| dc.description.abstract | In the zinc smelters at Boliden Odda, the zinc is extracted from the purified zinc sulphate solution by electrowinning. If the purified zinc sulphate contains more than 35 mg/L fluoride, the extracted zinc cannot be removed from the electrodes after electrowinning. At Boliden fluoride is removed by adsorption on iron oxide-hydroxide in the neutral leaching process. The neutral leaching process at Boliden consists of 6 tanks. Calsine from the roasting is mixed with the solution from jarosite part, JFF11 and spent acid H2SO4 from the cell house, in the neutral leaching process. The pH in each tank is increased by adding more calsine in each tank.
To find out which compound who is contributing most to Fluoride adsorption at Boliden, samples from neutral leaching tank 2 and 5 (NLT 2 and NLT 5), and thickener (Dorr) have been analysed. Through XRD was it found out which compound the samples consist of, and performed analyses to find out which of these compounds are contributing mostly to the Fluoride adsorption. The calsine has also been analysed in fluoride adsorption experiment to see how much calsine is contributing to fluoride adsorption. Data from Boliden shows that most Fluoride is adsorbed in tank NLT 2. The amount of adsorbed mg Fluoride per gram iron hydroxide is 25 mg/g for tank NLT2 according to data given from Boliden.
The software from XRD suggest that the Boliden samples could consist of iron hydroxide fluoride (Fe3O3.5F0.5), cobalt zinc chorium zinc titanium oxide (Zn0.423Ti0.25Cr0.227Fe0.927CoO4 ), magnetite, franklinite (ZnFe2O4), zinc oxide (ZnO), cobalt iron zinc sulfide (Co0.105Fe0.1Zn0.795S), lepidocrocite, copper nickel zinc indium, iron Oxide(Cu0.25Zn0.25Fe1.7Ni0.5In0.3O4), copper iron Oxide fluoride (Cu2FeO3F) , sodium , zinc sulfide, willemite (Zn2SiO4) and beaverite(PbCuFe1 5Al0.5(SO4)2(OH)6). XRD patterns of Boliden samples show that the sample contains some amorphous/poorly crystalline compound, which could be schwertmannite and ferrihydrite
Through BET analysis the surface area of sample NLT 2 was measured to be in the range 10 - 19 m2/g, sample NLT 5 was measured to be in the range 87 - 150 m2/g and sample Dorr was measured to be in the range 37.8 - 138 m2/g. The increase of surface area can be explained by a formation of flakes, which are seen in SEM pictures of the samples.
The results from Fluoride adsorption experiments with Boliden samples show that samples from NLT2, NLT5 and Dorr adsorb on average similar of fluoride per gram adsorbent. NLT 2 adsorb on average 190 mg/g (12 mg/m2) , NLT 5 160 mg/g (1.2 mg/m2) and Dorr 160 mg/g (1.2 mg/m2). Samples from NLT 2 adsorb more fluoride then sample from NLT 5 and Dorr per m$^{2}$. NLT 2 adsorb ten times more fluoride than NLT 5 and Dorr per m2. The compound which contributing mostly to fluoride adsorption have a low surface area and are very efficient. The calsine samples adsorbed on average 10 mg/g, which much less than NLT , NLT5 and Dorr. From the calsine Fluoride adsorption experiment can it be concluded that calsine is not contributing to Fluoride adsorption at Boliden.
The XRD patterns of the Boliden samples indicated that the samples could contain the iron hydroxide oxides lepidocrocite and magnetite. From the literature and industry is it understood that iron hydroxide oxides are contributing most to fluoride adsorption. Since magnetite and lepidocrocite are the iron hydroxide oxide which is present in the sample, was these compounds analysed in fluoride adsorption experiment. To understand how much magnetite and lepidocrocite is contributing to Fluoride adsorption, magnetite and lepidocrocite were prepared in the laboratory, and analysed in fluoride adsorption experiment and compared the results with data at Boliden. The results from the fluoride adsorption experiment show that lepidocrocite adsorbed 0.05 mg Fluoride per m2 ( 6mg/g), and magnetite adsorbed 0.25 mg Fluoride per m2 ( 2mg/g). From the results of Fluoride adsorption experiment with Magnetite and lepidocrocite can it be seen that lepidocrocite and magnetite are not adsorbing as much Fluoride as Boliden samples. Based on these results can it be concluded that magnetite and lepidocrocite is not the main contributor for Fluoride adsorption at Boliden.
The compound which contributes most Fluoride adsorption must have been precipitated in tank NLT 1 or NLT 2 since all sample adsorbs as much Fluoride per gram adsorbent. This compound could be an amorphous/ poorly crystalline compound since the XRD pattern of Boliden samples indicated that the samples contain some amorphous/ poorly crystalline compound. The amorphous/ poorly crystalline compound could be schwertmannite and/or ferrihydrite. According to literature are schwertmannite and ferrihydrite adsorbing 8 and 15 mg/g fluoride respectively. From the results of fluoride adsorption on lepidocrocite and magnetite, and literature value of how much fluoride schwertmannite and ferrihydrite adsorbs. Can it be seen that adsorption on iron hydroxide is not enough to remove the fluoride at Boliden. There must be another mechanism or adsorption with an unidentified compound. XRD software suggest that sample could contain the crystals iron oxide fluoride and copper zinc iron oxide fluoride, the fluoride in the solution could have been removed by precipitation of crystals. | en |
