Get the best out of it – Challenges and opportunities in OCC stock preparation
What factors determine the quality and efficiency of stock preparation for recovered paper? What are the typical weak points and sources of error? As the world's leading experts for stock preparation, Voith shows what it takes to create a robust OCC stock preparation process that delivers the best results even under challenging production conditions.
More than 120 years of experience in paper production, over 85 installed complete OCC (old corrugated container) systems and more than 70,000 individual components sold – the Voith Group based in Heidenheim, Germany is the world's leading specialist for OCC stock preparation plants. One of the company's great strengths is its holistic expertise, which enables the manufacturer to develop high-performance systems and components for stock preparation as a full-line supplier.
“We consider OCC stock preparation both at the product level and as an overall process,” explains Andreas Heilig, Vice President Product Management for Fiber Systems at Voith. “On the one hand, we consistently look for optimization potential in each individual process step and for all components used in it. On the other hand, with our cross-component engineering at the process level, we have a keen eye for the big picture, so all process steps are optimally coordinated with each other,” explains Heilig. “The result is an extremely robust and seamless process that delivers on efficiency, yield and quality.”
A robust process despite major challenges
Unexpected downtimes are one of the worst cases for every paper mill. Therefore, a robust and “trouble-free” production process is one of the most important requirements for OCC stock preparation. Voith machines for example, eliminate contaminants consistently and at an early stage so that the subsequent process steps are affected as little as possible from the dirt level upstream. The coarsest material is already sorted out in the pulper using the TwinPulp system or the IntensaDrum. Typical contaminants in this stage include stones, cans, plastic bags, larger plastic parts, adhesive tapes and wood. In addition to the “usual suspects,” however, frequently larger rejects such as curtains, brick stones, larger metal pieces and other extremely heavy contaminants that are normally unexpected in recycled paper can also be found in the pulper. Extremely heavy rejects – such as big stones and metal parts – are sedimented in the Junkomat outside the pulper (TwinPulp system). In the next stage, coarse impurities are separated in the IntensaMaXX pulper detrashing machine. In the IntensaScreenDrum that follows, the remaining fiber material is recovered, and the reject is discharged from the process. In the TwinPulp system, larger and rope-like plastic parts are twisted together by the rotation of the suspension and form a rag with the help of cut bale wires. The rag is continuously discharged via a ragger.
The problem: If the rag breaks, production must be stopped. The reason: The broken rag further agglomerates and impairs pulper operation significantly or makes it even impossible. The broken rag has to be manually removed from the pulper. It not only reduces productivity, but also represents a safety risk for employees.
“In order to avoid breakage, we have designed the ragger in such a way that it changes its contact pressure depending on the pulling efficiency of the ragger wheel. This minimizes the risk of the ragger breaking and adds to the robustness of the process,” explains Sebastian Schuster, Product Manager OCC Process at Voith.
Fewer chests for increased reliability and efficiency
With its intelligent system design, the screening system further adds to the overall robustness and efficiency. In conventional systems, the fiber suspension is sorted and if needed deflaked in a three-stage system in several steps. The reject is temporarily stored in a chest or a stand pipe before being passed on to the next stage. In order to avoid the formation of a floating layer in the chests, the reject must be kept in motion with an agitator. However, agitators tend to spin the contaminants, which leads to blockages and wear in the pumps and machines used in subsequent process stages. For this reason, Voith’s coarse screening process works without intermediate stock chests. This reduces potential sources of error and makes the process more reliable. The system’s efficiency also increases due to a reduced number of components, which in turn leads to lower energy consumption.
Another point for robustness is the new added flushing sequence in the screening system. Pulping cannot eliminate all rejects and thus smaller plastics, cords, etc., come to the coarse screening system. The rotation movement of the pumps can create smaller rope-like agglomerations that can block the screens or pumps. The creation of such agglomerations can hardly be avoided but they can be kept at a harmless size by flushing the coarse screening system frequently. This is only possible due to the good-natured FiberSorter and IntegraCombiSorter that can swallow the temporarily increased flow without problems or additional losses. In this way, small agglomerations are eliminated before they can cause problems. Thanks to their design, FiberSorter and IntegraCombiSorter can cope well with these rejects without crushing them.
The Feed-Forward Principle as a central idea
Conventional systems often rely on recirculation of rejects. During recirculation, not only flakes and fibers but also contaminants and stickies enter the system again. Due to repeated treatment in the systems, they are crushed and sent forward unnoticed along with the accepts. This leads to wear on pumps and other downstream units and can have a negative impact on the runability and efficiency of the downstream paper machine due to increased stickie content of the fiber suspension.
One of the guiding principles in the design and optimization of Voith BlueLine OCC systems is the Feed-Forward Principle. The goal is to consistently move the accept forward wherever possible. This results in significantly lower volume flows in the systems and higher stock consistencies throughout the process, which benefits the entire plant. Thanks to the high stock consistencies, the pump energy required to transport the pulp further is reduced. In addition, the pipeline diameters can be vastly reduced. This saves space and minimizes costs, as it enables savings in valves, instrumentation, pumps, motors and energy consumption.
Focus on each component
In order to improve the efficiency of the entire OCC process, Voith identifies the optimization potential for each individual component. The Voith IntensaPulper IP-R is a good example of that. Thanks to its optimized flow geometry, the pulping and mixing process has been significantly improved, reducing energy consumption by up to 20 percent. In this way, an OCC system with an annual production capacity of 350,000 tons achieves savings of more than 100,000 Euros per year. A further example for Voith’s pioneering stock preparation solutions is the cleaning process. Standard systems only accommodate low stock consistencies to achieve required removal rates. However, this reduces the energetic efficiency of the process. For this reason, Voith has designed the EcoMizer to achieve excellent separation and outstanding results with significantly higher stock consistencies.
A further important aspect for an efficient plant is water management. Water loop separation results in better water quality in the paper machine compared with the stock preparation. Efficient water management allows the lowest fresh water consumption by controlling the mill’s internal process water distribution during all operating conditions. This also ensures stable conditions for biological water treatment, resulting in a constant and best-treated effluent water that can be re-used or send out.
Additionally, a very important issue is the quality of the process water. The levels of solids in the water loops should be kept as low as possible in order to prevent accumulation. The water quality also determines how much yield of fibers and fillers gets lost with the effluent. “A typical weak point in conventional OCC systems are the filter systems used in thickening,” says Sebastian Schuster. With plastic filter bags on disc filters, there is a risk of holes and cracks forming. This reduces the quality of the filtrate significantly. It leads to clogged cleaning nozzles, e.g., at the paper machine and can reduce the lifetime of press felts. The efficiency of the entire process decreases due to longer downtimes and more demanding maintenance requirements, resulting from the components’ shorter operating lives. This is why Voith relies on filter systems made of corrugated stainless steel. These rugged components have outstanding service lives and guarantee the best results in the long term.
Each unraveled fiber is a double gain
A significant factor for the profitability of a paper mill is its yield, i.e., the proportion of accepts that can be extracted from the OCC raw material. The process benefits in two ways: First, the amount of fiber material recovered for paper production increases. And second, the improved yield reduces the amount of reject that has to be disposed of. For this reason, Voith systems are designed in such a way that the fibers are washed out of the reject as cleanly as possible wherever fiber loss can occur. The significance of this is demonstrated by the Voith IntensaScreenDrum, part of the TwinPulp System. In an OCC line with a production capacity of 1,000 tons per day, the particularly effective washing of the reject helps to avoid fiber loss of up to 315,000 Euros per year.
High-performance components to meet the highest quality demands
Another challenge in the OCC process is the clean removal of stickies. With its specialized systems, such as the multi-foil rotor for particularly gentle stickie removal, Voith has been setting benchmarks for years. Slot widths and process variants can be precisely adapted to individual requirements. In high-speed paper machines with low basis weights, four-stage sorting has proven to deliver excellent results. It enables the best sticky separation with low fiber loss. For slower running paper machines and higher basis weights, three-stage sorting is usually sufficient.
Constant production conditions as a basis
To ensure consistent production quality, Voith BlueLine OCC systems are designed to compensate for fluctuations in the quantity and quality of the raw material as efficiently as possible. The raw material input itself is a frequently underestimated factor. However, fluctuations that occur with regard to quality, fiber material and stock consistency can no longer be completely compensated by the system later on, but only minimized. To counteract this, Voith systems regulate the raw material input by conveyor belt via the stock consistency in the pulper. At the end of the OCC process, systems that are thoroughly tested and have been refined by years of experience ensure production conditions that are as constant as possible. To compensate for process fluctuations, e.g., the screw press is controlled by the torque. Unlike many other systems, the Voith InfibraDisp disperser has an adjustable gap that opens or closes depending on the specific power consumption, thus guaranteeing uniform quality. In the final process step, the Voith TwinFlow refiner with energy-controlled gap adjustment ensures that the fibers are treated consistently even when process fluctuations occur. This guarantees a high and uniform strength development of the fibers, so that the paper machine can be operated safely at maximum speed.
Tailor-made systems from a full-line supplier
Paper manufacturers benefit from Voith’s BlueLine OCC process expertise on many levels. One main advantage is that the manufacturer handles the entire conception, planning, manufacturing, startup and optimization of the complete OCC plant. “It was important for us to partner with a single-source supplier who could provide us with everything from the conveyor system for waste paper to the winder,” says Armin Vetter, Managing Director at Schoellershammer and a Voith customer. Furthermore, Voith offers to identify opportunities for production increases and optimization in existing plants. In this way, significant improvements in robustness, quality, efficiency and yield can be achieved quickly and easily.