Most waste plastics have conventionally been disposed of by incineration and as landfill. Incineration involves damage to the incinerator owing to the large amount of heat generated, and, in the case of waste plastics containing chlorine, the issue of treating the chlorine in the exhaust gas. In addition, waste plastics are not decomposed by soil microorganisms or bacteria; there is a shortage of landfill sites and an environmental load has been stocked. In recent years, therefore, a call has arisen for adoption of environment-friendly recycling technologies to avoid incineration and landfill disposal. Current methods for recycling without incineration include methods for reuse as plastic raw material and for reuse of gas components and oil components obtained by thermal decomposition as fuel and chemical raw materials.
After being used as plastic products, polyvinyl chloride, polyvinylidene chloride and other chlorine-containing resins and the like are discarded along with other plastic products without being sorted out. Waste plastics therefore inevitably include a chlorine component carried in by chlorine-containing resins and the like. Sorted waste plastics recovered from households do in fact ordinarily contain polyvinyl chloride and polyvinylidene chloride, which, when calculated as chlorine, contain several wt % of chlorine. When thermally decomposed at high temperatures, polyvinyl chloride and other chlorine-containing resins generate chlorine-type gases such as hydrogen chloride gas and chlorine gas. When chlorine-containing resins or waste plastics containing them are processed for recycling at high temperature, therefore, the problem arises of the processing equipment and the like being corroded by the chlorine-type gases generated. Owing to this, conventional recycle-processing of waste plastics has been conducted by the method of, in advance, sorting out and removing chlorine-containing resins and other chlorine-containing waste plastics or removing only the chlorine component of the waste plastics and then reusing the gas components and oil components obtained by thermally decomposing the waste plastics as chemical raw materials and fuel.
Conventional methods known for recycle-processing waste plastics include, for example, the method of using a blast furnace, which is one process in iron- and steel-making, and utilizing waste plastics as an iron ore reducing agent (JP-B(examined published Japanese patent application)-51-33493). Various development efforts have recently been made in order to effectively implement this method (e.g., JP-A(unexamined published Japanese patent application)-9-170009, JP-A-9-137926, JP-A-9-178130, JP-A-9-202907, and Japanese Patent No. 2,765,535).
In the case of processing waste plastics with a blast furnace, decrease in blast furnace productivity and the effect of the chlorine component inevitably contained in the waste plastics must be taken into account.
Specifically, when the blast furnace is charged with an amount of waste plastics exceeding 10 kg per ton of pig iron produced, deactivation of the blast furnace core is induced to markedly degrade pig iron productivity. In the case of processing waste plastics with a blast furnace, therefore, the amount of waste plastics processed has conventionally been limited to 10 kg per ton of pig iron.
Moreover, waste plastics discarded as industrial waste and nonindustrial waste include so-called chlorine-containing resins, such as polyvinyl chloride and polyvinylidene chloride, and so-called chlorine-containing organic compounds such as polychlorinated biphenyls. Waste plastics, both industrial and nonindustrial, therefore on average include chlorine at about several wt % to several tens of wt % and, even after sorting, include chlorine at an average of several wt %. When waste plastics including such chlorine are charged into the blast furnace as they are, chlorine-type gases such as chlorine and hydrogen chloride are generated during thermal decomposition of the waste plastics, causing a problem of corrosion of the shell, stave coolers and the like of the blast furnace body and a problem of corrosion of furnace-top waste gas equipment and the furnace-top electrical equipment. In the case of processing waste plastics in the conventional blast furnace, therefore, there has been conducted pre-processing, such as in advance sorting out and removing chlorine-containing resins, chlorine-containing organic compounds and other chlorine-containing waste plastics or removing only the chlorine component of the waste plastics, and the waste plastics have been charged in the blast furnace after having their chlorine content reduced to 0.5 wt % or below.
Methods have also long been known for recycle-processing waste plastics by thermal decomposition using, instead of a blast furnace, a coke oven, which is one process in the same iron- and steel-making (JP-B-49-10321 and JP-A-59-120682). Recently, various development efforts have been made regarding methods for efficiently processing waste plastics, most notably waste plastic charging methods that take coke strength into account (e.g., JP-A-8-157834). In these cases, instead of coal, waste plastics, which are also hydrocarbons, are charged into the coke oven to obtain coke, tar, light oil and fuel gas by dry distillation. A coke oven can thus also be used as a waste plastic recycling facility.
However, in the case where a coke oven is used to process waste plastics, as in the case of processing in a blast furnace, it is necessary to give consideration to decrease in coke productivity caused by the charging of waste plastics, the effect on the equipment of the corrosion etc. by chlorine included in the waste plastics, and the effect on product quality.
Regarding product quality, when, for example, a blend of waste plastics and coal is charged into a coke oven, the amount of waste plastics charged into the coke oven is expected to be 10 kg per ton of coal, because the coke quality deteriorates sharply when the waste plastic charging amount exceeds 10 kg per ton of coal.
Regarding the effect of chlorine in the waste plastics, when waste plastics containing around several wt % of chlorine are charged into a coke oven as they are, a possibility exists of the chlorine component remaining in the coke after the waste plastics carbonize. Moreover, there is not only a danger that the chlorine-type gases produced by thermal decomposition of the waste plastics may mix into the tar, light oil and coke-oven gas that are byproducts at the time of coke production but also a danger that the generated chlorine-type gases will remain in the oven and/or corrode the oven body and the waste gas treatment system pipes. Conventionally, therefore, processes have bee effected for thermally decomposing only the chlorine component of the waste plastics before charging in the waste elastics in the coke oven, as taught by JP-A-7-216361, or for removing chlorine-system resins and other chlorine-containing waste plastics with a specific gravity separator or the like beforehand and charging the waste plastics into the coke oven after reducing their chlorine content to 0.5 wt % or below, as taught by JP-A-8-259955. Therefore, since the conventional methods of processing waste plastics using a coke oven actually involve complicated processing processes, no attempt has been made to put them to practical use.
As a method of recycle-processing waste plastics that does not use a blast furnace or a coke oven, there is the waste plastic processing method utilizing the gasification furnace proposed early by the present inventors in JP-A-10-281437.
However, this processing method is also yet to be implemented because the processing costs are high owing to the need for equipment for recovering the HCl gas and other chlorine-type gases generated.
In DE-40 12 397, a process is disclosed for the pyrolysis of chlorine containing waste material wherein ammonia is added to the halogen containing gases produced in a fluidization reactor in order to effect the precipitation of ammonium chloride/-halogenide after the gases have emerged from the heated parts of the reactor.
As pointed out in the foregoing, in conventional methods of recycle-processing waste plastics using a blast furnace or a coke oven, either of which is one process n the same iron- and steel-making, the problems of equipment corrosion and product quality degradation by chlorine-type gases generated from the waste plastics, which problems are encountered in either case, make it necessary that the charging into the blast furnace or the coke oven be done after first either sorting out and removing chlorine-containing resins, chlorine-containing organic compounds and other chlorine-containing waste plastics or removing only the chlorine component of the waste plastics. This has made the processing steps complicated and led to increased processing costs. Waste plastics that have been collected from throughout a city and subjected to magnetic sorting, aluminum sorting etc. ordinarily contain a chlorine component of approximate from 3 wt % to 5 wt %. This is because the collected waste plastics contain from 6 wt % to 10 wt % of chlorine-containing waste plastics, mainly polyvinyl chloride and the like. In the case of a blast furnace, it is generally accepted that a problem of corrosion by the chlorine-type gases in the blast furnace will arise unless the ordinary chlorine content is lowered to 0.5 wt % or below. Also in the case of a coke, owing to concern about corrosion of the oven body and the waste gas processing system and about the effect on product quality, the waste plastics is charged into the coke oven after first lowering the chlorine content thereof to 0.5 wt % or below.
As the method for lowering the chlorine content of the waste plastics to 0.5 wt % or below, there is adopted either the method, using a dechlorinator, of thermally decomposing the waste plastics by heating to around 300.degree. C. and removing the chlorine component thereof as chlorine-type gases, or the method of separating the waste plastics into light plastics and heavy plastics by specific gravity separation using a centrifuge or the like and sorting out and selecting only the light plastics of low chlorine content. Of these methods, the former method using a dechlorinator is very complicated because it is applied to all of the collected waste plastics. In addition, it is extremely difficult technologically by this method to reduce the chlorine content of the waste plastics from 3-5 wt % to 0.5 wt %. The method is therefore seldom adopted. The later method of separating into light plastics and heavy plastics by specific gravity separation using a centrifuge or the like and sorting out and selecting only the light plastics of low chlorine content is rather more generally adopted. The specific gravity separation method, however, also involves problems such as the following. Explanation will be made taking the method of specific gravity separation using a centrifuge as an example. Generally, when, for example, 100 kg of waste plastics removed of extraneous matter (including 10 kg of vinyl chloride and having a chlorine weight of 5 kg) is separated with a centrifuge, ideal separation, i.e., separation into 90 kg with a chlorine content of 0% as light plastics and 10 kg with a chlorine content of 50% as heavy plastics (the chlorine content of polyvinyl chloride generally being 57%), is impossible. The separation is generally into 50 kg with a chlorine content of 0.5% as light plastics and 50 kg with a chlorine content of 9.5% as heavy component. Even if the conditions are further optimized, the limit is separation into 70 kg with a chlorine content of 0.5% as light plastics and 30 kg with a chlorine content of 15.5% as heavy plastics. In this case, as the waste plastics of a chlorine content of 9.5-15.5 wt % separated as heavy plastics (accounting for 30-50% of the waste plastics before specific gravity separation) are impossible to lower to a chlorine content of 0.5 wt % by further dechlorination, they can only be treated as a residual to be disposed of as, for instance, landfill.
Treating them as residual involves processing costs and, what is more, this treatment is essentially indicative of the low recycle rate of the waste plastic recycle-processing method and cannot be called a practical recycle-processing method that responds to social requirements.