By Keith Vining
Australian iron ore is becoming increasingly goethitic. With the composition of ore outputs changing, how is your value chain adapting to remain efficient in this operating environment?
In the current mining environment there is a clear need to understand the impacts caused by variation in both the chemical and physical properties of goethitic ore types. In fact, at CSIRO we firmly believe driving positive business outcomes from your operation will depend on leveraging a textural classification scheme to optimise your production processes.
Because different ore types behave differently across the value chain, without a clear understanding of their chemical and textural variation, and corresponding process design you’re likely to face a host of problems and inefficiencies.
However, this same variability of texture also presents a number of potential opportunities for mines to improve their processes and drive operational performance.
Our blog on understanding textural classification systems to improve business outcomes described the different types of goethite present in iron ores. Outlined in the table below is an overview of key goethite properties and the impact these can have on the various stages of the iron ore value chain.
|Goethite Property||Process Stage|
|Mining||Beneficiation (crushing, grinding)||Agglomeration (sintering, pelletising)||Blast Furnace|
|Chemistry||Mine planning – effective only if the varying effects of different goethite textural and compositional types are understood.||Difficult to separate ochreous goethite (in particular) from gangue by gravity separation due to the small difference in specific gravity. This leads to lower Fe recovery in hydrocyloning, spirals etc.
Goethite is not amenable to magnetic classification.
|Al and Si are key elements in the formation of sinter and pellet bonding phases (SFCA). Their reactivity is influenced by mineralogy (e.g. kaolinite/gibbsite vs substituted in goethite).
Fine goethite can be very effective as a ‘granulation enhancer’, although increasing granulation moisture requirement.
|High Al increases melt viscosity and fuel rates.
Si increases the amount of slag and consumes more limestone – as such the ratio of CaO/SiO2 must be adjusted.
High levels of P (>0.08%) affect hot metal production.
|Hardness||Drilling – hardness is variable and dependent on goethite type.
Vitreous goethite undergoes brittle fracturing to form ‘shards’, whereas brown and ochreous goethite fracture more evenly.
|Decreasing hardness results in mechanised grinding causing a higher proportion of fines and ultrafines. Variable texture can influence product sizing (for example, ochreous goethite is easily broken down to ultrafines, whereas brown and vitreous goethite tend to produce coarser fines sizing).||Low physical hardness generates fines requiring agglomeration.
Goethite has relatively high reactivity during sintering leading to greater melt formation and assimilation.
|BF feed is now dominated by sinter (up to 85%) compared to traditional lump.
Goethite lump has good reducibility, but some susceptibility to reduction degradation. Decrepitation can be a problem in dense, goethite-bearing ore types.
|Water||Need for dewatering of wet processed fines.||Handling properties – ores with high absorbed water can cause sticking and blockages in crushing and screening plants.
Ochreous goethite can have very high microporosity and moisture holding capacity.
|Fine goethite increases the moisture requirement for granulation, requiring increased fuel rate at higher levels. Loss of volume on heating leads to higher productivity in sintering if well managed.
Dense goethite can form stable sinter nuclei.
Reactivity can enhance sinter matrix melt formation and strong sinter bonding.
|Loss on ignition (LOI) leading to loss of volume on heating.|
|Texture||Contrast of micro-roughness of ochreous goethite surface to very smooth surface of vitreous goethite.
Ochreous goethite assists in binding adhering fines on particle surfaces.
|Goethites with low porosity have high to medium hardness.
Porous goethites are soft and friable leading to ultrafines generation. Powdery or dusty ores have goethite with an appreciable portion of goethite crystallites less than 0.01mm diameter.
|Goethitic ores can enhance sinter productivity.
Fine ochreous goethite enhances granulation in sintering. Vitreous goethite does not granulate as well, due to smooth surfaces.
Potential as a binder in pelletising, briquetting.
|Affects lump reducibility, reduction degradation, decrepitation index.|
The properties of various types of goethite, and the effects these can have on the value chain can be challenging for current mine processes. However, an incorporation of the textural characterisation of these ore bodies allows your organisation to overcome possible inefficiencies in current processes for managing goethite. For example, sintering processes can be optimised by leveraging higher levels of goethite to drive sinter plant productivity.
The graphs below show the effect of incorporating texture into modelling of pot grate sinter strength, measured by industry-standard Tumble Index (TI), via a combined factor derived from the proportions of individual ore textural components.
In this case, incorporating texture into the modelling significantly improved the measured correlation. As a result the unit was able to optimise the sinter process around the higher level of goethite, and reduced outliers and variation from the model from 25.1 to 13.4%. Understanding which ore components affect a unit process is critical in understanding product behaviour at all processing stages.
The images below show a simulated ore nuclei fired at 1330°C in a laboratory sintering test. These show the contrast in behaviour between a dense hematite-goethite ore nucleus (at left), where reaction has occurred internally due to melt penetration along cracks formed during heating, with that of uniformly microporous ochreous goethite (at right), which has shrunk and reacted completely, but remained in situ.
As the level of goethite mined continues to increase, this presents an opportunity to improve our understanding of the consequences of this trend across all stages of the value chain, in order to improve processes and mitigate problems. Your ability to achieve this relies on a complete understanding of not just the chemical composition, but also the textural elements, of different ore types.
This is an area in which CSIRO’s Carbon Steel Futures team conducts comprehensive research.
Our research, in combination with our experience globally implementing our findings, means we’re well equipped to help you improve your mine processes. As goethitic ore becomes more prevalent in the Australian market, so too will the need to find efficiencies in your value chain – if you’re starting to feel the impact of goethitic ore in your operation contact the Carbon Steel Futures’ Geometallurgy team on +61 07 3327 4761.