MXPA01007127A - Process for the recovery of sulphur from lead-acid battery scrap - Google Patents

Abstract

A process for recovery of substantially all the sulfur in a spent lead-acid battery as Na2SO4 is disclosed. The process comprises (a) breaking the batteries to remove the acid, (b) separating the plastic from the lead bearing materials, (c) smelting the lead bearing materials in a reverberatory furnace in an oxidizing atmosphere to volatilize most of the sulfur in the feed as SO2, (d) scrubbing the SO2 from the off gas stream using a soluble alkaline material such as NaOH, Na2CO3, or KOH to produce a soluble sulfite solution, (e) oxidizing the sulfite solution to sulfate, preferably by turbulent mixing of the solution with air, (f) adjusting the pH by adding the sulfuric acid separated from the batteries, (g) removing the contained heavy metals, (h) crystallizing the sulfate as Na2SO4 or K2SO4, (i) separating a bleed stream from the crystallizer and removing the contained chlorides as a mixed sulfate-chloride product by evaporation of the bleed stream in another crystallizer. The process produces a high grade Na2SO4 or K2SO4 product, condensed H2O for recycle to the scrubber, and a mixed sulfate-chloride product while reducing sulfur dioxide, chloride and heavy metal emissions to extremely low levels.

Description

PROCEDURE FOR THE RECOVERY OF SULFUR FROM LEAD-ACID BATTERY WASTES BACKGROUND OF THE INVENTION The recovery of lead from spent batteries is of significant economic importance both as a source of raw materials and due to the problems of hazardous waste. For many years, battery recycling plants have attempted to solve the problems associated with the presence of sulfur and chloride in battery waste through the extensive benefit of spent battery disposal. The batteries broke, the acid drained, and the remaining material broke up to a much smaller size. The breaking and fragmentation released most of the portions of dough from the grids and broke the plastic components of the battery. In a series of hydrometallurgical processing steps, the paste was separated from the metallic lead and the plastic portions of the battery. Much of the plastic containing chloride along with other non-recyclable plastics, glass, and other inorganic battery components were separated from the paste and metal components. The material, however, contains a substantial amount of lead as finely divided lead or active material. Despite persistent efforts to remove lead, enough lead remains in this material to prevent waste in unregulated landing fields. The standard method for recovering valuable lead from spent batteries involves melting the lead-containing portions of the battery in a reverberatory, rotary, shaft, or electric furnace using standard pyrometallurgical procedures. These pyrometallurgical processes have several disadvantages or drawbacks. The main disadvantage of pyro-metallurgical processes is that they operate at elevated temperatures and generate quantities of sulfur dioxide gas as well as volatile powders. The powders carry substantial amounts of volatile metals such as lead, arsenic, cadmium, and the like. Expelled gases also contain chlorine or chlorides as a result of combustion of chloride-containing materials such as separators made of polyvinylchloride. With the SO2 restrictions, emissions from industrial smelting facilities must be reduced to very low levels. Spent lead-acid batteries contain a substantial amount of sulfur in the form of H2SO from the electrolyte and even more as PbSO4 in the active material as the product of the battery discharge. High volume battery recycling plants handle hundreds of tons of battery waste per day. The sulfur content from a waste battery is about 3.9% of the weight of the battery and therefore a plant could have an input of many tons of sulfur per day.

To control the S02, the rotary kilns collect most of the sulfur in the battery waste as a matte salt of FeS-Na2S, the cell kilns and the electric furnaces can recover the sulfur as a matte sulfur. Reverberatory furnaces can also use iron or sodium compounds to collect the sulfur in the scrap; however, the further processing of the scrap in the reverberatory furnace or the disposal of the scrap or of the matte substances can be a problem due to the dripping of the heavy metals from the soluble components of the scrap. In order to reduce SO2 emissions, the separated pulp has been treated with solutions of alkali materials such as NaOH or Na2C03 to make it react with PbS0 in the following reactions: PbSO4 + 2NaOH? Pb (OH) 2 + Na 2 SO 4 PbSO 4 + Na 2 CO 3? PbCO3 + Na2SO4 The resulting "desulfurized" material is recovered as a sediment or as a residue in a filter. Despite extensive efforts to wash the sediment and to desulfurize it with excess alkali reagentsSubstantial amounts of sulfur are frequently present in the desulfurized pulp as unreacted PbS04 or as Na2SO4 retained in the material. The sulfur content of the non-desulfurized pulp is about 6%, while the desulfurized pulp usually contains about 1% total sulfur or less. In addition to sulfur, the paste often contains a number of small PVC particles which are not released into the plastic removal system. When the desulphurized pulp and metal components melt in furnaces, however, the SO2 content of the gas flow is still at high levels, therefore requiring the addition of streams to collect the sulfur as a matte solution or a salt mate. With desulfurization, only the amount of these wastes decreases. To ensure compliance with the restrictions of SO2 emission regulations at low levels, battery recycling plants using reverberation furnaces have installed alkali or lime scrubbers to reduce the amount of S0, emitted despite desulfurization of the original material. Lime scrubbers generate substantial amounts of gypsum as well as CaS03, while alkali scrubbers generate mixed sulphate-sulfite solutions. In addition to SO2, the scrubbers also purify any chloride content. The scrap effluent from the lime scrubbers as well as the scrap from the neutralization by calcium of the battery acid is generally sent to airfields. In processes where the active material portion (pastes) of the battery is separated from the metal forms and desulfurized using a solution of ammonium, sodium or potassium hydroxide, carbonate or dicarbonate, lead carbonate or lead hydroxide and relatively pure Na2S04, (NH4) 2S04, K2S04) solutions are produced. These solutions frequently crystallize to recover the sulfate salt.

When alkali scrubbers are used to recover sulfur, a discharge solution is produced containing a mixture of sulfate, bisulfite, thiosulfate, sulfite, and other sulfur species together with chlorides and heavy metals. Due to the chlorides and heavy metals, the purification solutions after sulfate oxidation have not been able to be processed almost to sulphate products capable of being sold. These alkali sulphate solutions, when cleaned of heavy metals and when the level of total dissolved solids allows, are discharged as wastewater for sanitary discharges. Where the disposal of high levels of dissolved solids into wastewater is not possible, lime scrubbers have been used to remove sulfur from the gases expelled by the kiln. In these scrubbers the sulfur is trapped as CaS 3, CaSO 4, or mixed sulfur compounds. When it is oxidized to gypsum, the material has low solubility in the purification solution. Because scrap products are not soluble, accumulation inside the scrubber is a major problem. In addition, plaster produced from battery recyclers is a solid waste and can be a hazardous waste depending on the heavy metal content of the material. Gypsum is also produced as a waste which can restrict waste. An additional problem is chloride which can form soluble CaCl2 and form in the purifying solutions. These chloride solutions are very soluble and present problems of highly dissolved solids in wastewater discharges. An additional problem is the small amounts of magnesium in the lime. The magnesium reacts with S02 or Cl to form soluble magnesium salts which make up the dissolved solids problem of the lime scrubber discharges. The efflux from alkali scrubbers in general can not be used to produce a sulphate product due to the presence of heavy metals and chloride scrubbers from the gas flow. When cleaning heavy metals solutions should be disposed of in sewers despite their high salt content. Many municipalities have restricted the total dissolved solids in the efflux of the plants, thus reducing the capacity of the plant to discharge these depurated solutions. In contrast to the methods previously used, the method of the present invention ensures that more than 99% of the sulfur in the battery is recovered and that the heavy metal content, SO2, and chloride content of the expelled gases is reduced to insignificant values .

BRIEF DESCRIPTION OF THE INVENTION In the practice of the present invention, the batteries are broken to remove the acid and the plastic is separated from the materials containing lead. Lead-containing materials are melted in an oxidizing atmosphere to volatilize any sulfur present to SO2. SO2 is recovered from the gas flow by debugging with a soluble alkaline material to produce a soluble sulfite solution which in turn is oxidized to sulfate which crystallizes after the heavy metals have been removed from the container. The flow of draining from the crystallizer can then be subjected to additional evaporative crystallization to recover the chlorides as a mixture of sulfate-chloride product.

DESCRIPTION OF THE DRAWINGS Figure 1 is a flowchart of the preferred practice of the present invention.

DETAILED DESCRIPTION OF THE INVENTION The invention consists of a method for recovering valuable lead in lead-acid battery waste or other materials containing lead without substantial air pollution by SO2, chlorides or volatile powders. The invention also removes solid wastes containing sulfur such as gypsum, matte salt, or salt from scrap. It also eliminates waste water from waste containing high concentrations of dissolved solid, and recovers substantially all of the sulfur as a high quality product. In the process of the invention, spent lead-acid batteries are broken. The acid from the batteries is separated and can be used to feed the heavy metal removal step described below. The plastic is removed from the broken battery. The materials containing lead are then used to feed a reverberating homo. In the kiln, lead-containing materials melt in an oxidizing atmosphere. As a result, the sulfur present in the furnace feed is volatilized to S02. Lead is recovered from the reverberatory furnace. The flow of gas expelled from the furnace, which includes the volatile SO2, is passed to a scrubber. The gases are filtered in a dust collector before reaching the scrubber. Optionally, collected dust can be returned to the furnace for further processing. Soluble alkaline materials are used to feed the interior of the scrubber to convert SO2 to a soluble sulfite. Examples of suitable alkaline materials include NaOH, Na 2 CO 3, KOH, ammonium carbonate, bicarbonate or hydrate or any other material which will produce a soluble sulfite solution. The scrubber liquid containing the purified material is oxidized, preferably by vigorous stirring of the liquid while air is introduced. This process oxidizes any kind of sulphite to sulfates. After oxidation the sulphate solution is cleaned of heavy metals by a pH adjustment and co-precipitation with iron, sulfides and other materials. In this process the pH of the sulphate solution is preferably reduced by the addition of the separated, filtered acid recovered in the battery breaking stage. The pH of the sulphate solution is generated in several stages with alkali material and other materials to precipitate heavy metals and produce a clean sulfate solution that also contains chlorides. The heavy metal sediment can be returned to the furnace for metal recovery. The filtered sulphate solution is sent to a crystallizer where the water evaporates and, depending on the alkaline material used to solubilize the S02, the pure anhydrous crystals of Na2SO4 are produced.; K2S04, etc. The condenser water in the crystallizer can be used to wash the crystals to produce product with a low chloride content. The vast majority of the condensed water can be returned and used for water from the scrubber. The chlorides are the most soluble construction in the recirculation solution of the crystallizer. To prevent excess chloride formation and to remove them from the crystallizer, a draining flow can be taken from the crystallizer. This liquid is saturated with sulphate and brought almost to the boiling point. This liquid can be sent to an additional crystallization step to produce a mixed product of sulfate chloride. To recover more of the solution as a pure sulfate product, the liquid runoff can be sent to a freeze crystallizer where the temperature of the liquid is reduced from around 100 ° C to 1-5 ° C. The reduction and temperature reduces the solubility of the sulfate. In the freezer the low crystallization temperature of the sulfate produces a hydrated salt instead of the anhydrous sulfate produced in the high temperature crystallization process. The hydrated sulfate salts are separated from the solution and redissolved in the hot crystallization solution. The chlorides are concentrated by the process and recovered by producing a dry sulfate-chloride mixed salt produced via an evaporator such as a dessicator by spray. The volume of liquid sent to the spray dryer is reduced as the water of hydration is removed from the flow of the crystallizer in the crystallizer by freezing.

EXAMPLE One hundred (100) tons of lead-acid batteries containing about 54 tons of lead and alloy materials with lead, 2.5 tons of sulfur in the paste or active material and 1.4 tons of sulfur as H2S0 in the electrolyte of the battery. In the present process, the sulfur loss in the process in the SO2 emissions from the scrubber is 0.012 tons or 0.3% of the total sulfur input. The scrap from the process will trap 0.105 tons, or 2.7% of the input sulfur. The two crystallizers will recover 3.78 tons or 97% of the input sulfur as products from the scrubber and the neutralized acid. The amount of the sulphate-chloride mixed product depends on the amount of chloride input to the system and is estimated to be 0.2 tons in this example.

Thus, the process will recover and trap 99.7% of the sulfur contained in the battery or virtually all the sulfur in the battery. Figure 1 describes a flow sheet with a block diagram which shows the various steps. In the process, the spent batteries are broken (1) and the acid (2) is recovered for later use. The broken batteries are used to feed heavy / float subsidence systems (3) where the recyclable polypropylene from the box material (4) is separated and the materials containing lead (5) including other polymeric materials are prepared for the oven, In the '6 'oven the materials containing lead are reduced to metallic lead'7'. The gases expelled'8 'from the furnace contain substantial amounts of S02, volatile powders, and chlorides. S02 is generated by the following reaction: PbSO4 + C? Pb + SO2 + CO2. The gases are cooled and the powder is removed in a bag with a filter cloth (9). The powder is transferred back to the oven. The gases leaving the dust filter enter a '10 'scrubber where the SO2, chlorides, and any waste dust from the bag filter are removed from the gas stream by contact with an alkaline solution based on in NaOH or Na2CO3. The cleaning gases then come out of the plant via a chimney'11 '. The purification solution '12' is oxidized to Na 2 SO in a series of '13' oxidation tanks using air introduced into the high pressure purification solution. The air oxidizes Na2S? 3 and other compounds containing sulfur to Na2SO4. After the oxidation is complete, the Na2SO (14) solution is transferred to a heavy metal removal system '15 '. In this system, the pH of the solution is decreased with H2SO4 recovered from the battery breaking operation. Fe2 (S04) 3 is added with a co-precipitant and the pH is raised in a series of steps to precipitate the heavy metals. The treated solution is filtered 16 and the heavy metal drained 17 is returned to the furnace. The clean Na2SO solution '18' is sent to a '19' crystallizer, where the water evaporates and condenses as high purity condensate '20', and the Na2SO4 is recovered as high purity crystals '21'. The condensate can be discharged, used as wash water, or used as water for scrubber work. The recirculation solution of the crystallizer contains chlorides which will continue to form to considerable levels if they are not removed from the system. A runoff stream '22' is removed from the crystallizer and sent to a freeze crystallizer '23' where the temperature is reduced causing the Na2SO in the solution to precipitate as Na2SO4"10H2O (24 'which is returned to the crystallizer The discharge from the crystallizer by freezing is sent to a third crystallizer such as a desiccator by aspersion'25 'where a mixed product of salt is obtained (26) The mixed product of salt contains NaCl, Na2SO4, CaCl2 , MgCl, and other soluble salts in the scrubber not removed in the heavy metal cleaning process.

Claims (10)

NOVELTY OF THE INVENTION CLAIMS

1. - A process for the recovery of sulfur in spent lead-acid batteries that includes: a. Separate plastic and acid from a broken lead-acid battery; b. volatilize the sulfur present in the battery to SO2 by melting the lead-containing materials which remain after said separation in an oxidizing atmosphere; c. debug SO2 from the flow of gas expelled from the melting furnace in the alkali scrubber to produce a soluble sulfite solution; d. treat the solution by oxidation to convert the soluble sulfite to a sulfate; and. remove the heavy metals in the solution; F. crystallize the alkali sulfate from the solution as high purity crystals.

2. The process according to claim 1, further characterized in that it comprises the removal of a portion of the crystallizer solution to a second crystallizer to recover the soluble sulfate and the chloride species as a mixed product of salt.

3. The process according to claim 1, further characterized in that a portion of the crystallizer solution is sent to a freeze crystallizer wherein a substantial portion of the alkali sulfate is removed as a hydrated sulfate and returned to the primary crystallizer.

4. - The process according to claim 1, further characterized in that the furnace is a reverberatory furnace.

5. The process according to claim 1, further characterized in that the furnace is a rotary kiln.

6. The process according to claim 1, further characterized in that the furnace is an electric furnace.

7. The process according to claim 1, further characterized in that the alkali scrubbing material is Na2CO3.

8. The process according to claim 1, further characterized in that the alkali scrubbing material is NaOH.

9. The process according to claim 1, further characterized in that the alkali scrubbing material is a potassium compound.

10. The process according to claim 1, further characterized in that the method used to crystallize the mixed species of salt is a desiccator by spray.