coded as BCR-701, and their resulting liquid and solid fractions were subjected to an exhaustive chemical and radioactivity characterisation. In this sense, several material characterisation techniques were used for chemical, mineralogical, and radioactive characterisation, in order to gain basic information about the obtained fractions. In accordance with the results of this work, the BCR sequential extraction procedure has been validated for the most significant alpha-emitter natural radionuclides (²¹⁰Po, ²³⁴U, ²³⁸U, ²³⁰Th, ²³²Th, and ²²⁶Ra). It has been demonstrated that their mobility is related to the speciation under environmental conditions and the type of radionuclide; we have even found differences between radionuclides of the same element, such as the cases of the pairs ²³⁴U²³⁸U and ²³⁰Th²³²Th, for the BCR-701. In addition, we found that radium was mainly bound to the reducible fraction (Fe and Mn-oxyhydroxides), uranium to the oxidizable fraction (organic matter and sulphides), and that the polonium and thorium isotopes had a high affinity with the particulate phase (non-mobile fraction).

Polonium behaviour in reservoirs potentially affected by acid mine drainage (AMD) in the Iberian Pyrite Belt (SW of Spain)

The province of Huelva is one of the areas most affected by acid mine drainage (AMD) in the world, which can produce big enhancements and fractionations in the waters affected by AMD. There are very few studies on this issue, and none on polonium-210. Twenty-two water reservoirs were sampled, and the (210)Po was measured in both dissolution and particulate phases. The (210)Po concentrations in the waters were in the same order of magnitude to those ones for unperturbed systems, although the data published to particulate matter are very scarce. A mean value and standard uncertainty for (210)Po of 0.25 ± 0.03 mBq L(-1) in the dissolved matter, and 62 ± 9 mBq g(-1) in the particulate matter can be established as base line for the reservoirs of the Huelva area. The distribution coefficients (kd) range from 10(4) to 10(6) L kg(-1), in agreement to the found ones by other authors for the case of neutral waters, but being the lowest values for the more acidic reservoirs. It has been also found that (210)Po has a high tendency to be associated to the particulate matter for neutral-alkaline waters, however, under extreme acid conditions (pH < 3), increases the Po tendency to be associated to the dissolved phase. Therefore, the main conclusion obtained in this work is that AMD has no a significant influence on the total activity concentration of (210)Po in the waters of reservoirs, but the acidity has a clear influence on its distribution between both dissolved and the particulate phases.

Mobility of Po and U-isotopes under acid mine drainage conditions: an experimental approach with samples from Río Tinto area (SW Spain)

Under acid mine drainage (AMD) conditions, the solubilities and mobilities of many elements are vastly different from conditions prevailing in most natural waters. Studies are underway in the Río Tinto area (Iberian Pyrite Belt), in order to understand the behavior and mobility of long-lived U-series radionuclides under AMD conditions. A set of leaching experiments utilizing typical country rocks from the Tinto River basin, waste rock pile composite materials, iron-rich riverbed sediments and gossan (weathered naturally rock) were performed towards this purpose. Initial leaching experiments using distilled water kept in contact with solid material for 300, 100, 50 and 1 h resulted in very low concentrations of U with (234)U/(238)U activity ratios close to equilibrium and activity concentrations of (210)Po < 0.03 mBq/g. Leaching experiments performed with sulfuric acid media (0.1 and 0.01 M), and contact times between the solid and solution for 24 h were conducted to quantify the amount of U-isotopes and (210)Po leached, and the radioactive disequilibria generated between the radionuclides in the leachate. These experiments show that Po mobility in acidic conditions (pH around 1-2) is very low, with (210)Po activity in the leachate to be 6% in average for the solid sample. By contrast, mobility of U-isotopes is higher than that of Po, around 1.2%.

Radioactive characterization of leachates and efflorescences in the neighbouring areas of a phosphogypsum disposal site as a preliminary step before its restoration

After the recent closure of certain phosphoric acid plants located in the South-West of Spain, it has been decided to restore a big extension (more than six hundred hectares) of salt-marshes, where some million tonnes of phosphogypsum (PG), the main by-product generated by these plants, had been disposed of. This PG is characterized by its content of high activity concentrations of several radionuclides from the uranium series, mainly (226)Ra, (210)Pb, and (210)Po and, to a lesser extent, U-isotopes. The PG disposal area can be considered as a potential source of radionuclides into their nearby environment, through the waters which percolate from them and through the efflorescences formed in their surroundings. For this reason, a detailed radioactive characterization of the mentioned waters and efflorescences has been considered essential for a proper planning of the restoration tasks to be applied in the near future in the zone. To this end, U-isotopes, (234)Th, (230)Th, (226)Ra, (210)Pb and (210)Po activity concentrations have been determined by applying both alpha-particle and gamma-ray spectrometric techniques to selected water and efflorescence aliquots collected in the area. The analysis of the obtained results has enabled to obtain information about the geochemical behaviour in the area of the different radionuclides analyzed; and the conclusion to be drawn that, in the restoration plan under preparation, both the prohibition of outflowing waters from the disposal area to the neighbouring salt-marshes, and the removal of all the efflorescences now disseminated in their surroundings are essential.

Occupational exposures in two industrial plants devoted to the production of ammonium phosphate fertilisers

In order to fill a gap in the open literature, occupational exposures and activity concentrations have been assessed in two NORM industrial plants, located in the south-west of Spain, devoted to the production of mono-ammonium phosphate (MAP) and di-ammonium phosphate (DAP) fertilisers. The annual effective doses received by the workers from these plants are clearly below 1 mSv yr(-1) and the contribution due to external radiation is similar to that due to inhalation. The contribution to the maximum effective doses due to inhalation of particulate matter has been estimated to be about 0.12 mSv yr(-1), while the (222)Rn concentrations inside the plants are of no concern. Consequently, no additional actions or radiological protection measures need to be taken to decrease the natural radiation received by the workers in these facilities.

Physico-chemical and radioactive characterization of TiO2 undissolved mud for its valorization

In order to find a potential valorization of a waste generated in the industrial process devoted to the production of TiO(2) pigments, and as an essential and basic step, this waste must firstly be physically and chemically characterized. Moreover, the content of radioactivity is taken in to account due to it comes from a NORM (Naturally Occurring Radioactive Material) industry. With this end, microscopic studies were performed by applying scanning electron microscopy with X-ray microanalysis (SEM-XRMA), while the mineralogical compositions were carried out by means of the X-ray diffraction (XRD) technique. The concentrations of its major elements were determined by X-ray fluorescence (XRF), while heavy metals and other trace elements were ascertained through Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The results obtained for this waste have revealed several lines of research into potential applications. Firstly, with the refractory properties of mineral phases observed leading to a possible use in the ceramics industry or in thermal isolators. And secondly, attending to the characteristic particle-size spectra can be used as an additive in the manufacture of cement and finally, its high concentration of titanium may be used as a bactericide in brick production.

Modelling the behaviour of 210Po in high temperature processes

In several Naturally Occurring Radioactive Material (NORM) industries, relatively high temperatures are used as part of their industrial processes. In coal combustion, as occur in other high temperature processes, an increase of the activity concentration of every natural radioisotope is produced both, in residues and by-products. An additional increase can be observed in the activity concentration of radionuclides of elements with low boiling point. This work is centred in the increase of polonium, more precisely in its radioisotope Po-210, present in the natural chains, and with a half-life long enough to be considered for radiation protection purposes. This additional increase appears mainly in the residual particles that are suspended in the flue gases: the fly-ashes. Besides, scales, with a high concentration of this radioisotope, were observed. These scales are produced on surfaces with a temperature lower than the boiling point of the chemical element. Both, the accumulation in particles and the production of scales are attributed to condensation effects. When effective doses for the public and the workers are evaluated, taking into account these increases in activity concentrations, the use of theoretical models is necessary. In this work a theoretical description of those effects is presented. Moreover, a verification of the predictions of the model was performed by comparing them with measurements carried on in coal-fired power plants. The same description here presented is applicable in general to the behaviour of Po-210 in other NORM industries where high temperature processes involving raw materials are used, as can be ceramic, cement production, tiles production or steel processing.

Physicochemical characterization of raw materials and co-products from the titanium dioxide industry

The present study was conducted to characterize several raw materials and co-products from the titanium dioxide industry in relation to their elemental composition (major, minor and trace elements), granulometry, mineralogy, microscopic morphology and physical composition. The main objective was to gain basic information for the future potential application of these co-products in fields such as agriculture, construction, civil engineering, etc. Microscopic studies were performed by applying scanning electron microscopy with X-ray microanalysis (SEM-XRMA) while the mineralogical compositions were analysed by means of the X-ray diffraction (XRD) technique. The concentrations of major elements such as Na, Al, Si, Ca, Ti, Fe, S and K were determined by X-ray fluorescence (XRF), while heavy metals and other trace elements were determined by ICP-MS. The physicochemical characterization of the raw materials used in the titanium dioxide industry, in addition to the characterization of the co-products generated, has enabled the evaluation of the degree of fractionation of different elements and compounds between the different co-products, as well as the control of the possible variations in the physicochemical composition of the raw materials throughout the time and the study of the influence of these variations in the characteristics of the obtained co-products. As a main conclusion of our study, it is possible to indicate that the levels of the pollutant elements associated to the co-products analysed were, in general, within safe limits and, therefore, they could potentially be used in composites as fertilizers or for building materials in road construction, etc. Nevertheless, for the specific application of each of these co-products in agriculture, construction and civil engineering, additional studies need to be performed to evaluate their appropriateness for the proposed application, together with specific studies on their health and environmental impact.