Waste-To-Value: Innovative recycling of mill waste

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Waste-To-Value: Innovative recycling of mill waste

August 01, 2015 - 15:33

BRUSSELS (PPI Magazine) -Opportunities now abound for mills to recover, re-use or even sell waste by-products

Waste from paper mills is often sent to landfill. This can be a costly and environmentally sensitive operation. A new alternative - known as waste-to-value - collects, treats, and converts waste into energy or saleable by-products. Innovative technologies for sludge and reject handling, material separation and recovery, and production of renewable energy help you reduce dependency on purchased power or fossil fuels and eliminate the sending of waste to land thus improving your mill's profitability.

Waste is expensive. It costs money to transport it, handle it, and dispose of it. And waste can have serious environmental implications, if it contains potentially harmful chemicals. In recycled paper mills, waste can be particularly bothersome. Waste comes in with the raw materials: plastics, textiles, wood, scrap, wires, glass, and other contaminants are an unavoidable part of the waste paper collection process. Depending on the grade of the recovered paper, rejects can account for more than 10% of the raw material; they can have various sizes (from the size of a grain of sand to large and heavy particles) and characteristics (wet or dry; of different densities; loose, compacted, or twisted).

There are several "protection" systems in mills to detect and screen out these impurities before they make their way into the paper stock. On the other hand, rejects can also contain recyclable materials like metals or burnable material with a considerable calorific value, such as plastics and textiles.

When recycled fibers are processed and deinked, sludge is generated, as is when raw water and waste water are treated. These mill sludge can sometimes be difficult to handle and dewater. Since transportation costs, gate fees, and landfill costs are often assigned on a per-ton basis, the extra weight of water in the waste increases the cost. Much of this cost can be avoided, however, with a good dewatering system.

Creating An Opportunity

Where there is a problem, there is potentially an opportunity. If we talk about waste, the opportunity is that it becomes economically attractive to recycle the mill waste, Fig. 1, being the source of renewable energy (gas and oil) and valuable by-products, such as metals (like aluminum) and bio-char. In this way, an internal fuel source is created, disposal costs are avoided, landfill volume is reduced, and so is the burden on the environment.

Andritz develops innovative waste-to-value technologies for sludge and reject treatment as well as smart recycling concepts for mill waste. Modular system designs with scalable sizes can be adapted to the specific waste streams of any mill.

Turning Waste Into Fuel

Dewatered rejects, sludge, and other mill residuals can be combusted to produce steam. The steam can be used directly in the paper mill or fed into a turbine-generator to produce electrical power. The heat for steam production can be produced either directly by burning the waste in a controlled environment or indirectly by generating a fuel gas that can be burned or further processed.

There are various technologies for the purpose of turning waste into fuel: the front-end technologies are for handling the rejects and preparing them for conversion into fuel; downstream processes convert the waste into solid, liquid, or gaseous fuels. Andritz is working with various technologies for both front-end and downstream processes.

The modules available today developed by Andritz are for rejects handling, sludge handling, drying, and fuel preparation at the front end, as well as the downstream pyrolysis technology and combustion systems (boilers and gasifiers). This equipment can be used in a stand-alone arrangement, as "process islands," or can be integrated into a complete system.

How To Make Profit From Mill Waste

Waste-to-value processes, Fig. 2, enhance your mill's profitability by recycling the mill waste products. Each mill situation is slightly unique. Therefore, the Andritz approach is to design a solution based on the mill's specific goals and requirements. There is no standard solution, but there are standard modules that can be combined with some customized solutions. The basis is an analysis of the available waste streams - amount of valuable waste compounds and heating values of the potential fuels - the existing equipment at the mill, and the desired final result.

Let's take a closer look at the individual process steps:

Rejects handling:With few exceptions, practically all recycled paper rejects can be used as a fuel source. What Andritz does is to separate out contaminants that cannot be readily converted into energy - part of them, however, can be recycled as valuable goods. An early step is to remove coarse, iron-containing metals using a powerful magnet. With a ballistic separator, Fig. 3, bulky material is removed from the reject stream: unwanted debris like stones, bricks, and other heavy items as well as valuable compounds, such as plastic bottles or cans.

After mechanical dewatering, the rejects are reduced in size and made more homogeneous in a shredder, Fig. 4, before further components, such as non-ferrous metals or PVC, are sorted out. The metals removed can be sold as scrap to the metals industry. After this, the material may have to be dried before being combusted or pyrolyzed.

Sludge handling:After pre-thickening by gravity, fibrous sludge is dewatered on mechanical presses applying very high pressures. Sludge from biological water treatment is somewhat more difficult to dewater mechanically. This is why flexible belt presses are typically applied for this purpose. Andritz technologies for sludge dewatering include screw and belt presses, gravity tables, thickening drums, Fig. 5, and other dewatering devices.

Drying:Thermal drying is required to remove more moisture from pre-dewatered sludges. Quite often, belt dryer technology is used for rejects and sludge materials, and also for woody biomass. By drying in a "closed system", the heat in the drying air can be recovered and utilized - reducing overall energy consumption.

Pelleting:Due to their dense form and low moisture content, pellets are very efficient to transport and store. Pelleting is therefore an excellent technique for preparing the waste fuel. The waste is pretreated and fed at a higher temperature into an Andritz pelleting machine. The machine extrudes the material through a perforated die to form pellets of uniform diameter. The pellets are cooled and conditioned to prevent them from sticking together. While wood pellets can be used in household heating systems, the pellets made from rejects and sludge materials need official permission for waste combustion.

Pyrolysis and combustion:Andritz offers innovative pyrolysis technology for full recycling of plastic rejects and laminates (plastic-aluminum compounds). The basic technical principle of pyrolysis is a chemical process, a technology for high-temperature heating of organic material in the absence of oxygen. For combustion, the company provides proven power boiler technologies - well suited for biomass and alternative fuels - with high combustion efficiency. With both pyrolysis and combustion processes, complex waste products can be converted back into resources, thus making substantial contributions towards protecting the environment.

Trash To Treasure

There is an old saying that "one man's trash is another man's treasure". Just as craftspeople turn old metal and textile scraps into works of art, Andritz technology is turning once-disposed-of waste materials into valuable energy sources and products. Perhaps the outcome is not as beautiful as a work of art, but it is important as a cost-saving and environmentally sound solution. So, don't waste your waste!

 

Michael Waupotitsch is VP sludge, reject and recycling; Regina Puschnig is marketing manager, Pulping and Fiber Division, Andritz, Austria.