Generally speaking, the harmful substances in batteries mainly include Zn, Hg, CNi, Pb and other heavy metals; H2S04 lead storage battery; KOH in various alkaline batteries and IiPP6 electrolyte in lithium batteries. Mercury and its compounds, especially organic mercury compounds, have strong biological toxicity, rapid biological enrichment rate and long biological half-life of brain organs. Cadmium is easy to accumulate in animals and plants, which affects their growth and is highly toxic. Lead has adverse effects on human organs and systems, such as chest, kidney, reproductive and cardiovascular systems, which are manifested as mental decline, kidney damage, infertility and hypertension. The toxicity of zinc and nickel is relatively small, but when it exceeds a certain concentration range, it will have adverse effects and harm to human body. The acid-base solution in the waste battery will affect the pH value of the soil drainage system, making the soil and drainage system acidic or alkaline. The main components of battery electrolyte pollution are soluble heavy metals, especially a lot of lead sulfate in lead battery electrolyte and cadmium hydroxide in nickel-cadmium battery. Heavy metal ions in the battery are dissolved in soil or water and absorbed by the roots of plants. When livestock feed on plants, heavy metals will accumulate in their bodies. Human beings eat food, vegetables, meat and water containing heavy metals. Along this food chain, heavy metals will be enriched in the human body. Because heavy metal ions are difficult to excrete in human body, it will eventually damage human nervous system and liver function.
Study on recycling of waste batteries
1 recycling status of waste batteries
Lead storage battery is the most widely used industrial battery in China, and lead accounts for more than 50% of the total battery cost. Pyrolysis, hydrometallurgy and solid-phase electrolytic reduction technology are mainly used. The shell is made of plastic, which is renewable and basically realizes no secondary pollution.
Nickel-cadmium, nickel-hydrogen and lithium-ion batteries are widely used in small secondary batteries. Cadmium in Ni-Cd battery is one of the heavy metal elements strictly controlled by environmental protection. Organic electrolyte in lithium-ion batteries, alkali in nickel-cadmium and nickel-hydrogen batteries, and heavy metals such as copper, which are auxiliary materials for manufacturing batteries, all constitute environmental pollution. At present, the total number of small secondary batteries used in China is only several hundred million, and most of them are small in scale, so the utilization value of waste batteries is low. In addition, most of them are used for domestic waste treatment, and there are cost and management problems in recycling, and there are also some technical problems in recycling.
Civil dry batteries are the most widely used and dispersed battery products at present, and the annual consumption in China reaches 8 billion. There are mainly two series of zinc manganese and alkaline manganese, as well as a small number of varieties such as zinc, silver and lithium batteries. Zinc-manganese batteries, alkaline zinc-manganese batteries and zinc-silver batteries generally use mercury or mercury compounds as corrosion inhibitors, and mercury and mercury compounds are highly toxic substances. When waste batteries are incinerated as domestic garbage, some heavy metals such as mercury, cadmium, lead and zinc in waste batteries are discharged into the atmosphere at high temperature, and some become ashes, resulting in secondary pollution.
2 Waste dry battery recycling technology
A. Manual sorting and recycling technology
General dry cells are simply cut mechanically after classification, such as zinc skin, plastic cover, carbon rod, etc. Manually separating, and sending the remaining mixture of manganese dioxide and brucite back to a brick kiln for calcination to prepare dehydrated manganese dioxide. This method is simple and easy, but it takes up more labor and has little economic benefit.
B. pyrometallurgical recovery technology
Generally, dry batteries are classified and crushed, and then sent to rotary kiln. At the high temperature of 1 100~ 1300℃, zinc and zinc chloride are oxidized into zinc oxide, which is discharged with flue gas. Zinc oxide is recovered by cyclone, and the remaining manganese dioxide and brucite enter the slag, and then manganese and other substances are further recovered. The method is simple and feasible, and the general smelter can recover zinc without additional equipment.
C. Wet recovery technology
According to the principle that zinc and manganese dioxide are soluble in acid, waste dry batteries are classified and crushed, and then put into a leaching tank. Adding dilute sulfuric acid (100 ~ 120 g/L) for leaching to obtain zinc sulfate solution, which can be used for electrolytic preparation of metallic zinc. After washing and separating the filter residue into copper caps and carbon rods, the filter residue Mn02 and brucite are calcined. The methods adopted include roasting leaching method and direct leaching method.
Compared with pyrometallurgical method, wet method has the advantages of less investment, low cost, quick plant construction, high profit and flexible process, but it can not guarantee the complete recovery of harmful components.
3 Prevention and control of secondary pollution in the process of recycling waste batteries
The above three recovery methods are simple, but each has its own shortcomings and secondary pollution problems. Through a large number of experiments, we have obtained feasible methods to prevent secondary pollution.
Firstly, the waste dry batteries are classified, and after mechanical cutting, copper caps and zinc skins are separated, which can be recycled separately. After magnetic separation to remove iron, the remaining carbon coating was soaked in water at a solid-liquid ratio of 1:4/hour, and the supernatant was evaporated and crystallized. The main components of the precipitate are MnO _ 2, MnO(OH), acetylene black, carbon rods and other substances, which are added into a rotary kiln to be smelted to 600 degrees Celsius, and the generated flue gas can be condensed to obtain condensate, and pure mercury can be obtained by regular cleaning. Meanwhile, it also prevents mercury vapor from polluting the environment. During the calcination process, a large amount of acetylene black and carbon in the mixture will reduce MnO 2 to MnO. The reaction process is as follows:
2mn 0 2+C-& gt； 2MnO+C0 2
Adde that calcined product into a sulfuric acid solution with the concentration less than 2 mol/L accord to the solid-liquid ratio 1: 4, and soaking at 80 DEG C1hour to generate the following reaction:
MnO+h2so 4- > manganese sulfate+hydrogen peroxide
Manganese sulfate solution is obtained, and other soluble heavy metal sulfates are introduced at the same time.
The obtained zinc skin and copper can be directly remelted, ammonium chloride can be used as fertilizer or chemical reagent after purification, and manganese sulfate is a hormone component for animal and plant growth, which can be used as a desiccant for paint and ink and a catalyst for some organic synthesis reactions, and can also be used as a reagent for producing paper, ceramics, printing and dyeing and electrolytic manganese. Table 1 shows the components of recyclable substances in zinc-manganese dry batteries.
This recycling method has the advantages of low investment, simple equipment and easy implementation in small and medium-sized cities, thus eliminating the transportation problem of used batteries.
The solution recovered from waste batteries is concentrated and reacts with EDTA to form metal complex, which can completely eliminate secondary pollution. The results show that the content of heavy metals in the solution after recycling waste batteries meets the national environmental protection standards. If these metals are to be separated, they can be classified according to their different stability. Table 2 shows the stability constants of metal ions complexed with EDTA.
4 Problems and suggestions in the recycling process of waste batteries
(1) batteries cannot be disposed of after recycling, and they are generally piled up. During stacking, batteries may leak or toxic substances may spread.
② There are many kinds of batteries and counterfeit products, which also bring difficulties to battery recycling. Some batteries contain mercury, some contain cadmium, some use ammonium chloride as electrolyte, and some use zinc chloride as electrolyte. Therefore, it is suggested that manufacturers use unified standards to identify the types and main components of recycled batteries.
(3) Strengthen the development of high-performance environment-friendly batteries to realize mercury-free common civil batteries.
(4) The state should give policy support to the recycling of waste batteries.