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Laboratory Flotation and Chemical Purification Tests

13.12.2021

Ostrobothnia, Kupukka

  • Flotation concentrates of around 90 % C
  • Chemically purified end product up to 98 % C with a liberation degree of graphite 99.7 %
  • Total carbon recovery 82.4 %

Bench scale test work was conducted on a composite drill core sample in length of 12.90 m, at the GTK Mintec laboratories, Outokumpu, during May-November 2021. The mineralogy of the sample and beneficiation of graphite by flotation and chemical purification were investigated. The aim of the work was to get preliminary information about the beneficiation potential of graphite.

The graphitic carbon grade of the sample was 5.5–5.8 %. According to MLA, the main minerals in the sample were quartz, K-feldspar, chlorite, plagioclase, biotite, pyrite, graphite, cordierite, clay minerals, garnet, and muscovite.

Totally 3 flotation tests were made on the sample. Grind size, flotation reagents and their dosages, pulp pH and flotation time were the main variables studied in these tests. Unnecessarily fine grinding was tried to be avoided to recover graphite as large flakes as possible. Therefore, grinding was mostly performed in stages. Flotation concentrates with almost 90 % C were obtained. Chemical purification tests consisting of alkali roasting and acid leaching showed that the carbon grade of the flotation concentrates could be upgraded up to 98 % with high recovery.

If the whole process, i.e., combination of flotation and chemical purification is considered, the total carbon recovery was 82.4 %. According to the MLA study, the liberation degree of graphite was 99.7 % in the final product.

The results of this test work encourage to continue the exploration within the Kupukka reservation by Suomen Malmitutkimus Oy. However, more tests would be needed to optimize the process parameters, both flotation and chemical. In addition of this test work the degree of crystallinity and crystallization temperature of graphite will be studied in near future at the GTK Mintec by Raman spectroscopy. Below images of graphite-bearing mica gneiss, graphite concentrates of different grain sizes and chemically purified graphite concentrate.

Graphite-bearing mica gneiss. Graphite is the light-colored.

Graphite flotation concentrate of different grain sizes. One division of the reticle is 0.5 mm.

Chemically purified graphite concentrate.

Southern Savo

Bench scale test work on a bed rock sample containing flake graphite was conducted at the GTK Mintec laboratories, Outokumpu during May-November 2021. The mineralogy of the sample and beneficiation of graphite by flotation and chemical purification were investigated. The aim of the work was to get preliminary information about the beneficiation potential of graphite.

The graphitic carbon grade of the sample was around 0.8 %. One aim of the test work was to study if this kind of low-grade graphite-bearing rock can be concentrated. According to MLA, the main minerals in the sample were quartz, K-feldspar, plagioclase, biotite, cordierite, Fe-oxides and Fe-oxyhydroxides, garnet, and graphite.

Totally 6 flotation tests were performed on the sample. Grind size, flotation reagents and their dosages, pulp pH and flotation time were the main variables studied in these tests. Unnecessarily fine grinding was tried to be avoided to recover graphite as large flakes as possible. Therefore, grinding was mostly performed in stages. Flotation concentrates with almost 90 % C were obtained. Chemical purification tests consisting of alkali roasting and acid leaching showed that the carbon grade of the flotation concentrates could be upgraded up to 98% with high recovery.

If the whole process, i.e., combination of flotation and chemical purification is considered, the total carbon recovery was only 55.5 %. Higher total C recovery would have been expected if a flotation concentrate with higher recovery had been chosen for the chemical purification.

Based on this preliminary test work graphite beneficiation is possible also for a low-grade starting material. More tests would be needed to optimize the process parameters, both flotation and chemical. It should be noted also that the sample for this test work was from bedrock surface with some weathering and oxidation of some minerals. This may be reflected in the study results. Thus, a drill core sample is preferred for possible future test works. By drilling, probably also the sample grade would be higher (best EM anomalies locate outside graphite-bearing outcrops). The degree of crystallinity and crystallization temperature of graphite will be studied in near future at the GTK Mintec by Raman spectroscopy.

This test work, together with the previous studies in southern Savo by Suomen Malmitutkimus Oy, provides novel and important information on the occurrence and beneficiation potential of graphite, which now can be applied also to other areas in Finland with similar bedrock geology.

Below images of graphite concentrates by different grain sizes and chemically purified graphite concentrate.

Graphite flotation concentrate of different grain sizes. One division of the reticle is 0.5 mm.

Chemically purified graphite concentrate.

Update of graphite surveys in southern Savo

26.04.2021

During the summer of 2020, in the Puumala, Sulkava and Juva areas, outcrops were mapped in or near the regional airborne EM anomalies. Grab samples by hammer were taken from outcrops containing graphite. The aim was to obtain samples representative of the average graphite content in the outcrop. The samples were subjected to chemical analysis in two different laboratories, ALS Finland Oy and Eurofins Labtium Oy. Part of the samples were assayed for both graphitic and total carbon. Polished thin sections were prepared for microscopic studies, both from rock slices cut perpendicular and parallel to the main schistosity of the rock.

At the turn of September-October, geophysical measurements were made at a few sites to determine the electrical conductivity of the bedrock in the area. The measurements were made with portable GEM-2 equipment and the contractor was the Geological Survey of Finland (GTK), which has extensive experience in these measurements. A report of the measurements was received at the turn of the year. In early 2021, petrophysical measurements were purchased from GTK to determine the physical properties of individual samples, such as electrical conductivity, magnetic susceptibility, and density.

About the results

The host rock for the disseminated graphite occurrences is a garnet-cordierite gneiss. The most common main minerals are quartz, plagioclase, garnet, cordierite and biotite, in places also orthopyroxene and potassium feldspar. Pyrrhotite is the dominating sulphide mineral.

The graphitic carbon contents of the chemically analysed samples remained quite low, at highest slightly below 3 %. The samples do not contain carbonate so the difference between the graphitic carbon and total carbon contents is minimal. The sulphur contents of the samples are low, usually less than 1 %. However, this is since pyrrhotite in the bedrock samples is in many places partially oxidized and sulphur is dissolved away.

Based on GEM-2 measurements, the EM anomalies are located next to the observed outcrops. It is therefore possible that there are higher concentrations of graphite next to the outcrops, under the soil. The samples, measured for the petrophysical parameters, show high galvanic and inductive resistance. Sampling from outside of the outcrops would require drilling or excavation.

Based on preliminary microscopic studies, the quality of graphite is good. The length of the graphite flakes is about 1 mm in many places and up to 3 mm in some places. Impurities are minor. Below a few micro-graphs of graphite.

Graphite flakes in gneiss
Graphite flakes at the main schistosity plane of the gneiss
Close-up of a graphite flake

Continuation of surveys

Graphite samples are still subjected to chemical analyses and laboratory tests. Some places will be mapped and sampled in the field during the summer of 2021.