Optimizing Cupola System Design

Cupola melting cannot be optimized until the melting process has been deoxidized. Molten iron oxidation occurs naturally in cupola melting: It cannot be prevented but it can be countered – offset. Previous reports in this series described the methods and an only-recently-available deoxidation material, Mastermelt DeOX, for achieving total molten iron deoxidation – both initial primary oxidation and re-oxidation that occurs if molten iron is unprotected from atmospheric contact.

Very significant savings, many times exceeding millions of dollars annually, result when deoxidation is properly carried out but, it must be stressed: molten iron oxidation has to be stopped before fine-tuning of the cupola melt process can occur.

Mastermelt’s DeOX Metal Treatment effectively de-oxidizes molten iron melted in electric furnaces. It has a 100% success track record accomplishing deoxidation illustrating its overall deoxidation effectiveness. That same material, injected into iron-melting cupolas through the tuyeres, accomplishes total deoxidation throughout the cupola melt cycle; carbon, silicon, and manganese oxidation is stopped.

Carbon oxidation stopped – Stopping carbon oxidation is a feat never before accomplished in cupola melting. It has been suggested that cupola-melting operators monitor tap-out chemistry, meaning the molten iron chemistry before iron oxide contamination comes into play. At that point, carbon level is something DeOX maintains throughout the melt cycle. DeOX injection maintains the elevated carbon, silicon, and manganese present when melting first begins.

Melting personnel must determine the elevated chemistry present at tap-out and then envision what those chemistry levels allow regarding reduced coke rates and lower auxiliary-alloy additions. The potential savings are significant.

Carbon oxidation losses in the tuyere area have gone undetected in most cupola operations. Now, with DeOX injection, carbon-level increases beyond anything comparable in the past are possible.

Experienced cupola operators can transform the higher carbon levels into significant coke-rate reductions.

EF deoxidation eliminates nearly all slag formation during the EF melt cycle. With DeOX Metal Treatment, slag volumes produced during the cupola melt cycle decrease close to 75%. A minor amount of slag will always be created due to the SiO2 ash contained within coke. Limestone additions can be significantly reduced, possibly eliminated.

Reducing cupola slag formation increases sensible heat available for molten iron droplets. Higher metal temperatures can transform into further reductions in coke rates.

Cupola thermal efficiency – Years ago, malleable iron was melted in specially designed cupola furnaces. Malleable iron required low carbon levels. Carbon levels in molten iron increase as the metal droplet contacts incandescent coke below the tuyere level.

Carbon levels increase due to the graphitized coke present below tuyere level. Carbon is added to the iron droplets when it contacts the coke.

Malleable iron cupolas were designed with absolute minimum tuyere heights, above the tap-hole; the aim was for minimum coke contact with the iron, and the low tuyere height limited the contact time between the molten iron droplets and coke.

The consequence of low tuyere height was minimal carbon pick-up but dramatically higher metal temperatures. Molten iron temperatures rose above 2800°F consistently, and at times reaching 2900°F.

Cupola melting was employed to produce malleable iron for 50 years. Malleable iron cupola design was proven and its coke rates and molten metal temperature performance were firmly established over that same time.

Malleable iron cupola melting achieved low coke rates of 7%, or less, and produced molten iron temperatures in excess of 2800°F.

Modern cupolas are designed with much higher tuyere elevations, so as to gain carbon pick-up needed for the high steel metallic charges employed since the malleable iron era.

Today, some cupola operations essentially are re-melting high-quality cast iron, which does not require significant carbon pick-up during the melting process and are melting iron with 7% coke rates. Other low coke-rate cupola operations use minimal percentages of steel in their metallic-charge compositions, thereby minimizing needed carbon pick-up. These low coke-rate-melting operations suffer high silicon-oxidation losses, producing questionable economic performance.

DeOX Metal Treatment offers low coke rates, high melting temperature, no oxidation losses, improved metallurgical quality of the iron produced, increased fluidity, increased strength and ductility, improved machinability, and more. In fact, because it removes the free-oxygen atoms from the molten iron, DeOX Metal Treatment has few, if any, detrimental effects. And, importantly, it creates significant savings.

Cupola design thermal efficiency – No heat is produced below tuyere level. Descending iron droplets heat the coke and the cupola below tuyere level, robbing heat from the droplets. Lower elevation tuyere designs reduce the amount of heat robbed from the descending molten-iron droplet.

Mastermelt experimentally reduced tuyere elevations in production cupolas by increasing the thickness of the cupola bottom, effectively raising the tap-hole. Small reductions in effective tuyere elevation produced substantial metal temperature increases.

Increasing the cupola bottom thickness four inches effectively reduced tuyere height by four-inches, producing 75°F temperature increases in several cupolas.

Most iron melting cupolas have tuyere elevations in the range of 40 to 50 inches. The high elevation is needed to gain carbon, and the consequence is significant thermal inefficiency: higher coke rates result. If a four-inch decrease in tuyere elevation produced a 75°F molten iron increase, consider what a 20-inch reduction in tuyere elevation would produce.

History supports the significant metal temperature increases with lower tuyere height. Malleable iron had been cupola-melted for decades, and lower tuyere heights produce much higher metal temperatures by reducing the heat robbed from the descending iron droplet. Lower tuyere height significantly improves the thermal efficiency of the cupola.

Reviewing thermal efficiency – The presence of iron oxide within the cupola disrupts all aspects of the melting process. It is the most important issue to be resolved. Deoxidation is critical to melting performance and productivity because iron oxide is continuously forming during melting.

Many discussions are initiated blaming coke quality for substandard cupola performance, and most of these are false claims. Once the operators effectively de-oxidize the cupola melting operation the remaining quality and performance issues will begin to be addressed.

Coke rates can be reduced far below common current levels. The technology exists for that to occur today. It only requires melt leadership to accomplish the reduced coke rates. DeOX Metal Treatment simplifies melting-process deoxidation.

Casting scrap rates – The free-oxygen atoms within molten iron are very active and successfully combine with other elements within 1-2 minutes after entering the iron matrix. This continuous oxidation process causes the formation of solid oxide molecules, which agglomerate into larger oxide masses and form the oxide clusters that produce surface defects during casting solidification.

The oxidation process is ongoing as long as free-oxygen atoms are present. Precipitated oxides form continuously up to the point of solidification. Oxide masses form during the casting process, after passing through even the smallest filtration. The oxides are nano-sized until they agglomerate, and particles that small are very difficult to trap and be filtered out mechanically.

DeOX Metal Treatment eliminates the need to filter-out the oxide masses. It stops the oxides from forming in the first place by very effectively cutting off the supply of free-oxygen atoms to the molten iron matrix.

Cast iron vs. ductile iron scrap rates – A portion of all iron scrap castings result from surface or sub-surface precipitated oxides; more so in cast iron as compared to ductile iron. In ductile iron, magnesium rapidly combines with all free-oxygen atoms immediately after its introduction to the iron bath during the treatment process. Because the casting process occurs at a delayed interval after magnesium treatment, most magnesium-oxide precipitates have exited the molten iron prior to metal entering the mold cavity. For this reason, ductile iron casting processes have lower surface-defect formation compared to cast iron.

All melting personnel must realize the full effect and critical nature of free-oxygen atoms present in the molten iron bath. The only reasonable solution is to eliminate free oxygen’s presence in molten iron, and DeOX Metal Treatment achieves that.