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Utvinning av monazit från ett zirconavfall genom flotation vid Namakwa Sands, Sydafrika
2014 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

Namakwa Sands is a mining company operating on the west coast of South Africa, producing zircon, rutile and ilmenite. Heavy mineral sands are extracted by wet gravity, magnetic and electrostatic separation. In order to maintain their zircon product specifications, particularly with respect to radioactivity, the reject stream may still contain significant grades of zircon but is associated with penalty elements such as titanium dioxide (TiO2), uranium (U) and thorium (Th). The radioactive components U and Th are associated with the mineral monazite and cause issues in handling and disposal of the material.If monazite could be removed from the reject stream, the remaining material could potentially be sold as a low-grade zircon product. Furthermore, monazite is a valuable mineral in itself due to its content of rare earth elements such as cerium and lanthanum, which are valued in the nuclear and wind-power industries. The separation of monazite from the zircon reject stream is therefore desirable to Namakwa Sands from both an economic and an environmental perspective, by reducing the amount of waste and increasing revenue. Flotation of heavy minerals is rarely practiced due to the effectiveness of gravity and electrostatic separation of coarse particles. However, the physical separation at Namakwa Sands is challenged by fine particle sizes and surface coating issues affecting the surface properties and thus the separation. Froth flotation, which operates within finer particle sizes, is a possible separation method that in combination with surface attrition could selectively separate monazite.The objective of this project is to investigate the use of reverse froth flotation to separate monazite from the final zircon reject stream to obtain a higher-grade zircon product that meets product specifications. The accompanying investigation of the mineralogy and surface coatings of the material will also be critical in understanding the effect of the flotation factors on the success of monazite flotation. This project is broadly divided into two phases of flotation experiments with an accompanying mineralogical investigation of the flotation products. In the first phase of the project, the aim was to find flotation conditions where monazite could selectively be removed from the pulp, i.e. by using reverse flotation. A statistical screening design in the statistical software MODDE (©Umetrics) was used to find the significant parameters and the optimum flotation settings. The investigated parameters were pH, collector type, collector dosage, depressant type, depressant dosage and ultrasonication. The most common monazite collectors in flotation were reported to be oleates, hydroxamates and amine based collectors. More specifically, oleate and hydroxamate collectors have been used for the separation of monazite and zircon, and were therefore the collectors that were used in the first phase.In the second phase of the project, fewer parameters, and narrower intervals were used. More intense mechanical attritioning was also applied in this phase using a custom-built mini pin mill. In two full factorial designs, collector dosage, depressant dosage, pH and attrition intensity were studied.The hydroxamate collector was found not to be selective for any of the minerals, but the oleate collector could selectively recover monazite at a pH of 10 and a collector dosage of 180 g/ton. Further investigation showed that an oleate collector dosage of 315 g/ton resulted in a monazite recovery of 55.6%. The best monazite grade was 71.7% with the oleate collector at a dosage of 215 g/ton and a pH of 10, however the recovery was low with 28.4%. Results showed that selectivity was highly dependent on pH. At a pH value of 9, no selectivity was obtained, and more than 50 % of the pulp was recovered to the concentrate in three minutes. An increase of pH to 11 resulted in only foam recovery. Monazite selectivity was obtained between these two extremes, at a pH of 10. The flotation system was extremely pH-sensitive, which indicates that the point of zero charge for the Namakwa Sands zircon and monazite may lie within this narrow pH interval.The accompanying mineralogical study showed that the final zircon reject composed of mainly zircon, between 70% and 83%. The rutile content was around 7% and the monazite grade varied between 3% and 6%. The mineralogy investigation included SEM, QEMSCAN and grain counting results, and besides the bulk mineralogy, surface coatings were studied. Surface coatings on both monazite and zircon were highly associated with SiO2, which has previously been detected as a common surface coating in the deposit in the form of opaline silica. SEM and QEMSCAN were used as tools to detect the degree of surface coatings before and after attritioning. It did not conclusively show any effect of the surface attritioning, which therefore remains unknown. Furthermore, the bulk mineralogy of flotation products was studied. It was shown that the concentrates from the tests with high monazite selectivity were enriched in garnets. The oleate collector may therefore be selective for both monazite and garnets at a pH of 10.The key finding of this project is that monazite can be successfully separated from zircon with an oleate collector at a pH of 10. A monazite recovery of 55.6% was possible; however further research into staged flotation to further improve the zircon-rich tail is needed. The results of surface attritioning are not entirely clear, which gives scope for further investigation into mechanical attritioning methods and intensities.

Place, publisher, year, edition, pages
2014. , 118 p.
Keyword [en]
Technology, Mineralogy, Flotation, Surface coatings, Surface characteristics, Monazite, Zircon, Attritioning
Keyword [sv]
Teknik, Flotation, mineralogy, surface coatings, surface characteristics, attritioning, monazite, Zircon
URN: urn:nbn:se:ltu:diva-45794Local ID: 37554dcd-359d-45f9-a323-69e4b0d926b6OAI: diva2:1019091
External cooperation
Subject / course
Student thesis, at least 30 credits
Educational program
Sustainable Process Engineering, master's level
Validerat; 20141020 (global_studentproject_submitter)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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