Understanding the effects of goethitic iron ore
By Keith Vining
Using textural classification systems to improve your business outcomes
After preferentially mining high-grade, low-phosphorus Brockman ore for some decades, Pilbara iron ore producers are now making the transition to increasing outputs of Marra Mamba (MM), Channel Iron Deposit (CID) and high-phosphorus Brockman ores. As demonstrated on the plot below, where the balance of ore textural components is shifting to the right with time from hematite-dominated (blue, red) to goethitic (brown, yellow), rising proportions of these ore types are resulting in increasingly goethitic iron ore products. This has had a noticeable impact on the iron ore supply chain.
Compared with hematite and magnetite, goethite has a much more diverse range of textural forms and occurrences, and its influence on iron ore processing is less understood.
Currently, most analysis of the effects of goethite on mine performance considers only the chemistry of the ore and not its textural components. Through improved textural classification of goethitic ores we can reframe the perceptions the industry has of goethite, and optimise overall processing performance.
Types of goethite
A greater understanding of the textural differences between types of goethite is the first step towards optimising a mine’s ability to process goethitic iron ores. Below, is a table of the three basic types of goethite:
The importance of textural classification when managing goethitic iron ore
Failing to leverage a textural classification scheme for managing goethite ore can impact a mining operation in numerous ways:
Effective mine planning relies on an understanding of your deposit and its distribution of ore types.
Accounting for the chemical and textural composition of goethite in a mine allows for optimised planning and processing to account for the varying effects the types of goethite can have on mine performance.
Plant downtime and underutilisation
Goethite present in iron ore deposits affects how your mine manages the handling and processing of ore. Failing to understand how the different textural composition of goethite impacts processing performance can lead to downtime and reduced mine efficiency.
Textural classification allows mines to better predict where different types of goethite are present, allowing blending processes to be adjusted to address differences between ore types.
Fine (-1mm) goethite is very reactive during the sintering process and can be leveraged to enhance melting and matrix strength. Fine, ochreous goethite also plays an important role in enhancing granulation. Conversely, coarse (+2mm) goethite particles often show surprising resistance to reaction, forming relatively stable nuclei and increasing the overall efficiency of the process.
The plot below shows that a high grade ochreous goethite matrix (‘Ore G’) achieves high strength (expressed as matrix TI), compared with dense hematite (Ore ‘H’) and porous hematite-goethite (Ore ‘H-G’) samples, at a moderate sintering temperature.
However, if a producer fails to acknowledge the textural composition of fine goethitic ore during sintering, they’re unable to capitalise on this additional efficiency. If the goethite types are unbalanced in a sinter blend, the result can be higher sintering fuel rate, due to the energy required to drive off excess moisture, as well as reduced productivity, due to excessive melt formation. The key is knowing what goethite textural types are present and in what proportions.
Without understanding the textural composition of goethite in ore bodies, consistency throughout the supply chain cannot be maintained. Therefore, with the increased presence of goethite in Australian ore, it is important for supply processes to account for differences between goethite types. Failure to address these differences may lead to product inconsistency.
There is a common perception that ochreous goethite in particular is associated with both chemical impurities and problems in processing (leading to disruption in plant throughput). While it is true that excessive ultrafines can cause problems in plant operation and ochreous goethite can carry a high level of moisture, the real challenge is in controlling the proportions of ore components to maintain consistent product quality and anticipating problems before they occur.
Nature and deportment of impurities
The distribution and mineralogical association of impurities in goethite is critical in determining the feasibility of upgrading an ore and the optimum processing route. In the case of goethite, impurities may be present either substituted in the crystallographic lattice of the mineral, or as discrete minerals, usually kaolinite clay, quartz, and gibbsite. The form and occurrence of phosphorus is not yet fully resolved, with suggested mechanisms including adsorption and coupled Al-P substitution.
Loss of Fe units to tailings
A growing issue for Australian producers is maximising recovery of iron units, with the increasing need to beneficiate lower grade ores to meet customer requirements. At present, high grade, valuable hematite and goethite are being lost to waste streams during wet processing, leading to inefficient plant operation as well as inefficient resource utilisation. From a producer’s perspective, the simplest solutions to this issue appear to involve improved flexibility in selection of processing routes to cope with feed variation. This is an area where improved and higher resolution ore classification data would be of great potential benefit.
Implementing textural classification schemes in your organisation
Leveraging CSIRO’s expertise in textural classification schemes becomes even more valuable, as we see increasingly goethitic ore being mined and exported. An improved understanding of the effects of goethite on ore outputs will allow you to adjust processes to best deal with the behavioural characteristics of this ore mineral, to generate positive business outcomes.
As part of CSIRO’s continued contribution to the iron ore industry, we are proud to announce we will be co-hosting Iron Ore 2017. At the event, the Carbon Steel Futures team will present recent work around:
- Goethite classification (Manuel, J R and Clout, J M F, 2017. Goethite classification, distribution and properties with reference to Australian Iron Deposits),
- Microwave treatment of low-grade goethite ores (Venkata, N, Hapugoda, S and Pownceby, M I, 2017. Study of microwave-assisted magnetizing roasting and mineral transformation of low grade goethite iron ores), and
- Mineralogical characterisation of goethite (Pownceby, M I, MacRae, C M and Torpy, A, 2017. A comparison of X-ray and electron-based analysis techniques for characterising the mineralogy and associated alumina deportment in iron ores).
We will also have a booth, where you will be able to learn more about the significant benefits a textural classification system could have in your mine.
After Iron Ore 2017, we invite you to take part in our two-day workshop to further your understanding of ore characterisation and image analysis.
If you are unable to attend these events, but are interested in understanding more about how you can leverage a textural understanding of goethite to deliver improved processes contact the Carbon Steel Futures’ Geometallurgy team on +61 07 3327 4761.
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