Recycling = RDF = cement

Recycling is not just a one-step process. Here, we look at how Italian company Satrind is using its versatile recycling equipment in an innovative way to turn waste into RDF which is then used as fuel for the cement industry … by Barbara Mariani The use of cement has significantly contributed to worldwide economies. In fact, cement is considered to be the second most-consumed substance in our society, and the amount used in construction is more than double the total of all other building materials, including wood, steel, plastic and aluminium. Nowadays, environmental responsibilities are high on the cement industry’s agenda because the cement manufacturing process requires extremely high temperatures which consume about ten times more energy than the average amount required by other manufacturing processes. The average amount of energy required to produce one tonne of cement is equivalent to the combustion of approximately 120 kg of coal. Employing waste as alternative fuel has greatly helped the cement industry to manage its environmental impact. Today, many plants derive up to 70% of their energy from alternative fuels. In the past, cement companies have traditionally burned coal, petroleum coke and other fossil fuels in cement kilns, but many of them have turned to alternative fuels which substitute approximately 3 million tonnes of coal every year. Using waste as fuel Alternative fuels are frequently prepared and blended outside the cement plant by specialist companies. Waste materials which are generally reusable as RDF include tyres, rubber, paper, textiles, exhausted oils, wood, plastics, industrial waste, hazardous waste and solid urban waste. Cement kilns require a homogeneous RDF composition which is uniform not only in shape and size but also in calorific value. To achieve uniform size, it is generally required that waste has to go through an adequate process of size reduction to make it reusable as a quality RDF. The material has to be uniform in size to facilitate transportation, and in many cases must be selected by a screen to obtain the optimal final size for the kilns, depending on the feeding location. In terms of uniform heating value, it is important to guarantee an homogeneous composition of waste allowing the material to burn releasing the same constant quantity of heat. It is therefore important to blend the different material types before the size reduction process or when the final material is in a processing pit. Recycling equipment for production of RDF Satrind S.p.A. has been manufacturing industrial shredders and shredder systems for many years, and has played a part in solving recycling, waste reduction and material handling problems for a variety of companies across the world. Using its expertise in recycling equipment the company has come up with a variety of solutions enabling its customers to process waste and turn it into fuel for use in making cement. Shredding plant for RDF from Industrial waste Click here to enlarge image In order to obtain an homogeneous RDF composition in terms of shape and calorific value, many companies prepare and select the waste to be shredded before the treatment. Preparation of the material is required because the shredding of heterogeneous waste could damage the machine or cause an explosion or fire, but this process has been shown to be difficult and dangerous for the operator performing the task. Satrind machines are unique in being able to shred heterogeneous waste (like metal, inflammable and chemical material), and waste sealed into intermediate bulk containers and drums, without any previous selection. The technology can guarantee a small and uniform final size. It is interesting to note that other machines designed for the same task require the waste to be pre-sorted and, in the case of drum treatment, for the drums to be emptied first. One interesting point about Satrind machines is their slow blade speed. This has been added to avoid the possibility of explosion or fire. This recycling technology can give, under the same conditions, a fine homogenous particle size which is ideal for burning in cement kilns. Recycling tyres for RDF Satrind has supplied other solutions for the production of RDF for cement kilns. One of the processes involves recycling end-of-life tyres. It is important to remember that fuels like coal and coke are known to release great quantities of heat when burning because of their high levels of carbon. Tyres are also a great source of hydrocarbons (carbon and hydrogen) and burning tyres in a cement kiln produces 25% more energy than coal. Typical shredding plant for tyres with size selection system Click here to enlarge image A typical Satrind plant gives a 58 tonne per hour output with a predefined ‘chip’ size of 5080 mm depending on requirements. This type of plant would be composed of a first, and eventually a second, stage slow speed, high torque shredder(s). Since the two shaft models cannot guarantee a specific final size, a size selection system must then be used to return all the tyre pieces which are larger than the desired dimension to the shredder for further processing. Primary shredder being fed by crane Click here to enlarge image Traditionally, the selection system was completed using a rotating trommel which required frequent maintenance to free the holes that were clogged by tyre pieces. A more efficient size-selection system has now been adopted which uses disk screens to avoid the problems encountered with previous solutions. These rectangular screens are composed of several steel disks fitted on parallel shafts. Example of tyre ‘chip’ size 80mm Click here to enlarge image The desired chip size can be obtained using pre-determined disk thickness and specific shaft speeds. The chip suitable for RDF will fall by gravity under the screening table while the longer pieces are discharged at the end of the table and are further reintroduced into the shredder with conveyor belts closing the loop. Advantages of using RDF in cement manufacture The use of alternative fuel is a well-known and consolidated technology that guarantees numerous environmental benefits. Primarily, avoiding the consumption of non-renewable precious resources and consequentially lowering gas emissions since traditional fuel is actually replaced by a fuel derived from waste which would otherwise require other forms of disposal. The burning waste produces toxic gases such as hydrogen chloride and sulphur dioxide which usually require extremely expensive filters to avoid their release in the environment. Conveniently, the raw material used to produce cement contains a high degree of lime and alkaline material which, when it is introduced into a kiln at very high temperatures (16002000°C), absorbs and neutralizes hazardous gases in the kiln chamber. Moreover, cement kilns are able to use the energy generated by the waste material, while traditional waste incinerators are less efficient converters of the heat content of waste. By cooperating with different worldwide cement companies and by using its know-how and extensive experience, Satrind is able to offer innovative technical solutions, customized for individual customers as a complete system. The recycling process, and the use of good recycling equipment, is a vital component of the larger process which turns waste to fuel that is then used for cement production. Barbara Mariani, Marketing, Satrind S.p.Ae-mail: sales@satrind.it Case study Hazardous waste plant, Turkey One of the most recent facilities Satrind has engineered, manufactured and assembled is a plant designed to process hazardous industrial waste (such as drums, bins, contaminated rags, oil filters), with a Turkish cement company, to whom it supplies alternative fuel derived from waste. To satisfy the customer’s requirements the plant had to guarantee a homogeneous composition of the output material in size and in calorific value, to allow it to efficiently burn into their kilns. The RDF the cement company specified is called ‘RDF class 3’, which is the leftover waste after metal, glass and other unsuitable materials have been removed. The plant creating the RDF from waste has a capacity of six tonnes per hour. The process begins with a primary shredding stage using Satrind’s two-shaft shredder model, 2R20/220, and is followed by a secondary shredding stage using Satrind’s three-shaft shredder, 3R15/320, with a selection screen at the discharge. The material to be treated is loaded by crane into the large feeding hopper of the first shredder. This is fitted with an hydraulic ram to press the material on the blades which avoids any possibilities of material bridging. The output material falls into a screw conveyor to separate the liquid from the solid fraction. The solid material is then discharged onto a conveyor belt fitted with a magnetic separator designed to extract any metal residues. The waste is further processed by the 3R15/320 three-shaft shredder with a selection screen designed to obtain a uniform output material size (40 mm x 40 mm). A second screw conveyor and conveyor belt with magnetic separator are positioned after this second shredder to further eliminate any small metal parts still present in the waste. The waste is discharged into a pre-holding pit. In the pit the material is mechanically blended to obtain a homogeneous composition. This composition is finally transported from the pit to a storage centre via conveyor belt where the material is stockpiled ready to be burnt as alternative fuel in the kilns. More Waste Management World Articles Waste Management World Issue Archives