Pierpaolo Pecchiari, engineer, head of sales department of Sorema Company (Italy)
Innovation solutions applied in polymer films recycling allow processing a wide range of materials, including hard-contaminated ones, with no influence on product quality, recycling process efficiency and production capacity.
What should be taken in consideration while designing a system
We use polymer film every day - both in domestic sphere and for industrial purposes. No wonder this material makes up majority of polymer wastes. From the ecological point of view its recycling is among the most urgent problems. Fortunately, this share of polymer wastes contains relatively little contaminations, and with the help of modern technologies recycling of it can be economically beneficial.
Polymer film recycling system, that is the topic of this article, applies practically complete washing process. Before we start description of this system, we will consider the factors that should be taken into account during designing these systems.
First of all, while designing a new system we should remember that its configuration depends on properties of the material being recycled. Polymer film has a hundred of various applications that determine properties of this material. Most widely used are LDPE and LLDPE, then come HDPE and PP.
From a recycler's point of view waste stream is analyzed by the following categories:
- constituents (possible polymers);
- average film thickness;
- degree of contamination and presence of other materials (metal, paper, rigid plastics, dirt, stones, gravel).
Other important characteristic is presence of adhesive substances, viscidities, dyes, pigments, printing, tapes and labels.
While designing a recycling system it is essential to take into consideration all the factors mentioned, as they influence production capacity, production costs and quality of the final product. Technical, economic and environmental characteristics of the system should be well balanced. While designing machines with washing stages the main purpose is to achieve maximum friction on the surface of material recycled. This technological solution is most efficient in case of friction of one film on another. Friction on metal parts will lower the process yield as it crushes film wastes so much that they are flushed with other contaminants with water.
Another important thing is that it is essential to ensure access to the film surface, at the same time avoiding trapping of contaminants and moisture between film folds.
|Picture 1. Basic principles of designing equipment for polymer film recycling
Recycling stages. Material feeding and shredding
One should be especially careful in developing first stages of the process.
As a rule film comes for recycling in bales. Once metal wires are removed (if there are such) process operators can unpack the material and examine it. This is a preliminary sorting of raw material, during which large foreign materials are removed.
One should not underestimate need for manual sorting at this stage, especially when material had no selection or was received from waste landfills. In this case there may be need for a special selection platform. In other cases - for example when recycling post-industrial film wastes - manual selection is not required.
Then the material should be crushed in a shredder. Such recycling stage has a lot of advantages. Including this stage in the process it is possible to organize pre-washing before crushing phase in order to remove foreign contaminants heavier than the water. It helps to decrease depreciation of equipment and to ensure highly efficient recycling: feeding of pre-cut material increases production capacity and reduces energy consumption.
In addition, shredding helps to measure quantity and to provide well-balanced material feeding to the next stage of recycling.
Prewashing and crushing
Prewashing stage takes place in a separation tank of a float or sink type. Here takes place initial washing of material before the wet grinder. It is especial important here that heavy contaminations are removed (sand, stones, metal, gravel, glass), thus preventing the grinder from damages and extending life time of its knives.
At the bottom of separation tank there are two gate valves for discharge of dirt, stones, glass, gravel and sand. With the help of a special screw these solid wastes are dewatered. After this, water usually returns to the tank. Filtration module can be attached to the side of the tank, thus processing water left after wet grinding stage.
Grinder is applied first of all for crushing the material processed, but one should not forget about advantages of strong mechanical friction the material undergoes while being crushed. This friction removes contaminants from the surface of film particles, so that wet grinding can really be considered as a full-featured washing stage.
After wet grinding, solid particles and moisture are segregated in centrifuges. These machines are also designed with estimation of washing action on the film. Water flow intensity and high speed rotation within a centrifuge enable additional washing of the material. Centrifuge also allows the removal of contaminants particles of diameter less than 2 mm. Then the material is supplied for the next recycling stage.
|Picture 2. Recycling stages
Film washing: an innovative approach
After preliminary washing film is delivered to further cleaning that usually takes place in a separation tank of a float or sink type. At this stage in most advanced systems a dynamic settling tank is applied; it combines a separating unit, dirt collector and settling tank.
The tank consists of two integrated machines that significantly increase efficiency of washing process: a stirring tank and hydrodynamic separation tank.
The first tank is an agitated vessel absorber. Mixing creates additional friction on the film surface. With the help of it contaminants are removed from the surface (paper, adhesives). Besides, due to water treatment of main material one can achieve complete soak of such materials as wood, water-proof labels, paper and/or cardboard. After that they are easy to remove as the water absorbed decreases their buoyancy.
Stirring tank is also a buffer where material flow is stabilized before supply to the next tank, settling tank. Efficiency of hydrodynamic separation mainly depends on how the material is supplied to the separation tank. Usually it is supplied below the water level. Special attention should be paid to material distribution within the tank.
As a matter of fact particles of material should be floating or should sink depending on their buoyancy, but one should avoid adhesion of heavier particulate contaminants to floating particles of plastics.
In most technically advanced systems it is possible to control water flow pooling it from the bottom of separation tank where heavier contaminants are usually removed. Control over water flow enables to generate a descending stream. Then it is possible to remove contaminants with lower buoyancy properties as their speed is less than that of the descending stream.
The processing techniques described enable more efficient removal of such contaminants as fibres, soaked wood particles, water-proof paper, multilayer film, including metallized one, pieces of vegetables and other.
Water flow from hydrodynamic separation tank usually returns to the system, but anyway this happens only after filtration. For filtration and immediate water return to the system one can apply various technical solutions.
Water is pumped back to the stirring tank through a pump and the cycle starts again. In this cycle contaminants from the water in separation tank are immediately removed, which means more efficient washing and purification.
Drying, storage and blending
The film drying is performed in two stages: first mechanical one, then thermal treatment. Operating factors of drying stage are limited by properties of the material processed - mainly by its thickness.
In the system concerned treatment is carried out sequentially in a washing centrifuge, a filter press, a drying centrifuge, and then the material is exposed to thermal drying.
First of all recycled film should be separated from water, for that purpose washing centrifuge is applied. At the same time residual contaminants are removed, if there are any.
Then a screw press squeezes the material and some moisture is disposed from it. Water content in the material after treatment in the centrifuge and under screw press should make up 15-20%.
The next step is processing in a horizontal centrifuge. This machine combines cleaning and drying of the material. Due to high rotational speed and special paddle design the material is ejected to the centrifuge basket at a very high speed. Here residual contaminations and water are removed. A system with self-cleaning function, as a rule, enables to perform these operations for a long time without interruption. At this stage water content in material is reduced to 8%.
The film dried in such a manner is supplied then for thermal drying, where humidity is decreased to the required level. Operation of electric battery is adjusted depending on air temperature, which allows to minimize energy consumption. Alternative energy source may be gas, steam or thermic oil. Then film fluff proceeds to further drying in a flow of hot air; after that residual moisture rate may achieve 3%.
The dry film can be stored in a special fluff silo which is designed to prevent block of material within it and is equipped with a hydraulic extraction system of film fluff. The material is dosed to extruder with the help of auger conveyor. Installation of a buffer silo has two large advantages: application of it as a buffer for storage of material allows continuous supply of material to extrusion/agglomeration line, regardless operating conditions of the washing line; besides, it is possible to blend the recycled material to make it homogeneous substance.