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What is Powder Simulation?
―From Resource Recycling to Gender Equality

Chiharu Tokoro
Associate Professor, Faculty of Science and Engineering, Waseda University

Representing Powder flow by Computer

Powders can behave at times like "solids," and at other times like "liquids." To give a simple example, if a layer of powder is allowed to flow out of a small exit opening, there are instances when it will obstruct the area around the opening like a solid, as well as instances when it will not have such behavior and will instead show the fluidity of a liquid. These blockages occur at random, and their occurrence is very difficult to predict.

One simulation method for predicting this type of complex powder behavior is known as the Discrete Element Method, or DEM. With DEM, the equation of motion for each individual particle is solved over a short time step, and by repeated applications the changes in behavior of the entire powder over the passage of time can be analyzed. When considering collisions of particles with each other or against the walls of a container, their rebound behavior is expressed by calculating their impact force using a model known as the Voigt model. If many particles form an aggregate body and are in contact with each other, and another particle collides with that aggregate, the equations of motion for all particles in it would in actuality need to be solved simultaneously. Therefore, if an aggregate consisting of one million particles were formed, a resulting simultaneous equation of one million degrees would need to be solved. No matter how many steps would be taken to repeat this calculation, it would equate to a tremendous amount of work, extremely difficult to solve in a practical time frame. In 1979, Dr. Cundall, who initially proposed DEM, devised a method to analyze the problem of such aggregate collisions in a simulated manner; instead of solving a simultaneous equation of one million degrees, he suggested to only solve for the impact force of the two particles directly contributing to the collision, and then to gradually propagate this impact force to the other surrounding particles by analysis over extremely small time steps. Although this is a method of approximation based on a highly engineering-oriented idea, in the more than 30 years from its proposal until today, it has seen application in a wide range of areas, from industrial fields such as fluidized beds, pulverizing mills, and mixing machines, to the understanding of natural phenomena such as avalanches, landslides, and soil liquefaction, unquestionably proving its value and importance.

Detaching Parts from Printed Circuit Boards and Concentrating Rare Metals

Here, I would like to present one example of an application of DEM simulation in our study. Specializing in resource recycling, I am currently applying DEM simulation to the investigation of a crushing method that can be used to concentrate rare metals from waste products such as small-scale electronic equipment. Rare metals are elements added in small amounts to various parts of electronic equipment including personal computers and mobile phones, and are useful in the development of advanced functions for them. When rare metals are recycled from waste printed circuit boards, if boards themselves and the parts mounted on them are crushed into powder together, it would be nearly impossible to separate and concentrate minute amounts of rare metals from the powder. On the other hand, however, because specific rare metals are primarily used in specific parts, if those parts alone could be detached and removed from their boards, concentrating their rare metals to some extent would be possible. In other words, this task could be accomplished by a mill able to detach specific parts from waste printed circuit boards without destroying them during rare metal recycling. The figure below shows a DEM simulation developed based on this purpose. By joining small constituent particles together and arranging them to directly simulate printed circuit boards (gray) with mounted parts (black), and by incorporating a model into the DEM simulation by which the particles' adhesive forces would be broken when the tensile force at their adhesion points exceeds a specified threshold value, we have succeeded in directly analyzing the detachment of parts from circuit boards by a agitation mill. These simulation results have also been successfully replicated by experimental results for confirmation. Use of simulation tools such as this can make possible the design of mills specifically created to detach parts from circuit boards for the purpose of rare metal concentration.

Figure Simulation of Part Detachment from Printed Circuit Boards

Speculating on the Meaning of Gender Equality by Powder Engineering

The similarities between powder behavior, which at times can display random phenomena, and the unpredictable behavioral patterns of humans, have been recognized for some time. Techniques similar to DEM simulation are being used in research applied to urban planning and other fields, to predict events including group evacuation during disasters, the formation of traffic jam, and congestion generated in front of train stations. That is to say, they use the concept that if a single person could be considered to be one particle, then a group of people could be treated in the same way as a powder layer.

Incidentally, recent years have seen the move towards a time in which every organization is taking active steps toward gender equality, under such labels as "diversity" and "inclusion." For example, the number of female researchers in college science and engineering departments is still relatively small, and (almost-forced) efforts are being made to promote the employment of female instructors. As a woman, I am pleased to see the advancement of environmental arrangements which allow women to work more easily, as well as the increase in fellow female researchers, but on the other hand there are also not a few times when I feel strangely uncomfortable at the excessive attention and special treatment. At these times, I try to reconsider the meaning of gender equality from the viewpoint of powder engineering. Comparing an organization to a pile of sand, I know that ultimately, an arrangement and structure with a wide variety of particles of different shapes and sizes, as well as differing coefficients of friction, adhesive forces, and attractive forces, will make the pile more resistant to being destroyed by unforeseen external forces. However, no matter how varied the accumulated particles may be, if there are forces of repulsion existing among them, the sandpile they comprise will collapse instantly. This, I believe, is the meaning behind what is known as "inclusion," but what I find even more curious is how much more readily I can calmly accept this idea, once the concept of an organization has been replaced with that of a powder layer.

Chiharu Tokoro
Associate Professor, Faculty of Science and Engineering, Waseda University

[Profile]
Graduated Chiba Prefectural Chiba Senior High School; and Department of Resources Engineering, School of Science and Engineering, Waseda University.
2003: Completed doctoral course, major in Geosystem Engineering, Graduate School of Engineering, The University of Tokyo. Obtained Doctor of Engineering.
From 2004: Assistant, School of Science and Engineering, Waseda University. From 2007: Full-time lecturer, Faculty of Science and Engineering, Waseda University. From 2009: Currently-held position. Areas of specialization: Resource recycling engineering, powder engineering.

[Publications]
C. Tokoro and S. Owada: "Crushing and Grinding Methods Suitable for Effective Utilization of Urban Mines" [Toshi Kouzan no yuukou riyou ni teki shita hasai・ funsai houhou], Metals [Kinzoku], Vol.82, No.7, pp.36-41, 2012.
H.Sasaski, C.Tokoro and H.Hayashi: "Colloidal particle processing using heterocoagulation" in "Electrical Phenomena at Interfaces and Biointerfaces," Ed. Hiroyuki Oshima, John Wiley & Sons, Inc., pp.315-330, 2012.
C.Tokoro et al.: "Fundemental Study of Parts Detachment Mechanism from Waste Printed Circuit Boards in Agitation Mill," Resources Processing, Vol.59, pp.27-32,2012.
et al.