Simple Solutions That Work! Issue 6

SYSTEMS INTEGRATION for gas introduction. If chlorine is included in the gas mixture, the metal container must be of a material like Inconel to prevent corrosive attack. The body of the plug is currently a solid mass of ceramic, with a high apparent porosity to allow open pores for gas flow. The average pore size in the ceramic is 50-100 microns in diameter. While this system has been used in the aluminum industry for many years, it has significant disadvantages: • The metal container adds significant cost to the plug component • The solid body design permits no control of gas flow direction • The pore size in the body requires continuous gas flow to prevent metal penetration • The pore size generates a relatively large initial bubble size With a highly permeable ceramic body pores are in the 5-micron diameter range, less than 10 percent that of current materials. The process also allows the production of precise, intricate shapes. This unique combination of properties offers dramatic enhancements to static degassing systems: • The reduced pore size increases the efficiency of the plug by creating far smaller initial bubbles. Since the individual bubbles are smaller, the quantity of bubbles increases dramatically given the same gas flow. A key criterion for max. efficiency is a large number of small bubbles. • The pore size is small enough to prevent significant metal penetration into the ceramic, allowing for far lower gas flows during idle periods and a far greater probability of plug survival in the case of gas flow stoppage. • The ability to cast intricate, precise shapes eliminates the costly metal shell in many applications. Threads can be cast into the ceramic to allow direct pipe connection. Interior voids in the plug can be designed to direct the gas preferentially through the top of the plug and into the molten metal where it is intended. This is accomplished by adjusting the ceramic wall thickness appropriately. • The versatility of this process allows custom design of static degassing systems to optimize the user’s specific needs. Dynamic Systems Dynamic degassing systems vary more in complexity than their static counterparts, but the majority are relatively simple devices, consisting of a vertical shaft suspended in an aluminum melt and driven by a motor. Gas is pumped down through the shaft, which rotates at 200-400 rpm, and is delivered through and/or around a head at the end of the shaft. The head usually has some geometry designed to shear the bubbles, which exit either through it or beneath it. Figure 4 Static Degasser Deployment Figure 5 Dynamic/Rotary Degasser For many years, graphite has been the material of choice for both the shaft and head of rotary degassing units. It is relatively strong, and is easily machined, a distinct advantage for the complex geometry of some components. However, graphite oxidizes readily at molten aluminum temperatures, causing very poor life for the shafts. In cases where the heads have multiple exit holes to improve gas dispersion, these can readily plug when not in use. Silica components have been introduced to deal with these problems, and while that material will not oxidize, it reacts adversely with aluminum, reducing its effective life. 28