the refractories of the furnace with the materials it is going to be in contact with in the shaft, especially the oxides. If the cupola has an acidic lining, which all cupola operations in North America do, a basic slag will neutralize the refractory and eat it away—meaning it will end up in your slag and reduce campaign life and increase costs. Slag composition is also an indica- tor of operational health. Certain elements are more prone to be lost during melting. Ninety-five percent of a pound of chromium added to the charge will likely end up in the melt. But elements like silicon and manganese typically lose consider- ably more. Fluctuating chemistries in your iron could be related to changes in your slag. Like the iron, slag can be tested


through visual observations, such as color, appearance and texture, as well as through lab analysis. Slag analysis is put into terms of basicity. A typical target basicity for North American cupola operations is 0.5-0.6, although some very good operations may be as low as 0.3 or as high as 0.8. What- ever number your facility targets, you should be hitting it consistently. Tis target basicity is the point where you have balanced the composition and melting of your slag with the furnace operations. Te cupola needs to be hot enough to reach that melting point— the slag has to be fluid to get it all out of the furnace.


Effective and Efficient


Te goal is to be effective and efficient in your cupola operation. Effective cupola operation means delivering the correct quantity of iron at the correct composition and temperature for the pouring demands. Efficiency in cupolas is measured by how many tons of iron can be melted per ton of coke, which is the fuel used to heat the furnace. Cupola operators are armed with


a few tools for controlling the cupola, but success is limited by the accuracy of the measurements and the abil- ity to control these inputs. Primary importance should be given to the blast air. Where is it measured? Are


Cupola operations have to balance the refractories of the furnace with the materials it is going to be in contact with in the shaft.


there leaks in the ductwork? Is the airweight controller maintained and calibrated on a regular schedule? When changes are made to charge makeup, how close can the charg- ing system get to specified targets? How consistent in composition and cleanliness are the charge materials from load to load or during various times throughout the year? How well are the materials kept segregated in the charge yard? Te measures of iron chemistry, melt rate and tempera- ture are interrelated. Operators can’t change one of those things without affecting the other two. An efficient furnace will use the


lowest coke percentage to meet the required iron temperature. Te more air blasted into the furnace, the more iron can be melted in an hour. Many


cupolas running today were designed with a cross-sectional air blasting area to melt a designated number of tons in the furnace per hour. Tis does not change, even if present-day operations might call for a lower volume per hour to meet the iron demand of the mold- ing and pouring operations. To illustrate this, Fig. 3 is a blast


rate chart for a cupola furnace. Te x axis is a blast volume expressed as a ratio of the blast volume (CFM) divided by the cross sectional area of the cupola in the melt zone in square inches. Te red line intersecting the peaks of the various coke percentage curves is called the “sweet spot” as it represents the most efficient blast rate to run the cupola (approximately 2.0-2.2.) In this chart, the lowest coke percent is 7.5, but the iron tempera-


Fig. 4. Monitoring the temperature of the melt after increasing the blast by 10% is a way to deter- mine your furnace’s coke bed level.


August 2016 MODERN CASTING | 29


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