Simple Solutions That Work! Issue 16

52 REMOVING HYDROGEN GAS FROM THE MELT The process of removing hydrogen from molten aluminum is called degassing. While other exotic degassing methods exist, such as ultrasonic and vacuum degassing, the most widely used method of degassing in aluminum foundries involves the use of a lance (fixed or rotating) in combination with a purge gas. Purge gas is supplied to either a fixed or rotating lance which is inserted into the molten aluminum. With this method, equipment is not expensive and when performed properly, it is a very effective way to remove hydrogen from the melt and eliminate porosity in the casting. Both the Fixed Lance Degassing and Rotary Inert Degassing (RID) work by distributing gas bubbles throughout the melt. As the purge gas bubbles rise through the melt to the surface, the hydrogen in the molten aluminum diffuses into the gas bubbles and are floated to the top where it burns off. The goal is to distribute many small bubbles throughout a large area of the melt. The purge gas bubbles must be small because a given amount of purge gas in the form of small bubbles has a much greater surface area than the same amount of purge gas in the form of several large bubbles. In other words, fine purge gas bubbles distributed throughout the melt are much more effective in removing hydrogen from the melt than large bubbles rising locally in the melt. FIXED LANCE DEGASSING Fixed Lance or “Wand” degassing is accomplished by using a straight graphite or ceramic tube which is capped on one end with a series of small holes drilled on the side of the lance near the capped end. Purge gas is supplied to the open end and the wand is then inserted into the melt. The small holes at the bottom of the wand create the desired fine bubbles but they tend to stay localized in the melt near the wand. For this reason, the degassing wand is best used on small crucibles or ladles under 200 lbs. Figure #2 displays a 500 lb. crucible being degassed with a fixed lance. The flames at the surface of the melt are the result of hydrogen burning once it is carried to the surface by the purge gas. As you can see in the photo, the fixed lance is degassing only a single area of the melt. It would have to be moved to different areas in this crucible to fully degas this melt. Figure 2. ROTARY INERT DEGASSING (RID) The Rotary Inert Degassing apparatus consists of a motor connected to a graphite or ceramic lance that rotates while distributing purge gas throughout the melt. The lances for these systems are designed to chop and disperse the purge gas bubbles. There are two main types of rotary inert degassing (RID) lances. The Straight RID Lance and the Impeller Head RID Lance. 1. Straight RID Lance The Straight RID Lance is a one- piece graphite tube that does not have an attached impeller. Fingers extend off the bottom of the graphite tube while the top end of the tube is threaded to attach to the degassing unit motor. Once the graphite lance is lowered into the melt, the motor rotates the lance. The purge gas is supplied through the inside passage of the lance which flows out the bottom of the lance creating purge gas bubbles. As the bubbles flow from the lance into the molten aluminum, they are chopped into fine bubbles by the rotating fingers at the bottom of the lance and distributed throughout the melt. Figure #3 displays a Rotary Inert Degassing system using a Straight RID Lance. There are several advantages to the Straight RID Shaft. • Simple, cost-effective design with no impeller to wear out and replace • Does not create a harmful vortex around the shaft as do many impeller designs. • Works well on crucibles and ladles from 50 lbs. up to 1,000 lbs. The disadvantage of this impeller design is that it does not spread bubbles as wide as with some impeller lance designs. While easily degassing crucibles of 1,000 lbs. or less, crucibles larger than 1,000 lbs. may require the lance to be moved to different locations in the melt to fully degas the molten aluminum. 2. Impeller Head RID Lance The Impeller Head RID Lance has an impeller attached to the bottom of the rotating graphite shaft. With the Impeller Head Lance, purge gas flowing out the rotating shaft is chopped into fine bubbles and distributed throughout the melt by the impeller. The advantage of an