In the RAWMINA project, the critical metals antimony and cobalt are bound to sulphide minerals as trace elements in processing residues from copper extraction, so-called flotation tailings. There are microorganisms that are able to dissolve these sulphides, such as pyrite (FeS2) or chalcopyrite (CuFeS2), which are present in the secondary raw materials (tailings). As iron is the main metal in these sulphides, a technology is required to separate the high iron and low antimony and cobalt concentrations. This is done by downstream processes utilising the different chemical-physical properties and serves the aim of recovering as many of the valuable materials contained in the tailings as possible.
After microbiological leaching of sulphidic flotation tailings from the Tharsis deposit, a solution was obtained which, in addition to 20 - 45 g/L iron, 60 g/L sulphates, also contained 300 mg/L copper, 200 m/L arsenic, 30 mg/L cobalt, 10 mg/L antimony, 600 mg/L calcium and organic substances of the biomass. The pH value was determined to be 0.8 - 1.5.
To enable the separation of iron from the pregnant leaching solution, a technological idea first had to be developed. Therefore, a concept for a multi-stage process was developed to enable the highest possible recovery of all usable contents.
Iron separation stage
The following protocol was processed on a lab-scale and transferred into large-scale tests:
1. Addition of H2O2 up to Eh > 850 mV - Oxidation of (residual) Fe(II) to Fe(III)
2. Dilution of the solutions 1 : 2 to 1 : 4 to Fe concentrations < 10 g/l
- Dilution is necessary because the precipitated Fe phases are extremely voluminous (hardly compacted by sedimentation, centrifugation or filtration - dry matter at max. 15 %)
- Wash water from the Fe phases can be used to dilute the pregnant
3. Increasing the pH to 2.4 with 10% magnesium oxide-suspension - Precipitation of Schwertmannite
- Pre-precipitation - Preferential sorption of antimony(V) compared to arsenic(V) on a small amount of Schwertmannite (enrichment of Sb)
4. Solid-liquid separation - separation of antimony-containing Schwertmannite
5. Increasing the pH to 3.3 with 10% magnesium oxide-suspension
- Main precipitation - Total precipitation of iron as Schwertmannite
- Complete sorption of arsenic(V) to the iron phases (Schwertmannite contains approx. 0.35% As and could be further used for the sorption of arsenate, molybdate, phosphate, vanadate, chromate, selenate, antimonate (pH range 3 - 6)
- (maximum absorption of 8.0 mass% As)
6. Solid-liquid separation - Separation of the arsenic-containing Schwertmannite from the valuable solution with cobalt and copper (ready for sorption by e.g. ion exchange, solvent extraction)
Generation of iron products
Producing an iron solution that still contained low concentrations of arsenic made it possible to synthesise two iron products: Schwertmannite (Fe163+[O 16|(OH)10|(SO4)3]·10 H2O) und nano-Magnetite (Fe3O4).
Schwertmannite production
a) Original pregnant leaching solution 1 : 1 diluted (29 L, pH 1.21)
b) pH to 2.36 using magnesium oxide, (partial) pre-precipitation of Fe(III)
c) Sedimentation of sludge, clear supernatant, separation of precipitate
d) clear Fe(III)-solution, further pH increase (magnesium oxide)
e) pH to 3.4 (MgO), complete precipitation of Fe
f) Sedimentation of final Fe sludge, green supernatant (Cu with little Fe)
g) Filtration of residual solution (blue), containing Cu, Co, As
The principle schematic illustrates the production of Schwertmannite.
Characterisation of Schwertmannite as iron precipitation product
Super-paramagnetic nano-Magnetite production
In general, the production method depends on following parameters:
- the ratio: iron(II)/iron(III)= 0.5
- temperature: 50°C to 80°C
- pH: up to 8.0 or 9.0
The principle schematic illustrates the production of nano-magnetite.
Pregnant leaching solution solution pH 0.9 to 1.67
Addition of NaOH to pH 3.3
Addition of Fe(II)-sulfate solution - pH 3.7
Addition of NaOH to pH > 9.0 = nMgt
Separation of nMgt (NdBFe) by magnetic characteristics
Use of Fe for industrial products
Schwertmannite:
- Cleaning of water from mines, tailings and industry from oxoanions
- Mining in many regions leads to soil and groundwater contamination due to weathering and smelting of ores containing arsenic. The problem arises due to water shortages in many regions
- In the 20th century, agriculture (viticulture) used calciumarsenate against leaf pests (spraying) and thus contaminated the soil for a long time → washing out into the groundwater.
Nano-Magnetite:
- Due to the size and shape of its particles (1-100 nm), high surface area, magnetic properties, and inert characteristics can be deployed as:
- Catalyst: (sustainable heterogeneous catalysts for chemical synthesis, e.g., palladium, cobalt, nickel, ... with high range
of selectivity)
- Wastewater treatment: heavy metals and organic pollutants, e.g., lead, arsenic (removal of arsenic by magnetic
separation)
- Lithium-ion battery (due to high theoretical capacity, low cost and improved lithium storage used as anode material)
Author: Dr. Jana Pinka, Mirko Martin, Dr. Frank Haubrich