Method to form a stabilized heap structure made of mine tailings

Abstract

A method to form a stabilized heap structure made of mine tailings (2), wherein an alkaline material (1) is added to said tailings to form a heap structure (3), a reagent (4) being injected into said heap structure, said reagent (4) being a carbon-containing fluid, said carbon forming carbonates (6) with the alkaline material, said carbonates stabilising the heap structure.

Description

[0001] Method to form a stabilized heap structure made of mine tailings

[0002]

[0001] The present invention relates to a method to form a stabilized heap structure made of mine tailings.

[0003]

[0002] Mine tailings are defined as the material that is left over after a mineral has been liberated from gangue material, using notably beneficiation and / or concentration methods. Gangue material is the undesired material and the mineral is the desired material from a ore. Mine tailings is a term used both in hard-rock, soft-rock, surface and underground operations. With time, quality of the extracted ore decreases, and more and more tailings are generated, which cause the problem of their management.

[0004]

[0003] Untreated tailings are typically piled up in heap structures, also known as tailings torage facilities, and landfilled, but those heap structures are not stable and can cause a lot of human, environmental and economic harm, notably when they contain water. Main risk is liquefaction. Liquefaction is a phenomenon in which a soil loses much of its strength or stiffness for a relatively short time, but nevertheless long enough for slope or bearing capacity failures to occur. There are many recorded cases of liquefaction failure of mines wastes.

[0005]

[0004] In order to avoid risk of liquefaction, binders exist that can be mixed with tailings to cement otherwise unreactive particles together and reduce liquefaction susceptibility. In underground cemented paste backfill applications, tailings are blended with cementitious materials. The most used cementing material is ordinary Portland cement (OPC) which is made of artificially produced highly reactive calcium silicates and calcium aluminates, which react with water to produce a number of hydration products that bind particles together. This method is however expensive and requires use of a product whose production has a non-neglectable environmental impact. Indeed, it is estimated that in total, one ton of cement typically produces about 0.8 to 0.9 tons of CO2 emissions, resulting in approximately 8% of the world's anthropogenic CO2 emissions, and about 25% of all industry carbon emissions.

[0006]

[0005] The aim of the present invention is therefore to remedy the drawbacks of the method of the prior art by providing a method to form a stabilized heap structure made of mine tailings which has a reduced CO2 footprint.

[0007]

[0006] For this purpose, a first object of the present invention consists in a method to form a stabilized heap structure made of mine tailings, wherein an alkaline material is added to said tailings to form a heap structure, a reagent being injected into said heap structure, saidreagent being a carbon-containing fluid, said carbon forming carbonates with the alkaline material, said carbonates stabilising the heap structure.

[0008]

[0007] The method according to the invention may also have the optional features listed below, considered individually or in combination:

[0009] - the blending ratio between the alkaline material and the tailings is from 1 to 20 tons of alkaline material for 100 tons of tailings,

[0010] - the blending ratio between the alkaline material and the tailings is from 5 to 15 tons of alkaline material for 100 tons of tailings,

[0011] - the carbon-containing fluid is chosen among a gas comprising more than 5% in volume of CO2 and an aqueous solution of carbonates,

[0012] - the carbon-containing fluid is a gas comprising more than 15% in volume of CO2, - the aqueous solution of carbonates has a concentration of at least 1 mol per liter of carbonates,

[0013] - the saturation degree of the heap structure is below 80%,

[0014] - the carbon-containing fluid is chosen among a solution of K2CO3 and a solution of Na2CO3,

[0015] - at least 90% in number of the particles of the tailings have a size below 3mm,

[0016] - at least 90% of the particles of the tailings have a particle size above 25pm,

[0017] - the tailings are iron ore tailings,

[0018] - the tailings are non-ultramafic tailings,

[0019] - the tailings comprise less than 18% by weight of MgO,

[0020] - the alkaline material is chosen among iron slag, steelmaking slag, cement kiln dust, fly or bottom ash,

[0021] - the alkaline material comprises more than 5% by weight of CaO + MgO,

[0022] - the alkaline material contains at least 25wt.% of CaO,

[0023] - the alkaline material is steelmaking slag,

[0024] - the steelmaking slag is BOF slag,

[0025] - at least 90% of the particles of the alkaline material have a size of 500pm or below, - the carbon-containing fluid is chosen among a blast furnace gas, a coke oven gas, a sintering gas, a converter gas, a iron direct reduction process top gas, an electric arc furnace gas, an electrical smelting furnace gas, or a post-combustion gas resulting from the combustion of any of the previously listed gases.

[0026]

[0008] Other characteristics and advantages of the invention will be described in greater detail in the following description.

[0009] The invention will be better understood by reading the following description, which is provided purely for purposes of explanation and is in no way intended to be restrictive, with reference to:

[0027] Figure 1 , which illustrates a method according to the invention,

[0028] Figure 2 which illustrates an embodiment of a method according to the invention, Figure 3 which illustrates another embodiment of a method according to the invention.

[0029]

[0010] With reference to Figure 1, the method according to the invention first comprises a step of blending an alkaline material 1 with tailings 2 to form a heap structure 3. It is preferably an alkaline-earth material 1, more preferably containing calcium and / or magnesium.

[0030]

[0011] The alkaline material 1 preferably comprises more than 5 wt.% of CaO and MgO, more preferably 30 wt.% of CaO or more. It preferably comprises from 15 to 60 wt.% of CaO, from 3 to 20 wt.% of MgO, frol 5 to 40 wt.% of SiO2, up to 40 wt.% of iron oxides, up to 15 wt.% of AI2O3, up to 5 wt.% each of MnO, TiO2, P2O5, up to 2 wt.% of sulphur and up to 20 wt.% of moisture.

[0031]

[0012] The alkaline material 1 may be ironmaking slag, steelmaking slag, cement kiln dust, fly, bottom ash or a mixture thereof. It is preferably a by-product of the ironmaking and / or steelmaking industry as it allows internal valorization of those by-products, when considered tailings is iron ore tailings. Ironmaking slag is slag produced during iron manufacturing, notably blast furnace slag. The steelmaking slag is produced during steel manufacturing, according to the process used, it may be electric arc furnace (EAF) slag, converter (BOF) slag, smelting furnace (ESF) slag or secondary metallurgy or ladle slag.

[0032]

[0013] It is preferably a steelmaking slag which comprise more CaO and / or MgO than ironmaking slag and for which less valorization routes are currently available. The method according to the invention would thus furthermore allows valorization of those by-products.

[0033]

[0014] The alkaline material 1 preferably has a particle size of 500pm or below, preferably below 250pm and even more preferably below 100 pm. This allows to increase the reactivity of the material. To obtain this particle size, the alkaline material may be subjected to a size reduction and to a size classification step. Those steps may be performed using known devices such as jaw crushers, giratory crushers, high pressure grinding rolls (HPGR), tumbling mills, AG or SAG mills, vertical mills or any other devices well known in the mineral processing industry.

[0034]

[0015] The mine tailings 2 are preferably iron ore tailings and more preferably non-ultramafic tailings and preferably comprise less than 18 wt.% of MgO. Preferably 90% ofthe particles of the mine tailings 2 have a particle size of 3 mm or below, preferably from 25pm to 3mm, more preferably from 100pm to 3mm. This allows to produce a more permeable pile, where it is easier for carbon reagent to move through, whether it is as a gas or in aqueous solution.

[0035]

[0016] The tailings may be used as produced in the mineral processing plant, where iron is separated from the gangue, or subjected to size classification technologies where they are deslimed to remove finest particles. The iron ore tailings 2 are usually available as slurries.

[0036]

[0017] The blending can be performed before deposition in the heap or directly in the heap.

[0037]

[0018] As a matter of illustration, tailings 2 and the alkaline material 1 can be blended as a slurry directly or before being fed in a thickener, producing a thickened blend disposed as a slurry or dewatered further in a filter before disposal as a cake to form the heap structure 3.

[0038]

[0019] A slurry of dewatered tailings 2 can also be blended with dry alkaline material 1 particles in a mixer such as a pugmill producing a thickened blended stream as well used to form the heap structure 3.

[0039]

[0020] The tailings 2, preferably highly dewatered filtered tailings, can also be disposed in a layer, followed by disposal of the alkaline material 1 as another layer on top of the tailings. They would be subsequently blended by using a soil mixer / stabiliser machine.

[0040]

[0021] The saturation degree of the heap structure is preferably below 80%. The degree of saturation is the ratio of liquid to the total volume of voids in the material.

[0041]

[0022] The proportion of tailings 2 and alkaline material 1 particles is preferably from 1 to 20 tons of alkaline material 1 per 100 tons of tailings 2, but higher proportion is also acceptable.

[0042]

[0023] Then, a reagent 4 is injected into the heap structure 3. This reagent 4 is a carbon-containing-fluid. It may be a gas comprising more than 5% in volume of CO2 or an aqueous solution of carbonates, such as a solution of K2CO3, a solution of (NH4)2CO3 or a solution of Na2CO3.

[0043]

[0024] Minerals present in the alkaline material 1 , such as Ca or Mg, will dissolve and react with the carbon of the reagent 4 and precipitate as mineral carbonates.

[0044]

[0025] Following chemical reaction may for example occur when calcium is the mineral and reagent is a gas containing CO2:

[0045] CaO + 2H+Ca2++ H2O

[0046] C02g) + H2O H2CO3(a)

[0047] H2CO3(a) H++ HC03

[0048] HC03H++ col~

[0049] Ca2 ++ COl~ CaC03(s)The resulting carbonate minerals 6 would bind tailings, and the remaining unreacted alkaline material, particles together, thus increasing the strength of the heap structure 3 to form a stabilized heap structure 5. The reaction of formation of carbonates is an exothermic reaction.

[0050]

[0026] In a first embodiment, the reagent 4 is a gas comprising more than 5% in volume of CO2, preferably more than 10%, more preferably more than 15%. The reagent 4 is preferably an exhaust gas from the steelmaking or ironmaking industry. It is for example a blast furnace gas (BFG), a coke oven gas, a sintering gas, a converter gas (BOFG), an iron direct reduction process top gas (DRG), an electric arc furnace gas (EAFG), an electrical smelting furnace gas, a post-combustion exhaust gas of any of those gases or a mixture of any of those gases. Each of these gases may be preferably subjected to treatment steps, such as dewatering, dedusting or gas separation to remove hydrogen, carbon monoxide or other valuable components. A post-combustion stream is the resulting stream after burning, which involves transformation of CO and H2 in CO2 and H2O.

[0051]

[0027] Increasing the CO2 concentration allows to increase the kinetics of the reactions, as well as it reduces the volume of fluid that needs to be moved through the heap to complete the reactions, and therefore also the time needed to complete the process.

[0052]

[0028] As a matter of illustration composition of some of those gases is illustrated in Table 1.

[0053] Table 1 - Gaseous reagent composition

[0054]

[0055]

[0029] The CO2 gas reagent 4 is introduced into the heap by either natural or forced ventilation. This introduction may be performed, as a matter of illustration, with a method as illustrated in figure 2. The heap structure 3 is placed on top of a high permeability blanket 10 comprising injection means 11, such as perforated pipes distributed along the heaplength L and extending over its width W so as to be able to distribute the reagent 4 over a large part of the heap’s lower surface.

[0056]

[0030] Depending on the permeability of the heap and the temperature gradient it is possible that a chimney-type effect forces the gas phase from the bottom blanket upwards and outside of the heap, transporting CO2 throughout the heap. If natural effect is not enough, then forced ventilation with centrifugal or other types of fans or blowers may be introduced. The pressure drop on these blowers is preferably below 20 kPa.

[0057]

[0031] CO2 in the gas phase will equilibrate with the pore water and speciate into carbonate, bicarbonate and aqueous CO2 depending on the pH. Due to the presence of alkaline materials the pH will be high and mostly carbonate ions will be present.

[0058]

[0032] Water is not consumed in the carbonation reaction but there will be some water losses due to evaporation because of the exothermic reaction, the formation of hydration reaction products (Si, Fe, Al) and therefore, depending on the climate, moisture in the heap may need to be maintained by water addition with sprinklers, drip pipes or buried perforated pipes. Any leachate produced may be collected from the heap.

[0059]

[0033] In a second embodiment, the reagent 4 is supplied in form of an aqueous solution of carbonates.

[0060]

[0034] This aqueous solution may have been prepared by capture of gaseous CO2 with a dedicated solution, such as NaOH, KOH or NH4OH which produces a Na2CO3, K2CO3 or (NH4)2CO3 solution by combination with a selected cation, such as K+, Na+ or NH4+.

[0061]

[0035] The gaseous CO2 may come from any of the previously mentioned exhaust gases.

[0062]

[0036] Compared to the previous embodiment where CO2 is supplied as a gas that will dissolve into water to react with the mineral and form carbonates, in the present case the CO2 is already dissolved in the water. Supply of CO2 in aqueous form allows to have a prior separation of the impurities also contained in the gas before its supply to the heap structure, thus avoiding releasing of those impurities into the atmosphere.

[0063]

[0037] Supply of this aqueous solution may be done with a method as illustrated in figure 3. The carbonate solution is supplied on top of the heap structure 3 by injections means 20, such as sprinklers or drip pipes and goes down through the heap. The heap structure 3 is preferably located on a liner 21 and has a slope allowing to recover the carbonate-lean solution 24 at the bottom of the heap structure with a collection system 23. The recovered carbonates-lean solution 24 may then be recycled for the capture of gaseous CO2.

[0064]

[0038] In another embodiment, not illustrated, the aqueous solution comprising carbonates may also be injected by pipes buries below the heap structure.

[0065]

[0039] Once the reagent 4 is injected, the carbon will react with the minerals present in the heap structure 3, according to the previously explained reactions, and the formedcarbonates will create bridges between the present particles and reinforce the strength of the heap structure.

[0066]

[0040] Reaction time may vary according to different factors, including the permeability of the heap structure 3, its geometry, such as its height, the concentration of the reagent 4 and the ratio of alkaline material 1 in the heap structure.

[0067]

[0041] Compared to the method according to prior art using Portland Cement, cementation through carbonates would typically lead to lower-strength resulting materials but this would be enough to prevent liquefaction without the use of high CO2-cost products. The commonly agreed criteria to meet for liquefaction resistance is an uniaxial compressive strength (UCS) of at least 100 kPa.

[0068]

[0042] The method according to the invention allows to increase the safety of tailings storage facilities by increasing the UCS of the heap structure. It may also have the advantages of capturing CO2 from exhaust gases and avoid their release into the atmosphere. It may also allow usage of by-products of the steel and iron making industries which may not otherwise be valorized.

[0069] Trials

[0070]

[0043] Different samples were prepared using tailings form different origins, composition and particles size of which being indicated in tables 2 and 3. They were mixed with different proportions of alkaline material chosen among BOF slag and EAF slag, compositions of which being indicated in table 3.

[0071]

[0044] Blending ratio of alkaline material is expressed in tons of alkaline material for 100 tons of tailings. Size of the BOF slag was also varied as indicated in table 5.

[0072]

[0045] The samples were molded into cylinders and compacted at a low effort (2-3 hammers drops per layer over 3 layers). Height-Diameter ratios of the cylinders were varied as different specimen preparation procedures were followed in each case. To consider those differences in H:D ratios, a peak UCS normalized to a H:D ratio of 2 has been calculated based on formula described in literature, (H. T. Gebresamuel, D. Tsige Melese, Y. T. Boru, and A. M. Legese, ‘Effect of Specimens’ Height to Diameter Ratio on Unconfined Compressive Strength of Cohesive Soil’, Studia Geotechnica et Mechanica, vol. 45, no. 2, pp. 112-132, Mar. 2023, doi: 10.2478 / sgem-2023-0001.)

[0073]

[0046] Samples were then placed in closed chambers provided with holes to introduce the reagent for a curing step of seven weeks with CO2 content of the atmosphere in the chamber and temperature kept constant. The chambers were also provided with outlets to remove the introduced gas to prevent pressure build up.

[0047] For trials with 18%v of CO2, remainder was argon, while for trials with 0.4%v, ambient air was used.

[0074]

[0048] After curing samples were tested in a uniaxial compressive strength machine. The machine records the force supplied and the displacement. The strain has then been calculated based on the initial sample height. As suggested by UCS measurement standards, a “corrected area” has been obtained although for this analysis the same correction has been applied regardless of the failure pattern, by dividing the starting area by 1 -strain in all cases.

[0075]

[0049] The stress has then been calculated based on the applied force and corrected area.

[0076]

[0050] The carbonate content was determined before and after the curing step by the calcimeter Bernard method which measures the volume of gas released after reaction with chlorohydric acid. A mass balance is then used to determine the carbonate content increase. This method is notably described in patent FR 2920226 A1.

[0077]

[0051] Results and experimental conditions for each sample are listed in table 4.

[0078] Table 2 - Composition of Tailings

[0079]

[0080] Table 3 - Particle Size distribution of the tailings

[0081]

[0082]

[0083]

[0084] Table 4 - Composition of BOF and EAF slag

[0085]

[0086]

[0052] Results show that with a method according to the invention it is possible to increase the UCS above 100kPa and thus to stabilize the structure and avoid liquefaction. This is true even with very low content of CO2 in the atmosphere, such as illustrated in trials 4 and 11 but increasing the content of CO2 of the reagent allows to capture more CO2 and thus to reduce the carbon footprint.Table 5 - Results and experimental conditions (*According to prior art)

[0087] < < < < < < < < < < < < < < <

[0088]

Claims

CLAIMS1) Method to form a stabilized heap structure made of mine tailings, wherein an alkaline material is added to said tailings to form a heap structure, a reagent being injected into said heap structure, said reagent being a carbon-containing fluid, said carbon forming carbonates with the alkaline material, said carbonates stabilising the heap structure.2) A method according to claim 1 wherein the blending ratio between the alkaline material and the tailings is from 1 to 20 tons of alkaline material for 100 tons of tailings.3) A method according to claim 2 wherein the blending ratio between the alkaline material and the tailings is from 5 to 15 tons of alkaline material for 100 tons of tailings.4) A method according to any one of claims 1 to 3 wherein the carbon-containing fluid is chosen among a gas comprising more than 5% in volume of CO2 and an aqueous solution of carbonates.5) A method according to any one of claims 1 to 4 wherein the carbon-containing fluid is a gas comprising more than 15% in volume of CO2.6) A method according to any one of claims 1 to 5 wherein the aqueous solution of carbonates has a concentration of at least 1 mol per liter of carbonates.7) A method according to any one of claims 1 to 6 wherein the saturation degree of the heap structure is below 80%.8) A method according to any one of claims 1 to 7 wherein the carbon-containing fluid is chosen among a solution of K2CO3 and a solution of Na2CO3.9) A method according to any one of claims 1 to 8 wherein at least 90% in number of the particles of the tailings have a size below 3mm.10)A method according to claim 9 wherein at least 90% of the particles of the tailings have a particle size above 25pm.11 )A method according to any one of claims 1 to 10 wherein the tailings are iron ore tailings.12)A method according to claim 11 wherein the tailings are non-ultramafic tailings.13)A method according to any one of claims 1 to 12 wherein the tailings comprise less than 18% by weight of MgO.14)A method according to any one of the claims 1 to 13 wherein the alkaline material is chosen among iron slag, steelmaking slag, cement kiln dust, fly or bottom ash.15)A method according to any one of the claims 1 to 14 wherein the alkaline material comprises more than 5% by weight of CaO + MgO.16)A method according to any one of claims 1 to 15 wherein the alkaline material contains at least 25wt.% of CaO.17)A method according to any one of the claims 1 to 16 wherein the alkaline material is steelmaking slag.18)A method according to claim 17 wherein the steelmaking slag is BOF slag.19)A method according to anyone of the claims 1 to 18 wherein at least 90% of the particles of the alkaline material have a size of 500pm or below.20)A method according to anyone of the claims 1 to 19 wherein the carbon-containing fluid is chosen among a blast furnace gas, a coke oven gas, a sintering gas, a converter gas, a iron direct reduction process top gas, an electric arc furnace gas, an electrical smelting furnace gas, or a post-combustion gas resulting from the combustion of any of the previously listed gases.

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