A process for producing a construction material

Abstract

A process for producing a construction material comprises: (a) leaching a lithium containing material (11) to produce a lithium sulphate containing solution (24) and a delithiated residue (26) containing a proportion of sulphate; and (b) separating the delithiated residue (26) having thixotropic properties from the lithium containing solution (24) in a solid / liquid separation stage (23) The separated delithiated residue (26) is a precursor material processed to a construction material such as a supplementary cementitious material or geopolymer green cement. Neutralising, step (c) (25), the lithium containing solution (24) enables recovery of a precipitate from the lithium containing solution (24). With lime or limestone neutralisation, the precipitate is a product gyplime (29) which can also be used, for example as a soil amendment or construction material.

Description

TECHNICAL FIELD

[0001] The present invention relates to a process for producing a construction material, in a particular to a process for producing a supplementary cementitious material (SCM) or a geopolymer. BACKGROUND ART

[0002] The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

[0003] Each document, reference, patent application or patent cited in this text is expressly incorporated herein in their entirety by reference, which means that it should be read and considered by the reader as part of this text. That the document, reference, patent application or patent cited in this text is not repeated in this text is merely for reasons of conciseness.

[0004] Cement production has a high carbon footprint which may be reduced by use of supplementary cementitious materials (SCM) typically in the form of fly ash (FA) and ironmaking slag. However, as coal fired power plants close, the supply of fly ash (FA) is already dropping. Further, even when available as for example in Western Australia (WA), it may have poor properties. Ironmaking slag already has a significant demand and typically requires importation from neighbouring countries adding to the already high carbon footprint of cement production. The high demand for ironmaking slag is expected to increase further as more cement companies use it as fly ash (FA) generation significantly reduces with further closures of coal-fired power plants.

[0005] Leaching of lithium containing material, for example obtained by delithiating a mineral such as p-spodumene, recovers an aqueous solution containing lithium for treatment to recover a lithium salt. The lithium content is correspondingly depleted in the delithiated residue left behind after leaching. Delithiated residue, which has aluminosilicate mineralogy, is rich in silicon, aluminium, and calcium oxides. The delithiated residue having aluminosilicate mineralogy may be treated to recover valuable products such as alumina and silica, as described in the Applicant’s International Patent Publication Nos. WO 2019148233, WO 2021146768 and WO 2023044537, the contents of which are hereby incorporated herein by reference. Alternatively, the delithiated residue may be treated to produce high value zeolites. Such treatment schemes are aimed at enhancing the economics of lithium salt production, in particular for battery use. The potential for production of higher value products has replaced a previous alternative use of the residue as a construction material.

[0006] A pozzolan may be defined as a siliceous or siliceous and aluminous material that possesses little or no cementitious value, but which will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds having cementitious properties. While a delithiated residue may have pozzolanic activity, this is not sufficient of itself to make the material suitable as a supplementary cementitious material (SCM) under standards such as Australian Standard AS3582.4 Manufactured pozzolans (AS3582.4).

[0007] There have been multiple concerns about delithiated residue as a supplementary cementitious material (SCM). A first concern relates to the relatively high sulphur content (measured as SO3 content) of delithiated residue as much of this material is produced by a sulphuric acid leaching process. Its sulphuric acid extraction origins with elevated SO3 levels necessitate caution in its use to maintain cement stability. The AS3582.4 standard recommends maximum sulphite content of 3% for delithiated residue when used as an SCM. Higher sulphur content may significantly impact the strength of concrete due to sulphur attack over time. Currently, delithiated residue typically has substantially higher SO3 content, for example between 6.7 and 14 wt% and this is not acceptable as a supplementary cementitious material (SCM) under the AS3582.4 standard.

[0008] The present invention has been developed against the above background. SUMMARY OF INVENTION

[0009] In a first embodiment, the present invention provides a process for producing a construction material comprising: (a) leaching a lithium containing material to produce a lithium sulphate containing solution and a delithiated residue containing a proportion of sulphate; (b) separating the delithiated residue having thixotropic properties from the lithium containing solution in a solid / liquid separation stage, said separated delithiated residue being a precursor material to the construction material; and (c) separately neutralising the lithium containing solution and recovering a precipitate formed by neutralisation from the lithium containing solution, wherein said separated delithiated residue is processed to produce said construction material.

[0010] In a second aspect, the precursor material advantageously has a delithiated aluminosilicate structure and is substantially free of gypsum. By ‘delithiated’ is intended that the precursor material contains less than 0.5 wt% lithium, 0.3 wt% lithium or 0.1 wt% lithium.

[0011] In a preferred embodiment, the construction material is a supplementary cementitious material (SCM) produced by processing the precursor material by a combination of washing and drying steps to form a supplementary cementitious material (SCM) having a sulphate content below 3 wt.% measured as SO3.

[0012] Preferably, such processing of the separated delithiated residue comprises a plurality of washing steps which preferably follow an initial drying step of the separated delithiated residue. Washing is conveniently conducted with process water and use of demineralised water free of dissolved salts is not required. As well as reducing sulphate including acid content, such washing is also directed to recovery of entrained lithium and removal of soluble impurities.

[0013] Processing of the separated delithiated residue is conveniently conducted within the solid / liquid separation stage which may comprise a filter in which case the separated delithiated residue is comprised within a filter cake retained on a filter medium. This filter cake is conveniently subjected to the combination of washing and drying steps while retained on the filter medium. In this case, drying is conducted by blowing a gas - preferably air and optionally heated, if required to a selected drying temperature - through the filter medium and filter cake. In this case, the filter cloth has suitable air permeability to allow sufficient airflow to enable such drying, for example 1.5-6 l / min.dm2. Alternatively, the filter cake could be dried externally to the filter, optionally using other drying techniques. Oven or kiln drying may be used.

[0014] Following washing, the washing liquor - together with dissolved lithium, impurities and sulphuric acid - is drained from the delithiated residue. A further drying step follows washing. Preferably, drying is conducted by flowing a gas, conveniently air, through the substantially processed delithiated residue. Again, the gas may optionally be heated to a selected drying temperature.

[0015] The washing water volume is selected to reduce sulphate content while not overly diluting any wash stream that may be returned to lithium recovery steps, given its lithium content, as any consequential lithium concentration steps would represent an undesirable cost. The washing water volume may desirably be selected between 0.53m3 per tonne of delithiated residue.

[0016] The sulphate content of the processed delithiated residue - as assayed for example under Australian Standard (AS) 3582.4, is below 3 wt% SO3, preferably below 1 wt% SO3, most preferably below 0.5 wt% SO3. The delithiated residue may also have a SiO2+Al2O3+Fe2O3 content of greater than 90-97 wt% with SiO2+Al2O3 content being high at 80-95 wt%, supporting pozzolanic activity.

[0017] If desired, the size of the processed delithiated residue may be reduced, such as by ball milling, to mechanically activate it, increase its reactivity and provide further mechanical strength in product cement. However, this is an optional step.

[0018] The precipitate from step (c), where neutralisation conveniently involves lime / or limestone (typically in excess), substantially comprises a mixture of gypsum in hydrated form (CaSO4.2H2O) and unreacted lime. Such a mixture may be referred to as gyplime and it may be used as a soil ameliorant or fertiliser in agriculture or horticulture; or as a construction material, for example for blending with another material such as a geopolymer. The gyplime precipitate may be washed and / or dried as required to meet specification for use as a soil ameliorant or fertiliser; or as a construction material.

[0019] In a second aspect of the invention is provided a supplementary cementitious material (SCM) produced by the process as described above. If desired, the supplementary cementitious material (SCM) may be dosed with an alkali such as sodium hydroxide. Sodium hydroxide, for example, increases the solubility of aluminosilicate materials, aiding in their reactivity and incorporation into the cement matrix. It is used in the production of supplementary cementitious materials (SCMs) to enhance the compressive strength of concrete, promotes the early hydration of calcined clay (CC), accelerating the chemical reactions that contribute to the development of strength in the material.

[0020] In a third aspect of the invention is provided a supplementary cementitious material (SCM) comprising an aluminosilicate and having a sulphite content of less than 3 wt%, Na2O less than 1.0 wt % and a calcium oxide content of less than 1.1wt%. MgO content is also less than 5 wt%. The available alkali content is low as understood in the art of cementitious materials. The supplementary cementitious material (SCM) may have Blaine fineness about 250 m2 / kg to 500 m2 / kg, specific surface area of about 2.5 m2 / g to about 4.5 m2 / g and density of about 1.7 to 3.5 g / cm3. Particle size distribution (Dso) ranges from 40 to 195 pm.

[0021] In a fourth aspect of the invention is provided a cement or other construction material containing the supplementary cementitious material (SCM) produced by the process as described above. For example, the supplementary cementitious material (SCM) may replace between 20-35 wt% of cement dependent on the required strength grade.

[0022] In a fifth aspect of the invention, the separated delithiated residue is a precursor material processable to produce a geopolymer, for example for use in production of geopolymer green cement. A geopolymer cement is produced by activating one or more aluminosilicate materials, in particular the separated delithiated residue or precursor material produced by the process described above, with alkali or alkali silicate solutions. By combining a blend of aluminosilicate materials including the separated delithiated residue (e.g. the separated delithiated residue and one or more selected from fly ash, blast furnace slag, kaolin and red mud) and an optional aggregate and optimising factors selected from the group consisting of curing temperature, alkali concentration, additives and the Na2O / SiO2 ratio, high strength and durable geopolymer cements may be produced. Geopolymer and geopolymer cement production is not subject to the SO3 constraint discussed above and so processing of the separated delithiated residue by a combination of washing and drying steps, though advantageous, is not required to an extent to obtain SO3 content below 3 wt%.

[0023] Gyplime, where produced in step (c), may be blended in controlled proportion with the delithiated residue or precursor material with benefit in terms of control of geopolymer mix preparation in contrast to a prior scenario where lime neutralisation of lithium containing solution in the presence of delithiated residue introduced uncontrolled proportions of calcium. For example, a controlled amount of gyplime may be mixed with the processed delithiated residue, preferably from washing and drying, to accelerate setting rate. In this regard, too little gyplime addition or Ca source in any form can result in longer setting times.

[0024] Geopolymer green cement production is expected to generate significantly lower CO2 emissions than OPC. While OPC emits 5-7% of global CO2 emissions, geopolymer only contributes 20-30% of that amount. Geopolymer cements also exhibit superior tensile and compressive strength to OPC as well as the following further properties: • Durability: geopolymer is highly resistant to corrosion, extreme temperatures and a range of acids, alkalis and salts; • Waterproofing: with pores smaller than 50 nm, geopolymer is highly impermeable to water; • Fire Resistance: geopolymer concrete is fire-resistant, as the water within the material evaporates safely preventing internal explosions; • Thermal Insulation: Foamed geopolymer concrete provides excellent thermal insulation properties; and • Construction Speed: Geopolymer concrete gains about 50% of its strength within the first 3 days, enabling faster construction times.

[0025] The separated delithiated residue or precursor material may be divided into two fractions. A first fraction of the separated delithiated residue may be directed to processing to SCM including the washing and drying processes described above. A second fraction of the separated delithiated residue may be directed to processing to geopolymer, in particular geopolymer green cement, as described above. This allows process flexibility and the ability to respond both to SCM demand for cement production and demand for aluminosilicate feedstock for polymer green cement production.

[0026] In a sixth aspect of the invention is provided a precursor material from the process as described above, whether or not processed to an SCM or other construction material in a lithium extraction plant. For example, the precursor material could be processed to the construction material remote from the lithium extraction plant, for example by a construction material manufacturer.

[0027] The process as described above allows a specifically processed delithiated residue to be used as a construction material, in particular as a supplementary cementitious material (SCM) but alternatively as a geopolymer, reducing the carbon dioxide (CO2) footprint of cement production. The processed delithiated residue may also be used as a backfill for mining or other excavations. A further gyplime product -free of delithiated residue contamination - is advantageously obtained following neutralisation of the lithium containing solution in the absence of delithiated residue. Such gyplime product may be used as a soil amendment / ameliorant or fertiliser in agriculture or horticulture. Such by-product formation improves process economics and aids decarbonisation efforts by reducing the carbon footprint of lithium salt production through a sulphate route whether using sulphuric acid or an alkali metal sulphate typically in alkaline solution. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Further features of the process for producing a construction material of the present invention are more fully described in the following description of several nonlimiting embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:

[0029] Figure 1 is a schematic partial flowsheet of a prior art process for treating a lithium aluminosilicate involving formation of a delithiated residue.

[0030] Figure 2 is a schematic partial flowsheet of a process for producing a supplementary cementitious material (SCM) and gyplime from a delithiated residue according to a first embodiment of the present invention.

[0031] Figure 3 is a schematic partial flowsheet of a process for producing a geopolymer green cement and gyplime according to a second embodiment of the present invention. DESCRIPTION OF PREFERRED EMBODIMENTS

[0032] Referring to Fig. 1, spodumene concentrate 1 is directed to calcination 2 for calcining at 1050-1100°C to increase its reactivity by phase transforming a-spodumene into p-spodumene prior to extraction of lithium in process 10. Calcination 2 involves a rotary kiln in this embodiment. In alternative embodiments, other calciners - such as flash calciners may be used - though the choice of calcination equipment of influences the properties of product SCM material.

[0033] After phase transformation, p-spodumene 3 is directed to acid roast stage 4 where it is roasted with sulphuric acid, at for example about 250°C. The roasted mixture 5 is water leached to liberate lithium (i.e. by delithiation of the p-spodumene 3) as lithium sulphate solution. The whole pulp comprising lithium sulphate solution and leach residue is neutralised with limestone (CaCOs) in leaching / neutralisation stage 6. The result is separation of calcium as gypsum forming a substantial part of a delithiated residue 9 containing a mixture of aluminosilicates, gypsum and some unreacted p-spodumene. In other embodiments, a neutralising agent other than limestone may be used, for example an alkaline hydroxide (e.g. NaOH or KOH) or carbonate (e.g. Na2COs or K2CO3). In such circumstances, gypsum is not produced.

[0034] The lithium sulphate solution 7 is separated from the leach residue 9 in a single liquid / solid separation stage 8. This separation is standard in the industry, a key reason being that the bulking of gypsum with the leach residue 9 greatly facilitates its separation from lithium sulphate solution 7.

[0035] The delithiated residue 9 has an aluminosilicate structure and contains a large proportion of calcium (4.4-10.1 wt%). The assayed sulphite content, according to Australian Standard AS3582.4 Supplementary Cementitious Materials - Pozzolans, Manufactured, typically ranges from 6.7-14 wt% SO3. These elevated levels of assayed sulphite are a result of the sulphuric acid extraction methodology (forming gypsum and leaving residual sulphuric acid) and make the delithiated residue 9 unsuitable for use as a supplementary cementitious material under the AS3582.4 standard. That is because the presence of high SO3 indicates the presence of ettringite and flaky gypsum, which accelerates corrosion and can adversely impact the stability and performance of a cement matrix.

[0036] Referring to Fig. 2, there is shown part of a lithium extraction process 20, the front end of which is the same as that described with reference to Fig. 1. Thus, the same unit operation stages have the same reference number with the prefixT.

[0037] Spodumene concentrate 11 is directed to calcination 12 for calcining at 10501100°C to increase its reactivity by transforming a-spodumene into p-spodumene during extraction of lithium in process 30. After this phase transformation, p-spodumene 13 is directed to acid roast stage 14 where it is roasted with sulphuric acid, at for example about 250°C. A pulp of the roasted mixture 15 is water leached to liberate lithium in an acidic lithium sulphate solution 24 in leaching stage 20 leaving a delithiated residue 26.

[0038] Delithiated residue 26 is separated from lithium sulphate solution 24 in solid / liquid separation stage 23. Solid / liquid separation stage 23, which involves filtration conveniently in a filter press, is complicated by the thixotropic properties of delithiated residue 26, in the absence of gypsum, and by the rapid settling characteristics of particles of the delithiated residue. It is important to avoid settling and hardening when hydraulic pressure is applied during filtration and high-pressure filtration is preferred.

[0039] The delithiated residue 26 has an aluminosilicate structure (containing 63-74 wt% silica and 20-25 wt% alumina) and contains a negligible proportion of calcium in the form of gypsum with leaching stage 20 being conducted in the absence of added calcium salts contrary to the process of Fig. 1. The particle size distribution (PSD) of the material, as measured by P50, is 27-41 pm. While the moisture content is around 24 wt%, prior to drying, most of the moisture is interstitial. The delithiated residue 26 is a precursor material processable to a construction material as described below.

[0040] The pH of the material, having come from an acidic leaching process is 1.5-2.00 or, in some embodiments, 1-1.5. The particle size distribution, as measured by P80, is 60-86 microns. Sulphite content is for example 3.5-4.5 %, still above the 3 wt% limit specified by Australian Standard AS 3582.4. Gyplime Production

[0041] The acidic lithium sulphate solution 24 is neutralised, with lime or limestone (CaCOs) as neutralising agent in neutralisation stage 25. The resulting lithium sulphate solution 28 is separated from a gyplime by-product 29 in liquid / solid separation stage 27 - preferably adopting plate and frame filtration - and further processed to a lithium salt such as lithium hydroxide monohydrate or lithium carbonate. Gyplime by-product 29, comprises a mixture of gypsum and unreacted lime and, importantly, a substantial proportion of sulphur as sulphate. The gyplime by-product 29 provides an advantage for use of lime or limestone as a neutralising agent. Use of NaOH or KOH as neutralising agents would not allow the production of a comparable by-product and would therefore be disadvantageous.

[0042] The gyplime by-product 29 may be used as a soil amendment / ameliorant for the improvement of soil physico-chemical properties such as pH, soil texture, structure and availability of micro and macro nutrients in soil for plant growth and potential increase harvest yields. Gyplime by-product may also be used as a construction material, for example as a source of calcium with an exemplary embodiment of one such application being described below. Lithium Salt Production

[0043] Following further treatment of lithium sulphate solution 28, which is combined with the wash liquor from the processes described below, lithium hydroxide monohydrate (LHM) or lithium carbonate is produced. The product lithium salt is desirably battery grade for inclusion in lithium ion batteries (“LIBs”), in particular for electric vehicles and other electrical equipment. Approximately seven to ten tonnes of delithiated residue 26 is produced per tonne of LHM. In alternative embodiments, other lithium salts such as lithium phosphate or lithium iron phosphates etc. could be produced instead dependent on the requirements of LIB manufacturers. Production of Supplementary Cementitious Material

[0044] Processing of delithiated residue 26 as a precursor for a construction material, through a selected combination of washing and drying steps in the case of SCM demand, is now described in further detail. The processing is preferably conducted while the delithiated residue or precursor material 26 is retained as a filter cake on a filter medium in the filter used in solid / liquid separation, i.e. filtration, stage 23. In other embodiments, processing could be conducted externally of the filter though this would be expected to increase capital costs for the additional equipment and land required.

[0045] Delithiated residue 26 is subjected to a washing sequence involving a Dry-Wash-Dry cycle. In this embodiment, six washing and drying steps are used during the washing sequence.

[0046] First, delithiated residue 26 is dried to reduce its moisture content. This involves blowing air through the filter medium, here a filter cloth - for example a polypropylene filter cloth - with air permeability of between 1.6-5.8 l / min.dm2. The air may be heated to enhance drying rate if desired.

[0047] Once the delithiated residue 26 is dried, it undergoes washing with water in a second step, still conducted in the plate and frame filter press used, in this embodiment, for filtration. This second step involves using a certain volume of water (for example and preferably 0.5-1.5m3 / t of delithiated residue 26) to wash the aluminosilicate, likely to remove impurities or other substances, including soluble lithium and other soluble sulphates, from the material.

[0048] After the delithiated residue or precursor material 26 is washed with water, the resulting wash liquor (the water used for washing along with dissolved impurities, residual acid from leaching stage 20 and also resulting in scavenging of entrained lithium) is drained away from the aluminosilicate leach residue. The wash liquor is conveniently recycled and may be used for repulping of the roasted mixture 15 for leaching in leaching stage 20; or directed to lithium salt production dependent on lithium concentration. The volume of water is controlled so as not to make lithium recovery operations more difficult through low lithium concentration in the lithium sulphate solution 24. While concentration is possible, this adds capital and process costs and may not be justified by the value of the processed delithiated residue 26.

[0049] Following the draining of the wash liquor, the delithiated residue 26 undergoes another drying cycle. This secondary drying process involve blowing air through the delithiated residue 26 to further reduce its moisture content. Again, this air may be heated to increase drying rate if desired. Once the treated delithiated residue 26 has undergone the secondary drying phase, it is dropped into a bunker or storage area ready for supply as a supplementary cementitious material (SCM) for various applications.

[0050] A detailed analysis of the processed delithiated residue 26 (SCM) is as shown in Table 1: Table 1 Summary of Processed delithiated residue sample chemical properties Analytes Units Range AI2O3 %m / m 20.0-25.0 AS2O3 %m / m 0.01 -0.02 CaO %m / m 0.04-1.2 Cl %m / m 0 - 0.03 Fe2Os %m / m 0.03- 1.2 K2O %m / m 0.2-0.6 MgO %m / m 0.1 -0.3 Na2O %m / m 0.2-0.4 SiO2 %m / m 63.0-74.0 SO3 %m / m 0.4-2.4 MnO %m / m 0.03-0.8 P2O5 %m / m 0.1 -0.3 TiO2 %m / m 0.02-0.1 tot_Li %m / m 0.1-0.9 Moisture %m / m 16.0-24 LOI %m / m 3.2-5.2

[0051] Table 1 shows an SO3 content that meets the AS3582.4 standard as well as other favourable properties for use as a supplementary cementitious material (SCM) as 5 described further below.

[0052] As apparent from the above table, processed delithiated residue 26 has SiO2+Al2O3+Fe2O3 content of greater than 85-97 wt% with SiO2+Al2O3 content being high at 80-95 wt%, supporting pozzolanic activity and availability for use as a supplementary cementitious material (SCM). Such SCM also has Na2O content less than 1.0 wt% and a calcium oxide content of less than 1.1 wt%. MgO content is also less than 5 wt%. The available alkali content is low as understood in the art of cementitious materials. The supplementary cementitious material (SCM) also has Blaine fineness about 250 m2 / kg to 500 m2 / kg, specific surface area of about 2.5 m2 / g to about 4.5 m2 / g and density of about 1.7 to 3.5 g / cm3. Particle size distribution (Dso) ranges from 40 to 195 pm. The pH of the delithiated residue 26 rises from 1-1.5 to 3-4 during washing.

[0053] As an example, the SCM produced as described above may be included in a 40 MPa strength concrete at an OPC cement replacement ratio of, for example 25%. Replacement ratio is dependent on the required concrete strength though a replacement ratio of 20-35% is expected to be achievable. Further, compressive strengths with the supplementary cementitious material (SCM) produced as described above may, as measured according to the Australian Standard AS 2350 (relating to test methods adopted for the evaluation of Australian cements and the contents of which are hereby incorporated herein by reference), be at least 25% greater than fly ash (FA) replaced concrete at a 28-day aging time. Serviceability and durability performance, as measured under the AS 2350 standard, is also generally superior performance in comparison to fly ash (FA). Thus, the supplementary cementitious material (SCM) produced from treated delithiated residue 26 compares favourably with fly ash which makes it a more than viable replacement source even as fly ash (FA) supply declines in the future. Geopolymer Production

[0054] Delithiated residue 26 may also be processed, without reference to an 3 wt% SO3 constraint which is unnecessary for geopolymer applications, to produce a geopolymer green cement as now described with reference to Fig. 3.

[0055] Fig. 3 shows part of a lithium extraction process 110, the front end of which is the same as that described with reference to Fig. 1.

[0056] p-spodumene concentrate 11, produced following calcination of an a-spodumene concentrate to increase reactivity according to conventional practice, is directed to acid roast stage 14 where it is roasted in a rotary kiln (or other suitable vessel as known in the art) with an aqueous solution of sulphuric acid 13a and water 13b, at temperature for example about 250°C. The rotary kiln is heated by combustion of natural gas 13c though other heating alternatives may be adopted if desired.

[0057] A pulp of the roasted mixture 15 is water leached in leaching stage 20 with water 21 to liberate lithium into an acidic lithium sulphate solution 24 leaving a delithiated residue 26.

[0058] Delithiated residue 26, as a precursor material to a geopolymer green cement, is separated from lithium sulphate solution 24 in solid / liquid separation stage 23. Solid / liquid separation stage 23, which involves filtration conveniently in a plate and frame filter press, is complicated by the thixotropic properties of delithiated residue 26, in the absence of gypsum, and by the rapid settling characteristics of particles of the delithiated residue. It is important to avoid settling and hardening when hydraulic pressure is applied during filtration and high-pressure filtration is preferred as known in the art of filtration of lithium leach residues.

[0059] The delithiated residue 26 has an aluminosilicate structure (containing 63-74 wt% silica and 20-24 wt% alumina) and contains a negligible proportion of calcium with leaching stage 20 being conducted in the absence of added calcium salts contrary to the process of Fig. 1. The particle size distribution (PSD) of the material, as measured by Pso, is 27-41 pm. While the moisture content is around 24 wt%, prior to drying, most of the moisture is interstitial.

[0060] The pH of the delithiated residue 26, having come from an acidic leaching process is 1.5-2.00. The particle size distribution, as measured by P80, is 60-86 microns. Sulphite (SO3) content is for example 3.5-4.5 wt%.

[0061] The delithiated residue 26, processed following washing and drying following the same process as described above though without necessity to achieve a target of less than 3 wt% SO3 in the case of geopolymer cement production, may be used in one embodiment, as a full or partial replacement for fly ash (FA) which is becoming more difficult to obtain, from coal fired power stations, as decarbonisation proceeds. In this embodiment, the geopolymer green cement was produced through the alkali activation of an aluminosilicate feedstock containing either fly ash (FA) alone or a mixture of fly ash (FA) and processed delithiated residue 26 as described above. It is to be understood that, in other embodiments, alternatives to FA are used in geopolymer cement production and so references to FA herein are not intended to be limiting. For example, kaolin or ironmaking slag could be used along with other examples.

[0062] A highly alkaline medium, in the form of a combination of NaOH or KOH (as alkali activator) with Na2SiOs or K2SiO3 30 is used as alkali activator by mixing in the ratio of 1.0:1. This solution is then mixed with fly ash (FA), and 10-20% replacement of fly ash (FA) with processed delithiated residue 26 until a homogenous mix is achieved. Alkali activator is used to dissolve silica and alumina from the aluminosilicate feedstock and catalyse the condensation polymerisation reaction to form the geopolymer. The mixture is then poured into a mould 34 and may, in some embodiments, be vibrated or pressed, with pressing for example being conducted at 5 tons for 2 minutes. The geopolymer green body 36 is removed from the mould and, may in some embodiments, be wrapped with plastic film and kept at room temperature for 28 and 60 days. Accelerated curing may be employed through heat to accelerate strength development.

[0063] Table 2 shows a compressive strength comparison for a geopolymer green cement having different replacement levels of FA by processed delithiated residue 26 (PDR) cured at room temperature. Results are also provided for a geopolymer green cement formed by alkali activation (with the same alkali activator as described above) of FA alone. Table 2 Comparison of Compressive Strength (MPa) for Geopolymer at various levels of FA Replacement PDR (wt%) Replacement of FA 0 10 20 28-day strength 53 MPa 70 MPa 80 MPa 60-day strength 61 MPa 92 MPa 90 MPa

[0064] Table 2 shows that compressive strength of PDR replaced FA is substantially higher than where FA alone is used as alumino-silicate feedstock to geopolymer green cement production. This shows that PDR 26 has unexpected utility in acting as a replacement for FA in geopolymer green cement production.

[0065] In embodiments, a controlled amount of gyplime by-product 44 - as described above - may be added to processed delithiated residue 26 in order to accelerate setting rate. The ability to add a controlled amount of gyplime (and hence calcium) provides a benefit not possible when using delithiated residue 26 containing excessive gypsum as is the case where the lithium sulphate solution is neutralised by lime or limestone in the presence of delithiated residue. Such excessive gypsum can reduce the compressive strength of geopolymers.

[0066] The process flowsheet of Fig. 3 allows gyplime by-product 44 production by the same process as described above. In summary, acidic lithium sulphate solution 24 is neutralised, with lime or limestone (CaCOs) 41 as neutralising agent in neutralisation stage 40. The resulting lithium sulphate solution 42 is separated from a gyplime byproduct 44 by liquid / solid separation - preferably plate and frame filter press filtration -and further processed to a lithium salt such as lithium hydroxide monohydrate or lithium carbonate useful for lithium ion battery (LIB) production.

[0067] Processed delithiated residue 26, produced as described above, is therefore an effective feedstock, i.e. precursor material, for geopolymer green cement production providing at least improved (to an unexpected degree) compressive strength compared to the case where FA is used as feedstock. Further, the supply of FA will reduce in coming years as coal fired power stations close. Advantages of the Invention

[0068] The delithiated residue 26, as processed above, may be used as a precursor for producing an effective construction material whether as a supplementary cementitious material (SCM) or geopolymer.

[0069] First, the processed delithated residue 26 as a supplementary cementitious material (SCM) allows production of concretes or cements with improved strength, serviceability and durability compared with good quality fly ash, the supply of which will reduce in coming years as coal power stations close. Delithiated residue 26 represents a new class of pozzolan that has the potential to improve compressive strength, mitigate ASR expansion, reduce sorptivity, reduce chloride diffusion and the potential for widespread use to partly replace general purpose (GP) cement and replace other SCMs in both domestic and structural concretes.

[0070] Alternatively, the processed delithiated residue 26 may be used as a geopolymer reducing the carbon dioxide footprint of cement production. The processed delithiated residue 26 may also be used for fabrication of other construction materials, such as geopolymers or used as a backfill for mining or other excavations.

[0071] A further gyplime product may be obtained following neutralisation of the lithium containing solution, using typically less expensive neutralising agents such as lime or limestone, in the absence of delithiated residue. Such gyplime product may be used as a soil amendment or fertiliser in agriculture or horticulture; or a construction material for example to be blended in controlled amounts with the processed delithiated residue 26 in particular for geopolymer cement production with a desired curing rate as described above. Such by-product formation improves process economics and aids decarbonisation efforts by reducing the carbon footprint of lithium salt production through a sulphate route.

[0072] Those skilled in the art will appreciate that the process for producing a construction material described herein is susceptible to variations and modifications other than those specifically described. The invention includes all such variation and modifications. The invention also includes all of the steps, features, formulations and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.

[0073] The invention described herein may include one or more range of values (e.g. temperature, concentrations and stages etc.). A range of values will be understood to include all values within the range, including the values defining the range, and values adjacent to the range which lead to the same or substantially the same outcome as the values immediately adjacent to that value which defines the boundary to the range.

[0074] Throughout this specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

1. A process for producing a construction material comprising:(a) leaching a lithium containing material to produce a lithium sulphate containing solution and a delithiated residue containing a proportion of sulphate;(b) separating the delithiated residue having thixotropic properties from the lithium containing solution in a solid / liquid separation stage, said separated delithiated residue being a precursor material processable to the construction material; and(c) separately neutralising the lithium containing solution and recovering a precipitate formed by neutralisation from the lithium containing solution,wherein said separated delithiated residue is processed to produce said construction material.

2. A precursor material having a delithiated aluminosilicate structure and being substantially free of gypsum produced by the process of claim 1.

3. A process for producing a construction material by processing said precursor material of claim 1 or 2 by a combination of washing and drying steps to form a pozzolanic supplementary cementitious material (SCM) having a sulphate content below 3 wt% measured as SO3.

4. The process of claim 3, wherein said processing of said precursor material to SCM comprises a plurality of washing steps which follow an initial drying step following separation of delithiated residue in step (b).

5. The process of claim 3 or 4, wherein washing of said precursor material is conducted with process water.

6. The process of any one of claims 3 to 5, wherein said solid / liquid separation step (b) comprises filtration by a filter and washing of a filter cake substantially comprising said precursor material is conducted within the filter.

7. The process of claim 6, wherein a combination of washing and drying steps to form said SCM is conducted within the filter.- 198. The process of claim 7, wherein drying is conducted by blowing a gas heated to a selected drying temperature through the filter cake.

9. The process of any one of claims 3 to 8, wherein a wash stream is directed for lithium recovery.

10. The process of claim 9, wherein volume of said wash stream is between 0.5-3m3 per tonne of said separated delithiated residue.

11. The process of any one of claims 3 to 10, wherein sulphate content of said SCM is below 1 wt% SOs.

12. The process of claim 11, wherein sulphate content of said SCM is below 0.5 wt% SO3.

13. The process of any one of claims 3 to 12, wherein said SCM has a SiO2+Al2O3+Fe2O3 content of greater than 90-95 wt% with SiO2+Al2O3 content being 80-99 wt%.

14. The process of any one of claims 3 to 13, wherein the SCM is milled to mechanically activate the SCM and increase its reactivity.

15. The process of any one of claims 3 to 14, wherein said SCM is dosed with an alkali.

16. The process of any one of claims 1 or 3 to 15, wherein neutralisation step (c) comprises neutralisation of the lithium containing solution with at least one of lime and limestone and said precipitate is gyplime substantially comprising a mixture of gypsum in hydrated form (CaSO4.2H2O) and unreacted lime.

17. The process of any one of claims 3 to 16, wherein the precursor material is activated with an alkali or alkali silicate to form a geopolymer.

18. The process of claim 17, wherein the precursor material is blended with at least one aluminosilicate material, said aluminosilicate material optionally being selected from the group consisting of fly ash, blast furnace slag, kaolin and red mud.

19. The process of claim 17 or 18, comprising blending a controlled amount of gyplime produced in neutralisation step (c) with the precursor material to accelerate setting rate.-2020. A supplementary cementitious material (SCM) produced by processing said precursor material according to any one of claims 1 or 3 to 15.x21. The supplementary cementitious material of claim 18, wherein said SCM comprises an aluminosilicate and having a sulphite content of less than 3 wt%, Na2O content less than 1.0 wt% and a calcium oxide content of less than 1.1 wt%.

22. The supplementary cementitious material of claim 19, wherein said SCM has Blaine fineness about 250 m2 / kg to 500 m2 / kg, specific surface area of about 2.5 m2 / g to about 4.5 m2 / g, density of about 1.7 to 3.5 g / cm3 and particle distribution (Dso) ranging from 40 to 195 pm.

23. A construction material containing a supplementary cementitious material (SCM) as claimed in any one of claims 18 to 20.

24. The construction material of claim 21, being a cement.

25. The construction material of claim 22, wherein said cement contains between 20-35 wt% of said SCM.

26. A geopolymer green cement comprising a geopolymer produced according to any one of claims 17 or 18.

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