image: Hockaday used a 2-meter diameter dish-type heliostat that generates highly focused intense heat at 955°C within a small solar furnace containing sample ores about a meter above the heliostat.
A small solar reactor with tightly focused solar flux highly concentrated on a single point can attain the range of extremely high temperatures that are required to achieve thermochemical reactions. Solar reactors can be designed specifically for their process applications, just like solar thermal energy plants are designed to integrate with power blocks.
Credit: Lina Hockaday, Mintek
Lina Hockaday, Senior Engineer in Pyrometallurgy at Mintek's New Technology Group in South Africa, proposes that solar thermal reactors, able to reach temperatures up to 1200°C, could virtually eliminate emissions from processing manganese ore fines, by using solar sintering.
Manganese is the fourth-most used metal in the world, at 6 million tons a year, needed for everything from cars to skyscrapers, used in iron to make steel strong. South Africa mines 80% of manganese ores globally.
But the fossil energy used to sinter manganese ores makes it very carbon-intensive. By 2020, South Africa alone will produce 3.4 million tons of sinter, emitting nearly a million and a half tons of CO2 annually.
"My research question is, can we use solar to replace fossil fuel combustion? I'm trying to prove this concept on a small scale, and then describe it well enough that we can say this will work on a large scale," said Hockaday, whose research is the subject of her PhD at Stellenbosch University in South Africa.
Her concept is unique. Other scientists around the world are investigating solar heat for other mining processes, but not to use solar to sinter manganese ore fines.
She presented her paper Solar Thermal Treatment of Manganese Ores based on her experimental results, at the 23rd Annual SolarPACES Conference in Chile, on the proof of concept.
What are manganese ore fines and how are they sintered?
Fines are the fine particles of manganese ore created when crushing manganese rocks. The fines are heated to sinter them (make them stick together, or become "agglomerated").
"The mines produce a lot of fines," Hockaday explained. "But they can't sell them, as furnaces won't buy materials that are too fine."
So the mines separate out the fines, and agglomerate them together into bigger pellets between 6 mm and 75 mm, by wetting them to make them sticky, and rolling them into lumps called green sinter, mixing in about 10% of coke particles.
Then these green sinter "mud balls" are heated in a sintering machine on conveyor belts to about 600°C with diesel burners which makes the coke burn, causing the temperature to rise to 1200°C, at which point the outer surfaces of the mud balls partially melt, making them stronger and more cohesive.
As well as replacing diesel to heat the green sinter, Hockaday believes she can also eliminate the coke combustion, for a carbon reduction of up to 100%, although the traditional industry has been skeptical.
"Some have raised the question that we need a reducing atmosphere because that's the way it's always been done," she related. "But thermodynamically we should be able to get away with not adding coke fines, just sintering with solar energy. There's no reaction that we can see that this requires carbon. So I hope to actually show that we can do this without any coke."
Once the fines are sintered, they then go to a blast furnace or an electric arc furnace, to smelt the manganese iron into manganese alloys.
It is here that fines would be a big risk for literally gumming up the works or even causing explosions. In the blast furnace, hot gases are produced by the reduction reactions.
"These furnaces work with layers of chunky material, and you need spaces for the gases from the reactions to rise up through this material and escape at the top of the furnace. If you have fine particles, almost powder, you can imagine how this fills the pores, preventing the gases from escaping," she explained.
Solar reactors can get hot enough to sinter the world's 4th most used metal
Hockaday used a 2-meter diameter dish-type heliostat that generates highly focused intense heat at 955°C within a small solar furnace containing sample ores about a meter above the heliostat.
A small solar reactor with tightly focused solar flux highly concentrated on a single point can attain the range of extremely high temperatures that are required to achieve thermochemical reactions. Solar reactors can be designed specifically for their process applications, just like solar thermal energy plants are designed to integrate with power blocks.
"The more you can focus sunshine collected over a large area on that small point the higher temperatures you achieve," she said.
Initially, Hockaday found that directly exposing particles to the focused solar heat did not adequately heat and sinter particles in the back of the experimental furnace.
Now she is designing a setup that will instead use convection heat transfer with a closed loop with air flowing throughout the sample.
"This is similar to what happens in the traditional sinters, they draw air through the sinter to combust the carbon," she explained. "That leads to very fast heat transfer so they can melt the particles relatively quickly. And I'm hoping to demonstrate that if we apply the same principle to the small solar sinter experiment we will get better heat transfer throughout the sample, so that's my next step."
Switching from direct to indirect solar heating will bring new challenges: the heat of the process will now need to be higher, above 1200°C, because "in order to heat something through heat exchange, the heat transfer medium has to be hotter than your target temperature."
Inventing real life commercial solar sintering
Hockaday's 20-year dream is to see a commercial solar sintering industry.
Her research is the first step to realizing this dream, "to develop the technology so that when we need this we are ready." At commercial scale, the solar sintering plants she envisions would be the size of Crescent Dunes, the first utility-scale 100 MW CSP tower plant.
"At that scale we would need the right materials to handle the high temperatures because if you raise your manganese ore temperature to 1200°C the environment around it will be at similar temperatures, so your construction materials become more challenging as you go to higher temperatures."
With her career in pyrometallurgy, she believes that this can be overcome with the right materials and engineering: "I come from the background of furnaces where we work with temperatures from 1300°C to 1800°C," she related.
"These are the things that we have to demonstrate first on the small scale to figure out what materials will we have to use and what our efficiency is going to be, and as we discover this, along the road we will be able to draw a picture of our future solar sinter."
Solar sinters make sense in South Africa with abundant space and sun. Most of the remaining 20% of global manganese comes from Australia, China, India, and Brazil, also countries with abundant space and good solar resources for thermal solar processes.
"At this point, I'm just using about 500 grams of ores, so you can see when people are talking industrial scale and they are talking 500 kilotons a year, it is a faraway dream," she laughed.
"But I hope to, in my lifetime, push it through the pilot station stage and through the demonstration stage. Then my dream would be that the technology would be mature, and viable, and could be applied commercially. So my work is at the beginning of this journey."
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