Uranium Deportment Studies: Beyond the Assay

URANIUM DEPORTMENT STUDIES: BEYOND THE ASSAY

By

1,2Brandon Youlton, 1Louis Coetzee, 1Lisa Scott, 1Johan O’ Connell,3Ronel O’ Connell,
2Judith Kinnaird

1SGS, South Africa

2The University of the Witwatersrand, South Africa

3AngloGold Ashanti, South Africa

Presenter and Corresponding Author

Brandon Youlton

ABSTRACT

The nature and mode of occurrence of uranium minerals, as well as their associated gangue, control reagent consumption during leaching, the rate of uranium dissolution and uranium recoveries. Traditionally, mineralogy was used to resolve difficulties in metallurgical testwork programs; however this is no longer the case. Mineralogical investigations are now included in the early stages of exploration. Samples from new uranium prospects are often sent for mineralogical investigation as soon as the assays confirm grade. The uranium deportment study is adapted to obtain suitable information from exploration through to feasibility. In the early stages of exploration, qualitative or semi-quantitative mineralogical data is adequate; indicating whether or not a prospect contains high concentrations of refractory uranium minerals and/or deleterious gangue. During a feasibility study, quantitative mineralogical data, produced by modern automated techniques, can be used to guide the metallurgical testwork program. Characteristics such as uranium phase speciation, grain size distribution, mineral associations and degree of exposure are quantified. This makes it possible to determine optimal processing conditions more rapidly and cost effectively.

INTRODUCTION

While grade is a vital characteristic in assessing a uranium prospect, it is not the only characteristic of importance. If the uranium occurs within refractory minerals and/or is associated with deleterious gangue, even a high grade occurrence may be uneconomic. Alternatively an uneconomic uranium prospect may become viable if the uranium occurs with other economic minerals.

The processing response of an ore is determined by the nature of its constituent minerals(1). Maximum recoveries are largely determined by the uranium minerals, while the gangue, by virtue of its higher concentration, typically accounts for the majority of reagent consumption. Because of this link between mineralogy and processing response, it is possible to use the tools of mineralogy to predict the behaviour of a particular ore. Information could be gathered that could assist in each phase of the metallurgical testwork program.

While a quantitative deportment study, of the sort used to guide a metallurgical testwork program, is not desirable in early exploration, a rapid, and relatively inexpensive, semi-quantitative study could recognise deleterious characteristics early in the exploration program. This could potentially prevent expending valuable exploration budgets on deposits that make grade, but will ultimately prove impractical to process.

METHODOLOGY

Depending on the needs of the project, a uranium deportment study could include any, or all of the following processes; screening and grading, heavy liquid separation, gangue characterisation and ore characterisation.

Grading and Heavy Liquid Separation

Grading and heavy liquid separation (HLS) make is possible to assess the potential methods of upgrading the ore.

For grading analysis the sample is milled to a particular grind and screened into various size fractions. These fractions are then assayed to determine if the uranium is concentrated in a particular fraction. The grade of the various fractions is not the only consideration. If a particular fraction is very high grade, but contains a very small amount of the uranium present in the head sample, then the grading was unsuccessful. For this reason it is important to determine the uranium distribution through the various size fractions.

Similarly heavy liquid separation can be used to assess the potential to upgrade the ore based on density. Due to the high specific gravity of uranium minerals [uraninite (7.5-9.7) or carnotite
(4.7-5.0)2] it is occasionally possible to concentrate uranium in heavy liquid sink fractions. While both grading and density based separation techniques often form part of a metallurgical test work program, mineralogical examination of the various products usually offers valuable insight.

Gangue Characterisation

A number of tools are available for characterisation of the gangue. These include X-ray diffraction (XRD), optical microscopy and automated scanning electron microscopy. X-ray diffraction identifies minerals based on their crystal structure and is one of the most valuable tools for mineral identification. XRD analyses may be either semi-quantitative (making use of reference intensity ratios) or quantitative (making use of RietveldRefinement(3)). XRD is also the only mineralogical tool which can accurately characterise clay minerals. Unfortunately XRD is limited by a relatively high lower detection limit(4) (often in the order of a few percent) and it is therefore not possible to detect uranium minerals in the head sample, unless the sample is particularly high grade. However, since gangue minerals typically make up the bulk of the sample, and are the major reagent consumers, XRD plays an important role in a uranium deportment study.

If quantitative mineral abundances are needed (as fora full uranium deportment), the XRD may be augmented by automated scanning electron microscopy.Automated scanning electron microscopes (SEMs), such as the QEMSCAN, scan through polished sections of the sample and identify minerals using their chemical composition. The identification is based on the energy dispersive X-ray spectrum (EDS) produced by the electron beam at the mineral surface, and on the backscattered electron (BSE) intensity. The X-ray spectrum is a function of the chemical composition, and the BSE intensity is related to the average atomic mass of the mineral(5). Techniques such as QEMSCAN BMA (bulk mineralogical analysis) determine the major mineral composition of a sample by analysing tens to hundreds of thousands of points along line scans(6). The data resulting from these analyses is then processed to determine the modal mineral composition of the sample.

Because XRD and BMA use different characteristics to identify minerals, it is possible to distinguish minerals with very similar crystal structures (by BMA) and with very similar chemical compositions (using XRD). The BMA also offers a lower detection limit than is currently possible by XRD analysis.

While XRD and BMA offer convenient means of determining the major mineralogy of a sample, they conveylittle or no textural data. In order to understand the textural characteristics of the gangue it is often useful to combine XRD analyses with optical petrography.

Ore Mineral Characterisation

Determining which minerals host the uranium (and in what proportions) is the most important stage of a uranium deportment study. In exploration samples, it is usually not necessary to quantify the uranium speciation. In this type of studies manual SEM petrography is used to determine the size and mode of occurrence of the uranium minerals, and EDS analyses make it possible to identify the uranium mineral. It is not always possible to distinguish certain uranium minerals based on EDS spectrum aloneas there are several uranium minerals which have very similar chemical compositions,e.g.autunite [Ca(UO2)2(PO4)2.10H2O] and meta-autunite [Ca(UO2)2(PO4)2.2.5H2O] or uranophane [Ca(UO2)2(SiO3)2(OH)2.5H2O] and beta-uranophane [Ca(UO2)2(SiO3)2(OH)2.5H2O](7). However, such distinctions are usually of academic importance since the two possible minerals generally have very similar leach responses.

Figure 1: A SEM BSE micrograph showing a grain of autunite/meta-autunite (Aut/Meta-aut) (A),and an EDS spectrum showing the composition of the grain (B).

If it is essential to distinguish between uranium minerals with very similar chemical composition, thesegrains may be picked from the sample and identified by XRD.

Figure2 illustrates an X-ray diffractogram collected from a few grains of meta-autunite that had been milled and analysed. These were picked from a sample and analysed to determine if the sample contained autunite or meta-autunite. The X-ray diffractogram clearly shows that the uranium mineral is meta-autunite.

Figure 2: X-ray diffractogram of meta-autunite (with traces of quartz and dolomite). In order to determine whether a sample contained autunite or meta-autunite this material was picked from the sample and analysed. The XRD analysis indicates that meta-autunite is present. The arrow shows where the autunite peak would have been if it had been present.

In particularly high grade samples (greater than ~1% U3O8) it may be possible to detect uranium minerals by XRD.

Figure 3 illustrates a diffractogram collected from a very high grade sample. The diffractogram shows that the uranium occurs in uraninite and coffinite.

Figure 3: X-ray diffractogram of a very high grade sample. This sample was dominated by uraninite with lesser coffinite, minor albite and trace amounts of galena. Due to the comparatively high detection limit of XRD, it is usually not possible to detect uranium minerals by this method (except in particularly high grade samples such as this one).

Automated scanning electron microscopes make it possible to quantitatively determine the proportion of the total uranium hosted by each uranium mineral. This is of particular importance for samples that contain both readily leachable and refractory uranium minerals. Methods such as the QEMSCAN TMS (trace mineral search) scan polished sections to find minerals with a high BSE. Due to their high average atomic number, uranium minerals are readily found. Once the high BSE grains are detected, the QEMSCAN maps out the grain along with the particle in which it occurs(6). The area of each uranium mineral is used to determine volume and densities are used to determine masses. These masses, and the weight percent uranium present in each mineral, are then used to determine the proportion of the total uranium hosted by each phase. The data gathered by the TMS also makes it possible to quantify the liberation, exposure and association characteristics of the uranium phases.

LINK BETWEEN MINERALOGY AND PROCESS CHARACTERISTICS

Once the mineralogical data has been collected, it can be used to make predictions as to the process characteristics of a particular ore. The main advantage of a uranium deportment study is that it makes it possible to use a single method to gather information that can aid the entire processing flowsheet from comminution to environmental considerations associated with tailings disposal. While a uranium deportment is not a substitute for good quality metallurgical testwork, it frequently aids in rapidly determining the most efficient and cost effective process route for a particular ore. Mineralogical investigation also makes it possible to scope the metallurgical testwork for a particular ore, rather than having to rely on a generic methodology.

Comminution

The grain size and textural characteristics of the ore minerals determine the optimal grind. If the ore minerals are particularly coarse-grained, occur in porous gangue, or tend to occur along fractures or cleavage planes, minimal comminution may be needed. However for finer grains, that are not easily exposed, comminution is an essential step. The amount of amount of energy consumed in grinding, and the degree of wear on equipment depends both on the specific gangue minerals that make up the rock, and on their textural relationship.

Figure 4 shows X-ray diffractograms collected from an arkosic sandstone from a Karoo uranium prospect, and a granite from a Namibian uranium prospect. The similarities between these two patterns indicate that they have similar mineralogical compositions; however optical micrographs (Figure 5) show that they have very different textures. The sandstone is cemented by calcite (a soft, easily milled mineral), while in the granite the quartz and feldspar grains have an interlocking texture vaguely resembling a puzzle. Because of these textural differences Karoo sandstones mill quickly, while Namibian granites are hard, and can result in excessive wear on crushers and mills(8).

Figure 4: X-ray diffractograms showing the compositions of an arkosic sandstone from a Karoo uranium prospect (red solid pattern) and from a Namibian granite hosted uranium prospect (blue dashed pattern). The similarity between these two patterns indicates that the samples have similar mineral compositions.

Figure 5: Optical micrographs of an arkosic sandstone from a Karoo uranium prospect (A) and granite from a Namibian granite hosted uranium prospect (B). Although XRD analyses show that these samples have similar mineral compositions, texturally they are completely different. The quartz (Qtz) and plagioclase (Plag) in the sandstone is cemented by soft calcite, while the quartz and K-feldspar in the granite have an interlocking texture vaguely resembling puzzle pieces.

Preconcentration

Preconcentration can be achieved by a number of different methods including radiometric sorting, flotation, size classification and dense media separation. The method that produces the best results will depend on the nature of the ore.

Since radiometric sorting simply uses radioactivity as a proxy for uranium grade, there is little scope for mineralogy in assessing the potential for radiometric sorting. However, the degree of secular equilibrium in a particular ore strongly influences the relationship between radioactivity and grade. For this reason, secular equilibrium should be investigated before attempts are made to use radiometric sorting as a means of preconcentration.

Flotation

In order to achieve effective upgrading of an ore by flotation, it must be possible to selectively float the ore minerals, while excluding the gangue to the maximum extent possible. Optical and scanning electron microscope examination of Witwatersrand ores(Figure 6) show that there is a strong association between uranium minerals (particularly uraninite) and organic carbon, in the form of kerogen. Because the carbon is easily floated it possible to use flotation to upgrade the associated uranium.

Figure 6: An optical micrograph (A) showing uraninite (Ur) and gold (Au) associated with carbon (C); and a BSE micrograph (B) showing partially altered uraninite and galena (Gn) occurring with carbon.

It is also important to recognise naturally floating gangue minerals. These report to, and dilute the flotation concentrate. XRD analyses show that the Witwatersrand ores contain significant amounts of pyrophyllite. The abundance of pyrophyllite is determined as part of a uranium deportment on these ores. Based on these abundances, it is possible to estimate the extent of dilution that the pyrophyllite will cause. If this proves to be excessive, measures can be taken to suppress or remove the pyrophyllite.