Kiln Shell Corrosion


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This paper proposes a simple solution to kiln shell corrosion problems.

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Kiln Shell Corrosion

  1. 1. KILN SHELL CORROSION Ricardo MosciINTRODUCTIONCorrosion of the rotary kiln shell behind the refractory lining has become anincreasingly serious problem for the mineral processing industries. Everyyear millions of dollars are spent in kiln shell replacement. The problem hasescalated with the use of waste fuels loaded with water, chlorine, sulfur,alkali and transition metals in the kiln.Several papers are available on the subject of shell corrosion, however, mostof them concentrate on the mechanism of chemical attack rather than on thesolution of the problem. This article summarizes the mechanism of kiln shellcorrosion and proposes practical solutions to the problem.ECONOMIC IMPACTThe cost of replacing 10 m (33 ft.) of kiln shell, in a 5.6 m (19 ft.) diameterkiln is illustrated in the following table. The kiln had to be stopped after ahole was detected on the shell. The figures correspond to a 2,000 metric tons(2,200 short tons) per day, long dry process kiln. Production Days Lost 15 Lost Production 30.000 metric tons Production Value US$ 2.145.000,00 Shell Replacement Cost US$ 750,000.00 Refractory Repair Cost US$ 182,000.00 Total Cost US$ 3.077.000,00 2002 Values Ricardo Mosci –
  2. 2. CORROSION MECHANISMChlorine, possibly from waste fuels, reacts with Potassium in circulationforming Sylvite (KCl). Sylvite has a very high vapor pressure and itpenetrates through the brickwork open porosity and joints, in vapor form.Next it condenses into liquid and the liquid migrates towards the kiln shellwhere it condenses in cristal form. The Sulphur follows a similar path butSO3 is known to attack CaO in the brick and carry it towards the kiln shellas Anhydrite or even CaS, if the kiln conditions are reducing.Once these salts collect against the carbon steel shell, it only takes a kilnshutdown to get the missing link in the corrosion process: water. Beinghygroscopic, these salts absorb and react with water, providing the anions[SO4]-2, [SO3]-2, Cl-, which in turn attack the steel forming magnetite(Fe3O4), hematite (Fe2O3), pyryte (FeS), goethite (Fe3+O(OH) ). Chlorine isthe oxidizing element and takes part in the corrosion process, but it is rarelyfound attached to iron in corrosion products.Usually the products of oxidation show a layered composition: Wustite(FeO) on the cold face, Magnetite (FeO.Fe2O3) in the middle and Hematite(Fe2O3) towards the brick lining. The problem with these oxides is that theyare porous and consequently cannot protect the steel shell against furtherattack.Figure 1 shows a corroded kiln shell after removal of the brick lining.The corrosion process is in its preliminary stages since the oxides layer isrelatively thin. Figure 1 – Mild corrosion of the kiln shell. Ricardo Mosci –
  3. 3. Figure 2 is a thermal picture of the kiln shell. The lighter areas in the picturerepresent overheated areas on the shell, prone to severe scaling andoxidation. Although cooler than the lower transition, the area past the 90 ft.mark is more prone to oxidation due to the higher concentration of volatilesin that part of the kiln.Figure 2 – Overheated areas on the kiln shell.Scaling of the kiln shell is a chemical reaction that intensifies withtemperature. Running the kiln on a thin refractory lining, or worse, runningthe kiln with hot spots on the shell can only aggravate the shell corrosionproblem.Figures 3 and 4 show a kiln shell with evident signs of overheating pasttire 1, towards the discharge end. Ricardo Mosci –
  4. 4. Figure 3 – Hot spots on the kiln shell.Chart 1 displays actual residual shell thickness measurements taken with anultrasonic probe. The measurement was made after several cracksdeveloped in the shell in the vicinity of tire II. The chart confirms that thescaling problem is more severe in the hottest zones of the kiln. X-raydiffraction tests performed on the deposit showed high concentrations ofreduced sulfur and ferrous iron, indicating that the kiln was running underreducing conditions in the burning zone. It was later confirmed by the kilnoperator that solid waste fuel was falling on the clinker bed. Figure 4 - Hot spot seen from the inside of the kiln. Ricardo Mosci –
  5. 5. 24 22 20 mm 18 16 14 12 2 6 10 14 18 22 26 30 M E T E R S F R O M N O S E R IN G Chart 1 – Residual kiln shell thickness measure by ultrasound.The products of corrosion, hematite, magnetite and iron salts form a porouslayer of loose material under the brick lining, making it progressivelydifficult to properly install brick in the area. Figure 5. Figure 5 - Products of kiln shell corrosion.In areas of intensive shell corrosion, an oily film can sometimes be seen onthe oxidized surface. Figure 6. This film is a saturated solution ofhygroscopic salts. This solution contains the electrolytes that promote shellrusting during kiln shutdown. Ricardo Mosci –
  6. 6. Figure 6 – Oily film (salt solution) on the kiln shell.Figure 7 shows the cold face of a brick removed from the kiln after 2 yearsin service. The coarse deposit seen on the brick are iron oxides detachedfrom the kiln shell. Figure 7 – Iron oxide deposit on the brick cold face. Ricardo Mosci –
  7. 7. COMBATING SHELL CORROSIONThe specialized literature describes several products and techniques toinhibit kiln shell corrosion. The most popular techniques are plasma spray,acid passivation and metal coatings.High temperature alloy or ceramic spraying is not widely used because of itshigh cost and long application time. The protective deposit is highlyabrasion resistant and does not crack or craze during shell expansion.Steel passivation with phosphoric acid or phosphates has proven ineffectiveagainst alkali salts. Moreover the application involves corrosive chemicalsand the risk of explosions caused by large volumes of hydrogen resultingfrom the reaction between the acid and the steel shell.Metallic coatings are usually urethane- or epoxy-based. The sacrifice metal,usually aluminum or zinc is readily depleted, leaving the shell unprotected.During application organic coatings generate large volumes of volatileorganic compounds inside the kiln.WATER-BASED CERAMIC COATINGSWater base ceramic coatings look very promising in combating shell scalingand corrosion. These refractory coatings resist temperatures of up to 1260o C (2300 oF) and can be applied with brush, roller or spray. The inorganicbinder adheres well to carbon and stainless steel. The coating thickness isless than 10 mil, and its permeability is under 0.01%. It resists both dilutedand concentrated acids and bases.Solutes in these ceramic coatings are usually refractory materials such asalumina, zirconia, yttria, aluminum silicate, silicon carbide, boron nitride.Some coatings use aluminum, zinc and silicon as solutes.Unlike other methods and techniques, the application of the water-basedhigh temperature ceramic coatings is fast and safe. When applied with asheepskin roller, the theoretical coverage is 17 m2 (183 sq.ft.) per gallon.The required time between applications is just 40 minutes. The ceramiccoatings can also be applied by brush or spray gun.Water-based coatings require careful surface preparation and temperaturecuring. Prior to coating application, the kiln shell must be sandblasted toremove rust, salts, mortar deposits and, most importantly, greasy spots. The Ricardo Mosci –
  8. 8. shell temperature must be kept above the freezing point of water duringapplication and 1 hour thereafter. After the coating film dries to the touch, itmust be cured at 100 ºC (220 F) for proper curing of the inorganic binder.Curing can be accomplished with heating elements placed inside the kiln,and it may take several hours.Although effective, ceramic coatings require maintenance every time thelining is removed since the protective film is disturbed in service.SACRIFICE STEEL MEMBRANEOf all the methods tested so far, according to the author’s experience, thebest was a combination of ceramic coating and a thin stainless steel platepoint-welded to the kiln shell.The kiln shell is first sandblasted to white metal as shown in Figure 8. Figure 8 – Partly sandblasted kiln shell.Next, two layers of water-based ceramic coating are sprayed or rolled on theclean shell. Figure 9. Ricardo Mosci –
  9. 9. Figure 9 – Ceramic coating CP 3015 Al sprayed on the kiln shell.After the coating is dry, a 22 gauge sheet of AISI 304 stainless steel is tackwelded over the coating (Figure 11) and the brick is laid on top. Figure 11 – Fixing the steel plate to the kiln shell. Ricardo Mosci –
  10. 10. Figure 12 – Detail of the brick installationADDITIONAL MEASURES TO MINIMIZE SHELL CORROSION  Install the brick using mortar instead of correction shims and plates.  Mortar brick to brick and ring to ring.  Use our zero permeability basic brick to minimize infiltration.  Use bricks with low thermal conductivity.  Seal the finished lining with a refractory coating.  Do not operate the kiln with an overheated shell.  Install a powerful shell cooling system in the affected area. Ricardo Mosci –