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KILN SHELL CORROSION

Ricardo Mosci

INTRODUCTION

Corrosion of the rotary kiln shell behind the refractory lining has become an increasingly serious problem for the mineral processing industries. Every year millions of dollars are spent in kiln shell replacement. The problem has escalated 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, most of them concentrate on the mechanism of chemical attack rather than on the solution of the problem. This article summarizes the mechanism of kiln shell corrosion and proposes practical solutions to the problem.

ECONOMIC IMPACT

The cost of replacing 10 m (33 ft.) of kiln shell, in a 5.6 m (19 ft.) diameter kiln is illustrated in the following table. The kiln had to be stopped after a hole 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 15Lost Production 30.000 metric tonsProduction Value US$ 2.145.000,00Shell Replacement Cost US$ 750,000.00Refractory Repair Cost US$ 182,000.00Total Cost US$ 3.077.000,00

2002 Values

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CORROSION MECHANISM

Chlorine, possibly from waste fuels, reacts with Potassium in circulation forming Sylvite (KCl). Sylvite has a very high vapor pressure and it penetrates through the brickwork open porosity and joints, in vapor form. Next it condenses into liquid and the liquid migrates towards the kiln shell where it condenses in cristal form. The Sulphur follows a similar path but SO3 is known to attack CaO in the brick and carry it towards the kiln shell as Anhydrite or even CaS, if the kiln conditions are reducing.

Once these salts collect against the carbon steel shell, it only takes a kiln shutdown to get the missing link in the corrosion process: water. Being hygroscopic, 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 is the oxidizing element and takes part in the corrosion process, but it is rarely found 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 they are porous and consequently cannot protect the steel shell against further attack.

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 is relatively thin.

Figure 1 – Mild corrosion of the kiln shell.

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Figure 2 is a thermal picture of the kiln shell. The lighter areas in the picture represent overheated areas on the shell, prone to severe scaling and oxidation. Although cooler than the lower transition, the area past the 90 ft. mark is more prone to oxidation due to the higher concentration of volatiles in that part of the kiln.

Figure 2 – Overheated areas on the kiln shell.

Scaling of the kiln shell is a chemical reaction that intensifies with temperature. Running the kiln on a thin refractory lining, or worse, running the kiln with hot spots on the shell can only aggravate the shell corrosion problem.

Figures 3 and 4 show a kiln shell with evident signs of overheating past tire 1, towards the discharge end.

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Figure 3 – Hot spots on the kiln shell.

Chart 1 displays actual residual shell thickness measurements taken with an ultrasonic probe. The measurement was made after several cracks developed in the shell in the vicinity of tire II. The chart confirms that the scaling problem is more severe in the hottest zones of the kiln. X-ray diffraction tests performed on the deposit showed high concentrations of reduced sulfur and ferrous iron, indicating that the kiln was running under reducing conditions in the burning zone. It was later confirmed by the kiln operator that solid waste fuel was falling on the clinker bed.

Figure 4 - Hot spot seen from the inside of the kiln.

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Chart 1 – Residual kiln shell thickness measure by ultrasound.

The products of corrosion, hematite, magnetite and iron salts form a porous layer of loose material under the brick lining, making it progressively difficult 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 on the oxidized surface. Figure 6. This film is a saturated solution of hygroscopic salts. This solution contains the electrolytes that promote shell rusting during kiln shutdown.

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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 years in service. The coarse deposit seen on the brick are iron oxides detached from the kiln shell.

Figure 7 – Iron oxide deposit on the brick cold face.

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COMBATING SHELL CORROSION

The specialized literature describes several products and techniques to inhibit 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 its high cost and long application time. The protective deposit is highly abrasion resistant and does not crack or craze during shell expansion.

Steel passivation with phosphoric acid or phosphates has proven ineffective against alkali salts. Moreover the application involves corrosive chemicals and the risk of explosions caused by large volumes of hydrogen resulting from 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 volatile organic compounds inside the kiln.

WATER-BASED CERAMIC COATINGS

Water base ceramic coatings look very promising in combating shell scaling and corrosion. These refractory coatings resist temperatures of up to 1260 oC (2300 oF) and can be applied with brush, roller or spray. The inorganic binder adheres well to carbon and stainless steel. The coating thickness is less than 10 mil, and its permeability is under 0.01%. It resists both diluted and concentrated acids and bases.

Solutes in these ceramic coatings are usually refractory materials such as alumina, 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-based high temperature ceramic coatings is fast and safe. When applied with a sheepskin roller, the theoretical coverage is 17 m2 (183 sq.ft.) per gallon. The required time between applications is just 40 minutes. The ceramic coatings can also be applied by brush or spray gun.

Water-based coatings require careful surface preparation and temperature curing. Prior to coating application, the kiln shell must be sandblasted to remove rust, salts, mortar deposits and, most importantly, greasy spots. The

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shell temperature must be kept above the freezing point of water during application and 1 hour thereafter. After the coating film dries to the touch, it must 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 the lining is removed since the protective film is disturbed in service.

SACRIFICE STEEL MEMBRANE

Of all the methods tested so far, according to the author’s experience, the best was a combination of ceramic coating and a thin stainless steel plate point-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 the clean shell. Figure 9.

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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 tack welded over the coating (Figure 11) and the brick is laid on top.

Figure 11 – Fixing the steel plate to the kiln shell.

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Figure 12 – Detail of the brick installation

ADDITIONAL 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.

Chart1

Sheet1

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Sheet2

Sheet3