The invention relates to a method of recycling lead-containing waste and a method of recycling a lead-acid battery.

Lead-acid batteries are widely used in the automotive and other industries due to their rechargeable nature and relatively low cost. During discharge, the lead and lead dioxide that is present in the battery plates converts to lead sulphate. Recharging the battery converts the lead sulphate back to lead and lead dioxide. Although lead-acid batteries are rechargeable, over time lead sulphate can crystallize as large passivating crystals in the battery plates thereby reducing the propensity of lead sulphate to convert back into lead and lead dioxide. This results in a deterioration of battery performance. Eventually, the battery will have to be replaced.

A high proportion of waste lead-acid batteries are recycled. In a typical process, the used batteries are comminuted, and the lead-containing solids separated from other battery waste components such as plastic materials and the electrolyte. The lead-containing solids originate from the battery plate which is made up of a battery grid and a battery paste. Generally, the spent battery paste is passed to a smelter for pyrometallurgical processing into a lead ingot. The lead ingot may then be used for manufacturing new lead-acid batteries. For instance, lead ingot may be used to manufacture a new battery grid, or it may be oxidized using a Barton pot or heated ball-mill process to produce leady oxide, which is a mixture of lead oxide and free metallic lead. This leady oxide may then be reused as the active, redox material in lead-acid battery plates.

These traditional recycling processes are highly energy intensive, with temperatures of approximately 1100 °C required for the decomposition of lead sulphate. Traditional recycling processes can also be highly polluting. In particular, sulphur dioxide, nitrogen dioxide and, often, lead particles are produced in the high-temperature smelter. In order to prevent pollutants from being released into the atmosphere, specialist equipment and timeconsuming processes are required. These can represent a significant expense in the recycling process.

In recent times, methods have been developed which enable lead-acid battery waste to be processed more efficiently. WO 2008/056125 discloses a process in which spent battery paste is mixed with aqueous citric acid so as to generate lead citrate. The lead citrate may then be converted into leady oxide by calcination. This enables the direct production of leady oxide from spent battery paste, without handling an intermediate lead ingot.

There are a number of advantages associated with the method disclosed in WO 2008/056125. In particular, leady oxide is manufactured directly from spent battery paste thereby avoiding the cost and logistics associated with the downstream processing of lead ingot to leady oxide. Moreover, the citrate acts as a fuel in the combustion process, thereby reducing the amount of energy that is required to be supplied to the calcination furnace. The conversion of lead citrate into leady oxide also occurs at a much lower temperature than that required for the decomposition of lead sulphate, further reducing the energy burden of the recycling method. In fact, the method disclosed in WO 2008/056125 releases approximately 400 MWh of energy per 1000 tonnes of battery throughput.

In order for leady oxide to be re-used as a paste for preparing battery plates, it must have a very high purity level. Lead ingot produced by conventional smelting techniques typically exhibits an impurity level of less than 0.1 %, and often less than 0.01 %, by weight. Commercial methods for recycling lead-acid batteries via alternative routes would therefore ideally also provide a lead product having these purity levels. Unfortunately, the leady oxide that is produced following the method disclosed in WO 2008/056125 is not consistently pure enough to be directly used in batteries, and instead requires further processing before use.

Another drawback of the method disclosed in WO 2008/056125 is that it requires a large excess of citric acid to convert a sufficient proportion of the lead that is present in the spent battery paste into lead citrate. This means that the cost of carrying out the method is highly dependent on the cost of citric acid, leaving the economic viability of the method vulnerable to fluctuations in the price of citric acid.

In summary, whilst the method disclosed in WO 2008/056125 represents a significant step forward in the recycling of spent lead-acid batteries, the method is reliant on the use of a large excess of citric acid and the quality of the paste that is produced by the method is often not high enough for direct use in a battery plate.

A further method in which citric acid is used is disclosed in He et al., Minerals, 2017, 7(6), 93. This method addresses some of the shortcomings of the method disclosed in WO 2008/056125. However, the method requires the use of elevated temperatures, relatively long dissolution times and high reagent concentrations. As such, it does not represent an attractive, commercial and scalable method for recycling lead-acid battery paste.

A further method for recycling lead-acid battery paste in which citrates are formed is proposed in Zhu et aL, J. Hazard Mater., 2013, 250-251 , 387-396. This paper discloses reacting each of PbO, PbOa and PbSC individually with a mixture of sodium citrate and acetic acid, and subsequently using the mixture of sodium citrate and acetic acid on spent lead-acid battery paste. However, the purity of the lead citrate obtained by this method is not significantly different from that obtained following the method disclosed in WO 2008/056125.

Another method for recycling battery paste is disclosed in Sun et al., Journal of Power Sources, 2014, 269, 565-576. According to this method, spent lead-acid battery pastes are converted into lead acetate, the lead acetate is crystallised and purified using glacial acetic acid, and the purified lead acetate calcined to give lead oxide. However, the solubility of lead acetate in aqueous systems is very high. This means that considerable effort would be required to crystallise the lead acetate salt from the aqueous phase. A significant amount of the lead burden would also inevitably be lost in the aqueous phase. Accordingly, the method disclosed by Sun et al. is not scalable on an industrial level.

W02020/152457 relates to recycling of lead-containing waste to leady oxide. The method comprises: (a) dissolving the lead-containing waste in an aqueous solution of a first acid (e.g. acetic acid) to form a solution of a first lead salt; (b) adding a second acid (e.g. citric acid) to the solution of the first lead salt to form a lead-depleted solution and a precipitate of a second lead salt; and (c) converting the precipitate of the second lead salt into leady oxide, wherein the first lead salt has a higher solubility in water than the second lead salt. The method may result in the generation of high levels of lead hydroxide, from which it is difficult to remove water. The method also requires large amounts of energy, water and reagents, and therefore has a high cost and environmental impact.

GB2586582A relates to desulfurization of lead-containing waste.

The present invention seeks to tackle at least some of the problems associated with the prior art or at least to provide a commercially acceptable alternative solution thereto. In a first aspect, the present invention provides a method of recycling lead-containing waste, the method comprising: providing a slurry of a lead-containing waste, the lead-containing waste comprising lead sulphate; passing the slurry through a comminution unit while controlling the pH of the slurry to be from 12 to 14 to convert at least a portion of the lead sulphate to lead oxide to provide a comminuted, sulphate-depleted, lead-containing waste and a sulphate solution, separating the comminuted, sulphate-depleted, lead-containing waste from the sulphate solution, dissolving the comminuted, sulphate-depleted, lead-containing waste in an aqueous solution of a first acid to form a solution of a first lead salt; adding a second acid to the solution of the first lead salt to form a lead-depleted solution and a precipitate of a second lead salt, the first lead salt having a higher solubility in water than the second lead salt; and converting the precipitate of the second lead salt into leady oxide, wherein the comminution unit operates in a continuous manner.

Each aspect or embodiment as defined herein may be combined with any other aspect(s) or embodiment(s) unless clearly indicated to the contrary. In particular, any features indicated as being preferred or advantageous may be combined with any other feature indicated as being preferred or advantageous.

The inventors have surprisingly found that, in comparison to conventional methods of recycling lead-containing waste, the method of the present invention may be more efficient, has a lower energy demand and requires lower levels of water and/or reagents.

Slurries of lead-containing waste are typically acidic, or at least have a pH lower than that required during the comminution step. By operating the comminution unit (or comminuting unit) in a continuous manner, slurry added to the comminution unit is combined with material already present in the comminution unit having a pH of 12 to 14. This may result in less fluctuation in the pH in the comminution unit. Drops in the pH in the comminution unit may increase the formation of lead hydroxide (such as, for example, Pb(OH) ₂ , PbsO3(OH) ₄ and/or PbsO ₂ (OH) ₂ ) in the comminution unit. The formation of lead hydroxide may be unfavourable because it may lead to “caking”, i.e. the formation of lumps, meaning that desulphurisation, and subsequent reactions with the first and second acids, are less efficient. In addition, lead hydroxide may be difficult to dry and/or press, meaning that it may be more difficult to separate the comminuted, sulphate-depleted, lead-containing waste from the sulphate solution. This may increase the energy demand of the method and/or require larger amounts of reagents. In this regard, the comminuted, sulphate-depleted, lead-containing waste typically comprises less than 1 %, more preferably less than 0.5 %, and more preferably less than 0.1 % by weight of lead hydroxide forms. Furthermore, by operating the comminution unit in a continuous manner, any temperature rises that may occur as a result of comminution and desulphurisation may be levelled out to a controlled level.

Controlling the pH of the slurry to be from 12 to 14 may deflocculate the particles, which would normally stick together under acidic conditions. A flocculated slurry typically settles as “fluffy” material, which may be easier to remove the liquid from. A flocculated slurry will form a more porous surface within a press filter and hence it will be easier to remove entrained liquid from it. However, by deflocculating the particles, separation using differences in density may be more accurate. In addition, the deflocculation may release smaller particles from larger aggregates, which may result in less over-grinding and a reduction in energy wastage.

The method comprises providing a slurry of a lead-containing waste. The term “slurry” as used herein may encompass a mixture of solid particles, i.e. lead-containing waste, suspended in water. In other words, the slurry is typically an aqueous slurry.

The lead-containing waste comprises lead sulphate, i.e. PbSO4. The lead-containing waste will typically comprise other species, such as other lead-containing species. Examples of other lead-containing species include lead oxide, e.g. PbO, PbaOa and PbOa. When the lead- containing waste is derived from a lead-acid battery, the lead-containing wate may comprise battery acid, for example sulphuric acid.

Comminution units are known in the art and may be configured to carry out, for example, crushing and/or grinding and/or vibrating. Preferably, the comminution unit is configured to carry out grinding. Lead sulphate tends to be cementaceous, hard and difficult to dissolve, and therefore difficult to convert to lead oxide in a timely manner. To facilitate a timely, and therefore, economical conversion of lead sulphate to lead oxide, the solids need to be reduced in size to break up any cementations, provide increased surface area and be held at raised temperatures during the desulphurisation process. In the comminution unit the pH of the slurry is controlled to be from 12 to 14 to convert at least a portion of the lead sulphate to lead oxide. Typically, the majority of the lead sulphate is converted to lead oxide, more typically substantially all of the lead sulphate is converted to lead oxide. As discussed above, lower pH values may result in the unfavourable formation of lead hydroxide. In the comminution unit the pH of the slurry is preferably controlled to be from 12.5 to 13.5.

The method comprises dissolving the comminuted, sulphate-depleted, lead-containing waste in an aqueous solution of a first acid to form a solution of a first lead salt. Insoluble impurities may then be removed from the solution by, for example, filtration, prior to adding the second acid.

The method further comprises adding a second acid to the solution of the first lead salt to form a lead-depleted solution and a precipitate of a second lead salt. The second lead salt may be recovered from the lead-depleted solution, for example by filtration.

Preferably, the lead-depleted solution may be recycled for use as the aqueous solution of the first acid, thereby reducing the reagent burden of the method. Soluble impurities may be dissolved in the lead-depleted solution. The lead-depleted solution may undergo a regeneration step to remove at least some of the soluble impurities prior to recycling.

By use of the two acid solutions, it is possible to remove both soluble and insoluble impurities. Accordingly, the second lead salt may be of high purity.

The first lead salt has a higher solubility in water than the second lead salt. That is, the first lead salt has a higher solubility in water than the second lead salt at room temperature and pressure. The use of an acid which forms a first lead salt having relatively high aqueous solubility means that a higher proportion of the lead-containing waste is dissolved than when, for example, only an equivalent amount of citric acid is used. Unlike prior art methods, high levels of conversion may be achieved by then adding just stoichiometric amounts of the second acid (e.g. citric acid), which forms a relatively low solubility salt, to the dissolved lead.

The first acid is typically different to the second acid.

The method comprises converting the precipitate of the second lead salt into leady oxide.

The term “leady oxide” as used herein may encompass a mixture of lead oxide and free metallic lead. Converting the precipitate of the second lead salt into leady oxide typically comprises heating the second lead salt.

The comminution unit operates in a continuous manner. The term “continuous manner” as used herein may encompass a situation in which slurry is fed to the comminution unit and comminuted, sulphate-depleted, lead-containing waste and sulphate solution are removed from the comminution unit continuously. In other words, the term “continuous manner” as used herein may encompass a situation in which fresh slurry may be added to the comminution unit while comminuted, sulphate-depleted, lead-containing waste and a sulphate solution is being formed or has formed in the comminution unit.

The comminution unit is preferably a closed-circuit comminution unit. Closed-circuit comminution units are known in the art. The term “closed-circuit comminution unit" as used herein may encompass a system in which the comminuted (e.g. ground) material is discharged to a classifier, with the classifier returning any oversize material to the unit (e.g. mill) for further comminution (e.g. grinding). The use of a closed-circuit comminution unit is preferred since it allows for the continuous processing of different feedstocks having a varied, and unknown, sulphate content by simply maintaining the pH level of the system. Furthermore, the use of a closed-circuit comminution unit may reduce energy costs since material having the desired particle size is not kept in the comminution unit longer than necessary.

The comminution unit preferably comprises a mill, more preferably a ball mill or rod mill, even more preferably a rod mill. This may result in a particularly favourable narrow particle size distribution, which may increase the efficiency of the method, reduce the required energy input and/or speed up the method.

The preferred operating speed of the mill will be dependent on several factors, including its size. Preferably, the rod mill is operated at a speed below 90 % of the critical speed, and more preferably between 60 to 85 % of the critical speed of the mill, i.e. the speed at which the mill becomes a centrifuge. Since critical speed is related to the diameter of the mill cylinder, the preferred operating speed will be dependent on the scale on which the desulphurisation is performed. The mill may typically be operated at a speed of at least 5 rpm, preferably at least 10 rpm, and more preferably at least 15 rpm. The mill may be operated at a speed of up to 60 rpm, preferably up to 40 rpm, and more preferably up to 30 rpm. Thus, the mill may be operated at a speed of from 5 to 60 rpm, preferably from 10 to 40 rpm, and more preferably from 15 to 30 rpm.

The slurry preferably has a water to solids ratio by mass of from 0.3:1 to 1 :1 , preferably from 0.5:1 to 0.8:1 . Such a low water to solids ratio may reduce the amount of water required by the method. Such a low water to solids ratio is achievable due to the small particle size resulting from the comminution unit. Larger particle sizes may render subsequent reactions less efficient, thereby requiring more water. Such a water to solids ratio is lower than that typically employed in conventional methods, which is often in the region of 5:1 .

The comminuted, sulphate-depleted, lead-containing waste preferably comprises a particulate having: a D50 of from 50 to 90 pm, preferably from 65 to 80 pm, more preferably about 75 pm; and/or a D95 of from 100 to 150 pm, preferably from 110 to 130 pm, more preferably about 120 pm. Larger particle sizes may require longer reaction times. Smaller particles may not speed up the reactions to any significant degree but will require larger amounts of energy to be formed in the comminution unit. The term “D50" as used herein refers to the corresponding particle size when the cumulative mass percentage reaches 50%. The term “D95” as used herein refers to the corresponding particle size when the cumulative mass percentage reaches 95%. The D50 and D95 are on a volume basis and may be measured using, for example, a laser diffraction technique.

Preferably, the sulphate-depleted, lead-containing waste and the sulphate solution are passed to a conditioning tank prior to separating the comminuted, sulphate-depleted lead- containing waste from the sulphate solution. This may ensure that substantially all of the lead sulphate is converted to lead oxide. As will be appreciated, if a particle is small enough to pass through a classifier in a closed circuit mill but is the result of a fresh break of a large particle with high PbSC content, it is possible that not all of the lead sulphate is converted to lead oxide in the comminution unit. It will be appreciated that, where a closed circuit mill is used, base will be removed from the slurry with the classified particles thereby enabling desulphurisation to continue in a tank to which the classified particles have been passed. The conditioning tank may be part of a filter press used to separate the comminuted, sulphate-depleted lead-containing waste from the sulphate solution (see below). In contrast to conventional methods, the use of a conditioning tank is optional in the present invention. This is because in contrast to conventional methods, in the method of the present invention significantly higher levels of lead sulphate may be converted to lead oxide in the comminution unit. Preferably, the method further comprises adding a carboxylic acid, preferably acetic acid, to the conditioning tank. The addition of a carboxylic acid, preferably acetic acid, into the conditioning tank may have has a flocculating effect on the slurry, making it easier to remove the liquid. Surprisingly, the effect of flocculation far outweighs the additional cost of the acetic acid as it significantly reduces press times. Preferably, the sulphate-depleted, lead- containing waste and the sulphate solution are agitated in the conditioning tank. This may aid the flocculation.

The carboxylic acid (e.g. acetic acid) is preferably added to reduce the pH of the sulphate- depleted, lead-containing waste and the sulphate solution to between 6 and 8, preferably from 6.5 to 7.5, more preferably about 7. This may result in flocculation of the particles but with only a minimal dissolution effect on the waste.

Preferably, the conditioning tank operates at a temperature of less than 60 °C, more preferably from 30 to 50 °C; and/or the sulphate-depleted, lead-containing waste and the sulphate solution reside in the conditioning tank for a time period of less than 60 minutes, more preferably less than 30 minutes, prior to separating the comminuted, sulphate-depleted lead-containing waste from the sulphate solution. This may reduce the energy required to carry out the method, and therefore the cost and environmental impact. Surprisingly, keeping the temperature in the range of 30 to 50 °C may enhance flocculation but is not so high that it will increase dissolution prior to filtration.

Converting the precipitate of the second lead salt into leady oxide preferably comprises calcination. Calcination may be a particularly effective technique for converting the precipitate of the second lead salt into leady oxide.

The calcination is preferably carried out at a temperature of from 250 to 1000 °C, more preferably from 300 to 600 °C, more preferably from 325 to 450 °C. Lower temperatures may result in substantially incomplete conversion to leady oxide. Higher temperatures may require higher energy input without generating higher levels of leady oxide.

The temperature is preferably 450 °C or lower, more preferably 425 °C or lower. Such temperatures may favour the formation of alpha lead oxide, which is the preferred form of lead oxide for use in lead-acid batteries. Higher temperatures may result in the formation of beta lead oxide, which is the less preferred form of lead oxide for use in lead-acid batteries. Calcination may take place in an atmosphere which comprises oxygen. It will be appreciated that higher amounts of oxygen will generally favour the formation of PbO, while a low-oxygen environment will generally favour the formation of metallic lead. Accordingly, the relative amounts of PbO and Pb in the leady oxide may be controlled by controlling the amount of oxygen in the atmosphere during calcination.

Calcination may take place at an oxygen partial pressure of at least 0.01 atm, preferably at least 0.05 atm, and more preferably at least 0.1 atm. Calcination may take place at an oxygen partial pressure of up to 5 atm, preferably up to 1 atm and more preferably up to 0.5 atm. Thus, calcination may take place at an oxygen partial pressure of from 0.01 to 5 atm, preferably from 0.05 to 1 atm, and more preferably from 0.1 to 0.5 atm. For instance, calcination may take place in air at atmospheric pressure, i.e. without the application or removal of pressure.

The calcination preferably comprises flash calcination. Surprisingly, flash calcination may generate more alpha lead oxide in comparison to beta lead oxide because it removes the problem of inherent insulation properties of lead oxide and enables control of the thermal mass of the second lead salt (e.g. lead citrate) by minimising conglomeration and maximising the surface area available for calcination. Both of these are issues with bed dryer calcination, as used in conventional methods, which is why they typically only give beta lead oxide. While flash calcination is known in the art, it is not typically used on materials requiring temperature control for the desired outcome, but which generate their own heat when burnt. Flash calcination may be well known, but flash calcination of materials which require temperature control for the desired outcome, but generate their own heat when burnt, are not usually treated in this way.

The flash calcination is preferably caried out with: a heating rate of from 10 ³ to 10 ⁵ °C/s; and a holding time of less than 2 seconds. Such conditions may be particularly effective at generating higher levels of alpha lead oxide. The precipitate, for example citrate, has its own thermal mass, and when burnt during the process can take itself above 425 °C, thereby forming the undesired beta lead oxide rather than the desired alpha lead oxide. In order to be filtered, the precipitate must have a minimum particle size, but as the particle size increases so does the thermal mass. Such conditions are particularly suitable for forming alpha lead oxide from the precipitate of the method of the present invention. Separating the comminuted, sulphate-depleted lead-containing waste from the sulphate solution is preferably carried out using a filter press. A filter press is particularly suitable for separating the comminuted, sulphate-depleted lead-containing waste from the sulphate solution. Filter presses are known in the art. The method may further comprise washing the comminuted, sulphate-depleted lead-containing waste that has been separated from the sulphate solution.

Sulphate ions (e.g. in the form of dissolved NaaSO^ contained in the sulphate solution may be isolated, e.g. by crystallising as a result of evaporating the water, and optionally used in another application. For example, the recovered sulphate may be used in the production of glass or gypsum.

The lead-containing waste is preferably derived from lead-acid battery paste. Lead-acid battery paste typically contains high levels of lead sulphate.

The first acid preferably comprises an organic acid, more preferably acetic acid. Such acids are particularly suitable for dissolving the comminuted, sulphate-depleted, lead-containing waste to form a solution of a first lead salt, i.e. lead acetate. Lead acetate has a favourably high solubility in water.

When the first acid comprises acetic acid, the comminuted, sulphate-depleted, lead- containing waste is preferably dissolved in the aqueous solution of the first acid at a temperature of at least 5 °C, more preferably at least 10 °C, and even more preferably at least 15 °C, preferably at a temperature of up to 90 °C, more preferably up to 50 °C, and even more preferably up to 30 °C. Thus, the comminuted, sulphate-depleted, lead- containing waste is preferably dissolved in the aqueous solution of the first acid at a temperature of from 5 to 90 °C, more preferably from 10 to 50 °C, and even more preferably from 15 to 30 °C. Dissolution may be accelerated by agitation of the mixture, e.g. by stirring or ultrasound.

The second acid preferably comprises an organic acid, more preferably citric acid. Such acids are particularly suitable for forming a precipitate of the second lead salt, i.e. lead citrate. Lead citrate has a favourably low solubility in water.

The citric acid is preferably added to the solution of the first lead salt (e.g. acetic acid) in an up to stoichiometric amount for the formation of the lead citrate from the lead ions in the solution of the lead acetate. By using a stoichiometric, or slightly under stoichiometric, amount of the citric acid complete conversion of the citric acid into the lead citrate is ensured. Preferably, the Citric acid may be added to the solution of the first lead salt in up to 100%, more preferably up to 90 %, and even more preferably up to 80 % of the stoichiometric amount required for the formation of the lead citrate. Preferably, the citric acid may be added to the solution of the first lead salt in at least 40 %, preferably at least 50 %, and more preferably at least 60 % of the stoichiometric amount required for the formation of the lead citrate. Thus, preferably the citric acid may be added to the solution of the first lead salt in an amount of from 40 to 100 %, more preferably from 50 to 90 %, and even more preferably from 60 to 80 % of the stoichiometric amount required for the formation of the lead citrate.

In a preferred embodiment, the first acid comprises acetic acid and the second acid comprises citric acid.

The precipitate of the second lead salt is preferably separated from the lead-depleted solution, more preferably by filtration.

The lead-depleted solution is preferably recycled and used as the aqueous solution of the first acid. This may reduce the total amount of water and first acid used in the method.

The leady oxide preferably comprises PbO and Pb in a total amount of at least 99 %, preferably at least 99.5 %, and more preferably at least 99.9 % by weight. Such amounts may render the leady oxide particularly suitable for use in the manufacture of a lead-acid battery. Leady oxides having these purities are comparable to those obtained using a Barton pot or ball-mill process, in which a lead ingot is oxidised. Purity may be measured using known method, e.g. spectroscopic techniques such as ICP-AES (inductively coupled plasma atomic emission spectroscopy). ICP-AES may be carried out as detailed in the examples.

Preferably, the method further comprises processing the leady oxide into battery plates, and optionally incorporating said battery plates into a lead-acid battery.

The comminuted, sulphate-depleted, lead-containing waste is preferably substantially free of sulphate, more preferably the comminuted, sulphate-depleted, lead-containing waste comprises less than 2 wt.% sulphate, even more preferably less than 1 wt.% sulphate, still even more preferably less than 0.75 wt.% sulphate. Controlling the pH of the slurry preferably comprises adding hydroxide ions (e.g. in the form of a NaOH) to the slurry, more preferably in an amount of from 1.5 to 3 moles, even more preferably from 1.75 to 2.5 moles, and still even more preferably from 2 to 2.25 moles of base per mole of lead sulphate in the lead-containing waste. The provided slurry may have a pH in the region of 6 to 8. Hydroxide ions may be particularly suitable for increasing the pH of the slurry to the required range of 12 to 14. Higher levels of hydroxide ions may increase the pH beyond 14 and/or result in the unfavourable formation of lead hydroxide. Lower levels of hydroxide ions may be insufficient to control the pH to be in the desired range.

The method preferably further comprises contacting the comminuted, sulphate-depleted, lead-containing waste with a redox reagent during the step of dissolving the comminuted, sulphur-depleted, lead-containing waste in an aqueous solution of a first acid to form a solution of a first lead salt. The redox reagent may assist in the dissolution of lead-containing materials that are not in the +2 oxidation state by conversion into the +2 oxidation state. For instance, a redox reagent may convert lead-containing materials in the +4 oxidation state such as PbOa into PbO, which then readily reacts with the aqueous solution of the first acid to form a soluble salt. Without the use of a redox reagent, the conversion of PbCk into a salt will generally proceed fairly slowly. A redox reagent may also assist with the conversion of metallic lead into PbO, though Pb may also form a salt with the first acid without contact with a redox reagent.

Preferably, the redox reagent will be a reducing agent, for instance for lead compounds in the +4 oxidation state. Preferably, the redox reagent will also be an oxidising agent, for instance for metallic lead. Particularly preferred redox reagents include hydrogen peroxide which functions both as an oxidising agent and reducing agent.

The redox reagent is preferably added to the comminuted, sulphate-depleted, lead- containing waste slowly, preferably at a rate of from 10 to 20 litres per minute per ton of slurry, more preferably from 12 to 18 litres per minute per ton of slurry. Faster additions may result in the generation of unfavourable bubbles. Slower additions may increase the time taken for conversion to the +2 oxidation state.

Preferably, the first lead salt has a solubility in water at 20 °C of at least 100 g/L, preferably at least 200 g/L, and more preferably at least 300 g/L; and/or the second lead salt has a solubility in water at 20 °C of up to 10 g/L, preferably up to 1 g/L, more preferably up to 0.1 g/L.

In a preferred embodiment, the comminuted, sulphate-depleted, lead-containing waste comprises material that is insoluble in the aqueous solution of the first acid and the method further comprises recovering said insoluble material from the solution of the first lead salt, preferably by filtration, preferably wherein the insoluble material comprises one or more of metal compounds such as barium sulphate; carbon such as carbon black, graphene and/or carbon nanotubes; and fibres such as lignosulfonates.

In a further aspect, the present invention provides a method of recycling a lead-acid battery, the method comprising: providing a lead-acid battery; extracting a lead-containing waste from the lead-acid battery; and recycling the lead-containing waste by the method described herein.

The advantages and preferable features of the first aspect apply equally to this aspect.

The lead-containing wate may comprise lead oxide paste.

Extracting a lead-containing waste from the lead-acid battery can be carried out by known methods. For example, lead-containing waste may be extracted by a method in which one or more lead-acid batteries are comminuted utilising a hammer mill and the lead-containing waste separated from the other components of the batteries such as plastic materials and the electrolyte.

Preferably, extracting a lead-containing waste from the lead-acid battery comprises passing the lead-acid battery through a breaker circuit. A spray may add water to the breaker circuit to form a battery slurry. The battery slurry may comprise metallic lead, lead-containing waste and plastic fractions. Preferably the slurry has a density of greater than 3.5 kg/L This may be achieved, for example, by only using the spray water for dilution. Such a high density may improve the accuracy of separation between the metallic lead, the lead-containing waste and the plastic fractions. This is because it may reduce the effect of size on settlement rates. There may be less displacement of material into the wrong separation stream. Accordingly, the use of such a dense slurry may result in a higher purity metallic lead fraction with a lower level of entrained plastic. The invention will now be described in relation to the following non-limiting drawings in which:

Figure 1 shows a flow chart of shown a method of recycling lead-containing waste according to the present invention.

Figure 2 shows a flow chart of a method of recycling a lead-acid battery according to the present invention.

Referring to Figure 1 there is shown a method of recycling lead-containing waste according to the present invention (shown generally at 1), the method comprising: 2 providing a slurry of a lead-containing waste, the lead-containing waste comprising lead sulphate; 3 passing the slurry through a comminution unit while controlling the pH of the slurry to be from 12 to 14 to convert at least a portion of the lead sulphate to lead oxide to provide a comminuted, sulphate-depleted, lead-containing waste and a sulphate solution, 4 separating the comminuted, sulphate-depleted, lead-containing waste from the sulphate solution, 5 dissolving the comminuted, sulphate-depleted, lead-containing waste in an aqueous solution of a first acid to form a solution of a first lead salt; 6 adding a second acid to the solution of the first lead salt to form a lead-depleted solution and a precipitate of a second lead salt, the first lead salt having a higher solubility in water than the second lead salt; and 7 converting the precipitate of the second lead salt into leady oxide. The comminution unit operates in a continuous manner.

Referring to Figure 2 there is shown a method of recycling a lead-acid battery (shown generally at A, the method comprising: B providing a lead-acid battery; C extracting a lead- containing waste from the lead-acid battery; and D recycling the lead-containing waste by the method described herein.

The invention will now be further described with reference to the following examples.

Example 1 : recycling a mixture of battery paste waste

Desulphurisation: A representative mixture of waste battery paste was ground in a 150 kg/hr rod mill and the pH of the classifier was controlled at pH 13.2

A sample of the ground product was collected, dried and sieved to a size of less than 250 pm to establish the amount of unground plastics and other materials. The resultant dried, sized powder comprised mainly PbO (90 wt%).

X-ray diffraction analysis could not detect any remaining lead Sulphate (PbSC , PbO.PbSOzi, 3PbO.PbSO4 or 4PbO.PbSO4) in the product. The main phases detected were PbO, PbO.HaO and PbO2, as expected. ICP-AES measurement on samples of this powder showed a content of Sulphate ion of 0.2 wt% (versus 11 .23 wt% for untreated paste) which can be attributed to the PbSO4 in the feed.

ICP-AES analysis on the filtrate revealed that it indeed retained the SOz" ions that were extracted from the paste as well as most of the Na+ ions from NaOH.

Dissolution:

A sample of the desulphurised lead-acid battery paste (10.00 g) was dissolved in a solution of glacial acetic acid (5.2 ml) in water (100 ml), followed by the addition of H2O2 (2.0 ml, 30 wt%). The dissolution of the majority of solids could be directly observed within tens of seconds, producing a clear and colourless solution with a minute proportion of insoluble material suspended in the liquid phase. The mixture was stirred at a rate of 500 rpm, at room temperature, for a period of 5 minutes.

The clear and colourless solution was then filtered. The cake (3.4 wt% of the paste) was analysed and showed to be mainly BaSO4, carbon and fibres.

Precipitation of lead citrate:

Solid crystals of citric acid (5.17 g) were added to the filtrate from the dissolution step. The precipitation of white lead citrate began instantaneously but the solution was left to react for 1 hour at 80 °C under stirring at 400 rpm. The solution was then filtered and the cake (lead citrate) recovered, dried and weighed. The mass obtained was 13.12 g, which is very close to the expected figure of 13.32 g. X-ray diffraction was used to confirm that the powder obtained was exclusively Pb3(C ₆ H ₅ O7) ₂ ; thermogravimetric analysis on both pure PbafCeHsO?^ and the synthesised powder showed a perfect match. ICP-AES analysis on the powder showed complete absence of S, Ba, Sn, Al, Fe, Zn and Sb (0 %); a Cu content in the order of magnitude of 10 ppm; and Na content in the order of magnitude of 500 ppm.

With a more stringent washing protocol, the Na content was reduced to figures below 100 ppm, thereby demonstrating that Na content in the above examples is a function of the quality of water and efficiency of washing.

Calcination of lead citrate:

Lead citrate (200.00 g) was heated in a small oven at 250 °C for 1 hour in air then left to cool down to room temperature. The resultant grey/orange/yellow/green powder (lead monoxide and metallic lead) was then recovered and analysed.

The expected mass for a total combustion of PbgfCeHsO?^ to PbO was 134 g. The actual mass of material obtained by the process was 110 g; this is because some of the PbO was reduced to lead metal (Pb) during the calcination-combustion process. The presence of Pb was observed by the formation of droplets of metallic Lead in the sample

The powder was analysed with XRD to confirm that the phase obtained was PbO. ICP-AES was used to confirm the high purity of the material, which was equivalent to the purity levels seen in the case of the lead citrate above.

Example 2: Production of lead citrate in Pilot Plant

Experiments were conducted on spent lead-acid battery paste having a PbSO4 content of approximately 20 % by weight.

Desulphurisation :

Lead paste was initially desulphurised using a stoichiometric amount of NaOH for the presence of 20% lead sulphate. The classifier circuit was controlled at pH 13.0 using an amount of 50% sodium hydroxide solution. Dissolution:

A 5000 kg sample of desulphurised paste was generated in the pilot plant on a continuous basis. The sample was combined with glacial acetic acid (220 litres), and H2O2 (100 l,~35%). The resulting mixtures were stirred for about 30 min at 900 rpm at elevated temperature (50 degrees) due to the heat of neutralisation in the milling circuit.

The samples was filtered in a small filter press and the insoluble components recovered.

Precipitation of lead citrate:

A stoichiometric amount of citric acid was added to the filtrate. The formation of lead citrate as a fine white precipitate was immediately observed. The mixture was stirred using a tank stirrer for 20 minutes at 600 rpm.

The mixture was filtered in a small filter press, and the white precipitate allowed to air dry overnight. XRD analysis showed characteristic peaks of Pbs^eHsO?^ in both experiments.

Calcination:

A sample of the lead citrate was introduced into a flash calciner with an air temperature of 350 degrees centigrade at the entry point. The resultant mixture of lead oxide and lead metal was collected from the cyclone in the flash calciner circuit.

The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.