Two Acids and a Rock A Cement Process That Sends the Sulfur Back Round Instead of Venting Carbon

This application covers a chemical route to lime and Portland cement that pulls the calcium out of a rock with one acid, precipitates it with a second acid as a different salt, and then bakes that salt instead of baking limestone. The gas that comes off the baking step is sulfur dioxide rather than carbon dioxide, and the specification recycles it straight back into the second acid, closing the loop. It was filed jointly by Brimstone Energy Inc. and the California Institute of Technology, names Cody E. Finke and Hugo F. Leandri as inventors, and is a divisional of Australian application 2020330562.

The Problem

Cement is made by heating limestone until it falls apart. The specification writes the reaction out and gives it a number: CaCO3 goes to CaO plus CO2, with a heat of decomposition of 178 kJ per mole, and it labels the reaction non-spontaneous. That step happens at about 900 degrees C, and the resulting lime is then fired with alumina and silica at about 1450 degrees C to make Portland cement, which the specification describes as roughly 60 per cent CaO by mass. The stated cost of doing it this way is 2.7 to 6 GJ per tonne of cement and 0.7 to 1.3 tonnes of CO2 per tonne of cement. The specification puts the total at more than 5 per cent of global CO2 emissions.

The important part of that carbon number is where it comes from. Roughly half is fuel, and fuel can in principle be swapped for electricity or hydrogen. The other half is the carbon that was chemically bound in the rock, and no amount of clean energy removes it, because the carbonate has to be broken to release the calcium. Carbon capture is the usual answer and it is expensive, because the gas has to be separated, compressed and stored somewhere permanently.

The specification also deals with the obvious alternative feedstock and explains why it is not used. Cement can be made from calcium sulfate rather than calcium carbonate, because CaSO4 heated hard enough also decomposes to CaO, giving off SO2 and oxygen instead of CO2. Variants of this exist, including the Mueller-Kuehne process, which reduces some of the CaSO4 to CaS with coal and then co-decomposes the two. The specification is blunt about why none of it is done commercially: SO2 cannot be released into the atmosphere, and the global demand for SO2 is far lower than the demand for cement. The sulfur has nowhere to go. Gypsum cement solves the carbon problem by creating a sulfur problem.

The third constraint is feedstock range. The specification notes that conventional processes use only a limited range of feedstocks, primarily simple calcium-based materials such as limestone or mined gypsum, and that complex rocks in which calcium sits alongside silicon, aluminium, iron and magnesium are effectively off the menu. Those complex rocks, anorthite and the other calcium feldspars, wollastonite, basalt and the mafic and ultramafic rocks generally, are vastly more abundant than limestone and contain no carbon at all.

What This Invention Does

Claim 1 has three steps and is shorter than the title suggests. First, react a calcium-bearing starting material with a first acid to produce an aqueous first calcium salt. Second, react that aqueous first calcium salt with a second acid to produce a solid second calcium salt, where the second acid is different from the first acid and the second calcium salt is different from the first calcium salt. Third, thermally treat one or more calcium salts to produce a first cement material. That is the whole of claim 1.

Note what it does not say. It does not say the starting material is a rock, or a silicate, or free of carbonate. Claim 19 adds the natural rock or mineral limitation and claim 21 lists the candidates, and limestone and natural gypsum are both in that list alongside basalt, wollastonite and anorthite. Claim 22 goes further and allows the feed to be cement, concrete, fly ash or slag. Claim 1 also does not require that the acid is regenerated, that the process avoids CO2, or that the calcium salt being baked is the one made in step two. Those are claims 3, 4 and 2 respectively. The title of the application says calcium bearing rocks and minerals, and the marketing story around this technology is about making cement from silicate rock instead of limestone. Claim 1 is considerably broader than either, and the article follows the claim.

The chemistry the claim is built around is set out in the worked examples. With hydrochloric acid as the first acid and anorthite as the feed, the specification gives CaAl2Si2O8 plus 8HCl going to aqueous CaCl2, aqueous AlCl3 and solid SiO2. The silica drops out and is filtered off. Sulfuric acid is then added to the remaining solution, and this is where the selectivity sits: the specification states that calcium sulfate is the only one of the common sulfate salts, against MgSO4, Al2(SO4)3 and Fe2(SO4)3, that does not dissolve in water, so CaSO4 alone precipitates while everything else stays in solution. The written reaction is CaCl2 plus 2AlCl3 plus 4H2SO4 going to solid CaSO4, aqueous Al2(SO4)3 and 8HCl, which regenerates the hydrochloric acid used in step one. Claim 5 sets the purity target for that precipitate at 90 weight per cent or better.

The calcium sulfate is then calcined: CaSO4 plus heat gives CaO plus 1/2 O2 plus SO2. Claim 62 puts that step at 1100 to 1800 degrees C, or claim 63 at 800 to 1200 degrees C in the presence of a chemical reductant. The SO2 is not vented. Claim 9 requires converting a gas product of the thermal step back into the second acid, and claims 10 to 12 give three ways of doing it: the contact process route, SO2 plus 1/2 O2 plus H2O to H2SO4, which claim 57 runs at 400 to 1800 degrees C over a catalyst and describes as exothermic; a sulfurous acid route, SO2 plus H2O to H2SO3; and an electrochemical route, SO2 plus 2H2O to H2SO4 plus H2, which produces hydrogen as a co-product. Claim 13 then feeds that hydrogen back to power the calciner.

Add the steps together and the specification’s net reaction for the silicate case is CaAl2Si2O8 plus 2H2O going to CaO plus Al2Si2O5(OH)4, with an enthalpy the specification writes as approximately zero. Both acids appear on both sides and cancel. In the electrochemical variant the net reaction also yields oxygen and hydrogen and the specification puts the enthalpy at 50, which it calls slightly uphill. Claim 58 states the intended envelope as a net energy between minus 2 and plus 2 GJ per tonne of cement produced, against the 2.7 to 6 GJ per tonne the background gives for conventional cement.

The leftovers are the other half of the argument. The silica filtered out in step one can be isolated as silica-fume grade, nano or micro silica under claim 42. Claims 43 to 47 recover alumina by saturating the leach liquor with HCl so AlCl3 precipitates, converting it to Al2(SO4)3 with sulfuric acid and calcining that to Al2O3, again giving back SO2 for regeneration. Claims 48 and 49 dry the remaining solution and heat the iron sulfate at 500 to 700 degrees C to insoluble iron oxide, with the magnesium sulfate washed away afterwards. Claims 100 to 103 go further still and electrowin aluminium and iron metal from those intermediates. Claims 40 and 41 then point out that the alumina and silica are exactly the additives needed to turn the lime into Portland cement in the first place.

Two other independent claims sit in the same document. Claim 83 is the one that actually recites a calcium silicate feed: it requires a starting material containing at least Ca and Si, a first acid reaction that produces a silicon salt as a solid byproduct, a separation, and then treating the first calcium salt to produce cement, with no second acid at all. Claim 70 is a different process entirely, reacting a calcium-bearing material with a chemically reducing gas to produce methane and a cement material, with claim 79 adding methane pyrolysis to regenerate the hydrogen and deposit the carbon as a solid.

That third route is the only one with laboratory data in the document. The specification reports putting 2.011 g of CaCO3 powder in a tube furnace, ramping at 7 degrees C per minute under forming gas at 0.3 litres per minute, measuring methane at the outlet with a gas analyser, and finding by XPS that the product was more than 99 per cent lime, with integration under the curves giving approximately 100 per cent decarbonisation. It also gives 13.1 kJ per mole for that reaction against 178 kJ per mole for conventional calcining. There are three figures in the specification and all three belong to this experiment. The two-acid process of claim 1 is presented as chemistry and stoichiometry only. No yields, no runs, no mass balances.

Key Features

• A salt swap that changes which gas comes off. The calcium is moved out of the rock as one salt and re-precipitated as a second, so the salt that finally gets calcined releases SO2 rather than CO2.
• Both acids regenerated in the loop. The second reaction gives back the first acid, and the gas from the calciner is converted back into the second acid, so the specification’s net equations have no acid on either side.
• Selectivity by solubility. Calcium sulfate is the only common sulfate among the magnesium, aluminium and iron sulfates that will not dissolve, so adding sulfuric acid to the leach liquor precipitates calcium and nothing else, at a claimed purity of 90 weight per cent or better.
• Feedstock range as the point. Claim 21 lists basalt, wollastonite, anorthosite, montmorillonite, bentonite, diopside, pyroxene, garnet, skarn and more, and claim 22 adds fly ash, slag and crushed concrete, none of which conventional cement making can use.
• Co-products drawn from the same rock. Silica, alumina and iron oxide are recovered from the streams left behind, and claims 100 to 103 electrowin aluminium and iron metal, with the alumina and silica also serving as the additives that convert the lime to Portland cement.
• Two further independent claims on different routes. Claim 83 covers a single-acid process specific to a calcium and silicon bearing feed, and claim 70 covers reacting a calcium-bearing material with hydrogen to make methane and cement, which is the only route in the document with experimental results attached.

Who Is Behind It

Brimstone is a genuine and well funded company rather than a filing vehicle. Its own About page says it was founded in 2019 to make cement, steel, aluminium and other critical minerals more efficiently and at lower cost, names Cody Finke as chief executive and Hugo Leandri as a senior advisor, and states that the company has completed pilot work in Oakland, California and is advancing toward a manufacturing facility in Reno, Nevada. Its technology page describes the Brimstone Rock Refinery, a co-production process that refines a single abundant rock into cement, supplementary cementitious materials, alumina and metals, which is recognisably the claim set above with the co-product claims given equal billing. CNBC reported in April 2022 that Breakthrough Energy Ventures and DCVC had put 55 million US dollars into the company.

The academic half is straightforward. Finke took his doctorate at Caltech and the two founders overlapped there as graduate students, which is why the California Institute of Technology is a co-applicant rather than a licensee. Co-assignment between a university and a spinout is the normal shape when the underlying work started in a laboratory and the provisional applications were filed while that relationship was still live.

On priority the specification is explicit. The cross-reference paragraph claims the benefit of four United States provisional applications: 62/886,137 filed 13 August 2019, 62/913,620 filed 10 October 2019, 62/932,200 filed 7 November 2019 and 63/019,916 filed 4 May 2020. The priority country is the United States and the earliest priority date is 13 August 2019. The drawing sheets reproduced in this specification still carry their original headers, WO 2021/030529 and PCT/US2020/046063, which is how the international application that became Australian application 2020330562 can be identified from the document itself.

Why It Matters

Lime, the specification says, is the primary constituent of cement, which it calls the most consumed human made material on the planet. Its emissions have resisted every fix that works elsewhere. A steel mill can switch to hydrogen. A power station can switch to renewables. A cement kiln can switch fuel and still emit more than half of what it emitted before, because the carbon is in the rock. That is why almost every serious decarbonisation plan for cement ends in carbon capture, and why a process that simply does not liberate any fossil carbon is worth a patent family this size.

The honest reading of this document is that the chemistry is credible and the claim is broader than the story. Every individual reaction in the two-acid loop is known industrial practice. Acid leaching of silicate rock is routine. Selective sulfate precipitation is routine. The contact process has made sulfuric acid from SO2 since the nineteenth century. What is new is assembling them so the sulfur cycles indefinitely between the precipitation step and the calciner, which converts the sulfur from a waste product into a reagent and removes the reason gypsum cement was never commercialised. The difficulty, which the specification does not address, is entirely in the engineering: hot concentrated hydrochloric acid attacks almost everything, the solids handling is substantial, and none of the numbers in this document come from a plant.

It is also worth noting what the claim set says about the business. A process that only made cement would have to compete on the price of cement, which is low, heavy and local. Claims 42 to 49 and 100 to 103 turn the same rock into silica, alumina, iron oxide, aluminium metal and iron metal, and that is the co-production argument Brimstone now leads with publicly. Whether the environmental case or the co-product economics is what eventually justifies a plant, the patent is drafted to cover both.

Related Concepts

• Calcination – the high temperature decomposition step that this process applies to a sulfate rather than a carbonate.
• Gypsum – calcium sulfate, the intermediate the process manufactures synthetically at 90 weight per cent purity before baking it.
• Sulfur dioxide – the gas released by the calciner, which the claims require be converted back into the second acid rather than emitted.
• Basalt – one of the abundant carbon free calcium bearing rocks the specification lists as a feedstock in place of limestone.
• Environmental impact of concrete – the broader context for a cement process whose stated aim is a net reaction free of acid forming gases.

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AU 2026202169 was published in the Australian Official Journal of Patents on 9 April 2026 and is open for public inspection. Patent applications represent inventions that are sought to be protected and do not necessarily reflect commercially available products.