Atık pillerin değerlendirilmesi

Abstract

Dünyada çeşitli amaçlar için önemli miktarda pil kullanılmaktadır. Piller deniz motoru ve deniz araçlarında, elektronik cihazlarda, saatlerde, kameralarda, hesap makinelerinde, kulak cihazlarında, kablosuz telefonlarda ve daha pekçok ev aletlerinde kullanılmaktadırlar. Son yıllarda pillerdeki ağır metallerin çevreye ve insan sağlığına zararlı etkileri dikkatleri çekmiştir. Bu konudaki endişeler atık pillerin yeniden değerlendirilmesine yönelik programların düşünülmesine sebep olmuştur. Yakın zamana kadar pillerin toplanma ve yeniden değerlendirme çalışmaları sadece otomobil pilleriyle sınırlıyken, son yıllarda ilgi evsel atıklardaki pillerin yeniden değerlendirilmesinde odaklanmaya başlamıştır. Yapılan çalışmalar pillerin zehirliliklerini ortadan kaldırmak amacıyla güvenli depolanmalarına, geri kazanılmalarına ve bertaraf edilmelerine yöneliktir. Kaynaklarda atık pillerin pirometalurjik olarak değerlendirilmeleri konusunda yapılmış az sayıda çalışmaya rastlanmıştır. Hidrometalurjik olarak değerlendirme konusunda ise sınırlı sayıda çalışmaya rastlanmaktadır. Bu çalışmada özellikle nikel-kadmiyum pillerindeki demir, nikel ve kadmiyumun geri kazanılmasında metallerin farklı pH'larda çökmeleri ve ayrıca tiyoürenin kadmiyumu CdS halinde çöktürme özelliğinden yararlanmaya yönelik yeni bir hidrometalurjik yöntem üzerinde çalışılmıştır. Ön araştırmalar sonucunda alkali ve çinko-karbon pilleri %37'lik HCI, nikel-kadmiyum pilleri %37'iik HCI, %30'luk H2S04 ve %55'lik HNO3 çözeltilerinde çözündürülmüştür. Alkali ve çinko-karbon pilleri ile yapılan deneylerden metallerin seçici olarak geri kazanılamayacağı anlaşılmıştır. Nikel-kadmiyum pilleri ile yapılan deneylerde HNO3 ile hazırlanan çözeltiden Fe(OH)3, NİCO3 ve CdS halinde üç metal ayrılmıştır. Elde edilen ürünlerin saflıkları sırasıyla %96, %88 ve %92'dir. işlem sonunda ele geçen atık çözeltilerdeki ağır metal konsantrasyonun yüksek olduğundan arıtma gereklidir. Atık pillerin değerlendirilmesinde, bu çalışma ile ilk defa kullanılan tiyoürenin endüstriyel olarak kullanılabilmesi, süreç optimizasyonu ve atık su ağır metal giderme çalışmalarının başarısına bağlıdır.

People use considerable quantities of batteries to power a variety of household and industrial products. Batteries are used in motor and marine vehicles, electronics, watches, cameras, calculators, hearing aids, cordless telephones, power tools, and countless other portable household devices. Recently, much attention has been focused on the potential environmental and human health risks associated with the heavy metals present in batteries. Such concern has caused many municipalities to consider programs for recovering the large number of batteries discharged in municipal solid waste. Historically, residential collection and recycling efforts have been limited to used automebile batteries. In recent years, states and municipalities have begun to focus on the recovery of used household batteries. Such activities have coincided with a number of legistative and industry initiatives to reduce the toxicity of batteries and to promote their safe collection, reclamation and disposal. Batteries are complex electrochemical devices, composed of distinct cells, that generate electrical energy from the chemical energy of their cell components. A battery cell consists primarily of a metallic anode (negative electrode), a metallic oxide cathode (positive electrode), and an electrolyte material that facilitates the chemical reaction between the two electrodes. Electric currents are generated as the anode corrodes in the electrolyte and initiates an ionic exchange reaction with the cathode. The electrical energy produced from this reaction is sufficient to power a variety of consumer and industrial devices. Batteries are classified and distinguished according to their chemical components. Batteries are referred to as wet or dry cells. In wet cell batteries, the electrolyte is a liquid. In dry cell batteries, the electrolyte is contained in a paste, gel, or other solid matrix within the battery. Primary batteries contain cells in the chemical reactions are irreversible, and they therefore cannot be recharged. This is in contrast to secondary batteries in which the chemical reactions are reversible and external energy sources can be repeatedly applied to recharge the battery cells. Batteries are manufactured in a variety of sizes, shapes, and voltages. They are produced in rectangular, cylindrical, button, and coin shapes. Batteries differ in their chemical composition, energy storage capacity, voltage output, and life span. These factors affect their overall performance, utility, and cost. Lead-Acid Storage batteries contain sulfuric acid, polypropylene plastic casing, polyvinyl chloride rubber separators, and chemical sulfates and oxides to which the lead is bound. Alkaline (manganese) batteries contain mercury, lead, cadmium, arsenic, chromium, copper, indium, iron, nickel, tin, zinc and manganese. Zinc-Carbon batteries contain mercury, lead, cadmium, arsenic, chromium, copper, iron, manganese, nickel, zinc and tin. Nickel-cadmium batteries contain nickel and cadmium. The disposal of used automobile and household batteries into waste must be assessed for its potential human and environmental health impacts. The disposal of used batteries in solid wastes is problematic for two reasons. First of all, batteries contribute to the total quantity of potentially hazardous waste that is disposed in solid waste. Second, and more importantly, batteries contain many potentially toxic chemicals that can have adverse environmental and human health impacts. Batteries contain a variety of heavy metals that may become toxic contaminants in landfill leachate, incinerator emissions, incinerator ash and compost. Much concern has been directed to the high percentage of mercury, cadmium and lead in waste that is attributed to used batteries other potentially toxic metal that may be present in batteries include silver, zinc, nickel, manganese, lithium, chromium and arsenic. Many accumulate in aquatic sediments and soil and may be metabolized by indigenous microorganisms to more toxic organic forms. Of most concern is the potential for uptake and accumulation of heavy metals or their metabolites in the food chain. Recent attention has focused on potential human and environmental exposure risks from the metals present in used batteries, specifically lead, cadmium and mercury. Lead-acid batteries are recycled to reclaim the lead, sulfuric acid and polypropylene plastic housing. Batteries are processed by secondary smelters who rely on used batteries for more than 70 percent of their lead supply. Recycling programs for household batteries are not widespread and have been hampered by the limited number of processing facilities available. Currently, only three United States facilities reclaim household batteries. The Mercury Refining Company in New York recovers mercury from mercuric oxide and silver from silver oxide batteries. INMETCO in Pennsylvania recovers nickel and steel from nickel-cadmium batteries. After the batteries are processed, the residue is sent to another firm to recover cadmium. The Bethlehem Apparatus Company also in Pennsylvania, recovers mercury from mercuric oxide batteries. There are many studies about recycling waste batteries in the literature. A hydrometallurgical method was developed for Cd recovery from spent batteries. The main product consisting of Cd sponge is prepared by leaching the waste battery waste with H2S04, 2-stage precipitation of Cd by Zn, removal of Fe and recovery from the effluent by precipitation with NaHC03. The byproducts are basic zinc carbonate, Fe(OH)3 and Na2S04. Nickel and cadmium were recovered from battery waste by leaching with HCI, removal of Fe by extraction with TBP, and seperation of Ni and Cd by extraction with alkylmonothiophosphonic acid (MSP-8) diluted with n-heptane followed by selective stripping of Cd with HCI. The recovery of Ni, Cd and Fe was 100, 89 and 86 % respectively. A recovery process is simplified by heating the spent batteries in plasma jet from the mixture of air with 10-30 volume % C3H8. The melt is then processed conventionally for reduction and metal recovery. Another process involves leaching of Cd from the nickel-cadmium alloy scrap in aqueous solutions containing (NH4)2S04 or H2S04l seperating the nickel- containing residues and treating the leach solutions by electrolysis to deposit Cd on an anode. The porcess is suitable for recovery of high-purity cadmium from spent batteries at a low cost without generating poisonous cadmium vapor. In another recovery process, the spent batteries are mechanical comminuted, leached with an acid and the cadmium salt in the leached solution is reduced with a more electropos metal than cadmium. The obtained powder cadmium can be reused for the manufacture of battery anodes. Thus, a paste containing Cd 27.10, Fe 2.6, Ni 0.6, Co 0.05, Mn 0.02, graphite 0.80 and water 63 % was removed from comminuted batteries, leached with 30 % HCI for 15 minutes and the obtained CdCI2 was reduced with powder Mg to recover cadmium. In another process the leaching solutions, contain thermally stable NH4 salts such as (NH4)2S04, NH4NO3 or AcONH4 with respectively NH3 and NH4 concentrations of > 0.5 (1 - 2) and > 0.5 (1-3 mol/l). The leaching yield for nickel and cadmium was > 98 %. In a pyrolysis proses; after a sludge containing nickel and cadmium is roasted at 770-1270 °K in a weakly oxidizing atmosphere, the cadmium is selectively leached with a mineral acid by stirring at pH 1-4. Cd (OH)2 and Ni (OH)2 in the sludge feed are changed to CdO and NiO respectively when roasted, and cadmium is efficiently seperated by controlling the pH. The method is useful for sludge residue in manufacture of nickel-cadmium batteries. Thus, sludge 60 g containing 49.9 Cd and 7.41 % was roasted at 1270 °K under a weak flow of air. The roasted product 40.0 g was slurried with water 100 ml and H2S04 was added for pH 2.0 maintained for 1.5 h before filtering. Recovery of cadmium was 82.1. In a hydrometallurgical process nickel and cadmium were recoverd from spent batteries by crushing, acid leaching, 2-stage solvent extraction, successive seperation of Cd and Fe and NiCI2 crystalline from the aqueous phase after Fe removal. Thus recovery of cadmium, nickel and iron from spent nickel-cadmium batteries was reported. A prolysis process involves opening the batteries and emptying them of any free electrolyte, drying the scrap, removing the organic substances by pyrolysis at an initial temperature followed by condensation. The pyrolysis and distilling - off operations are performed in one and the same furnace by the successive and preprogrammed raising of the temperature inside the furnace, for example in stages, in order to produce a residue inside the furnace in the form of nickel and iron scrap. Pyrolysis takes places in a controlled atmosphere, requiring the introduction of an inert gas such as nitrogen with the addition of between 3 and 12 % of oxygen, if necessary in the form of air. Pyrolysis is made to occur by raising the temperature inside the furnace from about 370 °K to an initial temperature of about 670 °K or 770 °K. The vaporization of the cadmium takes place at a second temperature of up to about 900 °C in the presence of a reducing protective gas. The cadmium vapor is condensed and the liquid metal is cast into cadmium rods. In a hydrometallurgical process the waste is leached with an ammoniacal carbonate solution to form an aqueous ammoniacal carbonate solution containing cadmium and nickel ammine complexes and a leaching residue. The resulting aqueous ammoniacal carbonate solution is contacted with a substantially water insoluble organic solution which contains a hydroxyoxime which forms a nickel compound readily soluble in the organic solution with any nickel present but which does not affect cadmium. Nickel is thereby removed. The nickel compound is stripped by washing with an aqueous solution of sulphuric acid to strip the nickel as nickel sulfate. The organic solution stripped of nickel can there after be reused. Cadmium is precipitated as a carbonat by removing ammonia from the aqueous ammoniacal carbonate solution. XI In this study hydrometallurgical methods were used to recycling waste batteries. The prenciples which are used in this study are mentioned below. The hydroxides of the various metals vary greatly in their strengths as bases. On the one hand are the very strong bases such as sodium hydroxide and potassium hydroxide, the caustic alkalies which are soluble and highly ionized in water. On the other hand are the trivalent and quadrivalent metal hydroxides, such as aluminum hydroxide, ferric hydroxide and titanium hydroxide which are hardly hydroxides at all but hydrated oxides; they are insoluble and give up water so easily that their composition is more accurately expressed as AI203,.xH20, Fe203.xH20 and Ti02.xH20 Between these two extremes, the strong bases and the very weakly basic hydrated oxides, fall the hydroxides of the various common metals. It is convenient to arrange the metals on the pH scale on the basis of the pH at which they are precipitated as hydroxides. The variation in the pH at which the metals are precipitated as hydroxides or basic salts makes it possible to effect certain separation of one element from another by proper adjustment of the pH of the solution. Thiourea is a very good sulfide doner to copper, zinc, lead, cadmium and mercury. Thiourea does not react with nickel, iron, cobolt, chromium, manganese, magnesium, antimony or arsenic to form sulfides at room temperatures. In this study the variation in the pH at which the metals are precipitated and the effect of thiourea on metals were examined especially for seperation of metals in nickel-cadmium batteries. After battery scrap was leached with HCI solution, thiourea was added and pH was increased. The precipitants were analysed with an X-Rays. The elements which were determined in the zinc-carbon and alkaline batteries precipitants at different pH values are shown in below. Determined elements from zinc-carbon and alkali batteries at different pH values. XII The elements which were determined in precipitans at different pH values are shown below. the nickel-cadmium batteries Determined element from nickel-cadmium batteries at different pH values. Total iron concentrations were analysed with spectrophotometric technics. There are shown below. Iron concentrations in nickel-cadmium precipitants. Nickel-cadmium battery was boiled in H2S04 solution until all metals were dissolved then filtered. The total iron concentration in this solution was 22 gr/l. Concantrate NaOH solution was added to the filtrate until pH has a value of 7,5. Then it was filtered. The total iron concentration has became 9,6 mg/l in the solution, so most of the iron was removed from the solution. But other metals couldn't removed from this solution. Another solutions were prepaired with HN03. The batteries were leached in 55 % HN03. The leaching was made in two steps. The first leaching step was about 5 minutes, and the second one finished when the all metals dissolve. The pH was rised to 3,5 with 5 N NaOH solution and ferric hydroxide was filtered. The second leaching solution was added to the filtrate and the pH was rised to 3,2 with 5N NaOH solution. Ferric hydroxide was filtered again. XIII 0,8 M, 20 ml thiourea solution was added to the filtrate and pH was rised to 13. CdS and Ni(OH)2 precipitated. To separate the precipitants the pH was decreased to 1,3 with 65 % HN03 solution. Ni (OH)2 was dissolved and CdS was filtered. To precipitate the nickel, solid Na2C03 was added to the solution. At pH=8,7 NiC03 is filtered. The % purity of the products are shown in below. % Purity of the products An important problem of this recovery process is that big amount of waste water is occured. Thats why the utility of the process depends on the waste water treatment and time optimization.