2.3-1 LIME KILN CHEMISTRY AND EFFECTS ON KILN OPERATIONS Honghi Tran Pulp & Paper Centre and Department of Chemical Engineering and Applied Chemistry University of Toronto Toronto, Canada ABSTRACT A lime kiln is used to convert lime mud into lime for reuse in the causticizing plant of the kraft recovery process. Many of the problems encountered in lime kiln operations can be related to kiln chemistry, including TRS and SO 2 emissions, dusting, ringing and refractory brick thinning. Understanding the composition and thermal behaviour of lime mud and the major chemical reactions involved is critically important in developing preventive measures for the problems. INTRODUCTION In the causticizing plant of a kraft pulp mill, calcium oxide (CaO) is used to causticize sodium carbonate (Na 2 CO 3 ) in the green liquor to produce sodium hydroxide (NaOH). CaO(s) + H 2 O(l) Ca(OH) 2 (s,aq) ... Reaction 1* Na 2 CO 3 (aq) + Ca(OH) 2 (s,aq) = 2 NaOH(aq) + CaCO 3 (s) ... Reaction 2 The causticizing reaction precipitates calcium carbonate (CaCO 3 ) which is separated from the liquor, washed to remove the residual liquor and dewatered on a precoat filter to a solids content of 65% or higher. The resulting lime mud is fed into a rotary kiln where it is dried and heated counter- currently by combustion gases from an oil or gas burner at the other end of the kiln. As the mud temperature reaches about 800 o C (1470 o F) in the calcination zone of the kiln, CaCO 3 decomposes into CaO and CO 2 (Reaction 3). The resulting CaO or reburned lime is reused in the causticizing proces s. CaCO 3 (s) CaO(s) + CO 2 (g) ... Reaction 3 There are many problems in lime kiln operation. Of parti cular impor tance are ringin g, dusti ng, TRS and SO 2 emissions, and refractory brick thinning. These problems are directly or indirectly related to the chemistry of the kiln. * The letters l, s and aq in the bracket beside each compound respectively denote that the compound is a liquid, a solid and an aqueous solution. This paper first examines the basic lime kiln chemistry and major chemical reactions occurring in the kiln, and then examines how kiln chemistry may be used to explain the occurrence of the above problems and to minimize them. LIME MUD COMPOSITION The composition of lime mud varies from mill to mill depending on many factors: wood species, the impurities in the make-up lime and refractory bricks used in the kiln, the efficiencies of slakers, causticizers, clarifiers and mud washers, and the burning conditions in the kiln. On a dry basis, lime mud typically contains about 95 wt% CaCO 3 and 5 wt% of impurities (Table 1). Table 1. Typical Lime Mud Composition (dry basis) Average, wt% Range, wt% CaCO 3 95 92 - 97 MgO 1.04 0.4 - 1.6 SiO 2 0.50 0.1 - 1.0 Al 2 O 3 0.14 0.05 - 0.4 Fe 2 O 3 0.05 0.01 - 0.4 P 2 O 5 0.86 0.2 - 1.4 Na 2 O 1.14 0.5 - 1.6 K2 O 0.09 0.04 - 0.12 SO 3 0.91 0.3 - 2.5 I m p u r i t i e s The oxide components shown in Table 1 are for simplicity and comparison purposes only; they do not represent the actual compounds that exist in the mud. For instance, the calcium compounds in the 95 wt% CaCO 3 may include small amounts of calcium hydroxide (free lime), calcium sulphate, calcium phosphate and calcium silicates. Similarly, MgO may include magnesium hydroxide, magnesium carbonate, magnesium sulphate, and minerals such as dolomite (CaCO 3 •MgCO 3 ) and silicates. Na 2 O and K2 O represent total alkali compounds which are mainly hydroxides, sulphides, carbonates and sulphate, although they may also be part of complex alkali-calcium-silicate minerals. Among the impurities in lime mud, Na 2 O has the highest concentration, followed by MgO, SO 3 P 2 O 5 , SiO 2 , Al 2 O 3 , Fe 2 O 3 and K2 O. Since sodium compounds contain a large portio n of SO 3 (in the form of Na 2 SO 4 ), they are by far the largest and most troublesome impurities, due to their low melting temperatures (Figure 2). As lime mud moves through the kiln, the composition changes as the mud begins to decompose. The decomposition temperature of lime mud depends greatly on the local CO 2 partial pressure and the impurity content in the mud. Since the CO 2 concentration in the kiln gas varies from 12% CO 2 near the burner to about 25% in the back end, the decomposition temperature varies from 800 to 820 o C (1470 to 1510 o F). During decomposition, the temperature of the solids remains constant due to heat
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absorption. It increases only when most of the CaCO3 in the
solids has been calcined.
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Lime Mud Reburned Lime
W e i g h t P e r c e n t
CaCO3 CaO
Impurities0
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Lime Mud Reburned Lime
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Figure 2. Impurities in lime mud and reburned lime
The composition of the solids in the kiln lies between thecomposition of lime mud and the composition of reburned
lime, which consists of the same ingredients as lime mud
minus about 40 wt% CO2 that has been released in the kiln.On a weight basis, reburned lime contains about 1.6 timesmore impurities than lime mud, and has a much higher
sulphur content due to the sulphation reaction between lime
mud and SO2 in the kiln gas.
SODIUM COMPOUNDS
There are three types of sodium compounds (customarilyreferred to only as “sodium”) in lime mud: water-soluble
sodium, water-insoluble sodium and guarded sodium.
Water-soluble Sodium
Water-soluble sodium is derived from residual white liquor
in the mud, thus consists of mostly NaOH and Na2S, smallamounts of Na2CO3, Na2SO4 and NaCl. Water-soluble
sodium is routinely monitored at many mills due to the
simplicity of the analytical procedure involved.
The chemistry of water-soluble sodium changes as the mudmoves through the kiln. In the chain section of the kiln,
NaOH reacts rapidly with CO2 in the flue gas to form
Na2CO3 (Reaction 4), while Na2S react with CO2 and H2O
forming H2S and Na2CO3 (Reaction 5). In the highertemperature zone, Na2S, if it still exists, would be oxidized
to Na2SO4 (Reaction 6).
2 NaOH(s,l) + CO2(g)
Na2CO3(s) + H2O(g) .. Reaction 4
2 NaOH(s,l) + SO2(g) + 1/2 O2(g)
Na2SO4(s) + H2O(g) ... Reaction 5
Na2S(s) + CO2(g) + H2O(g)
H2S(g) + Na2CO3(s) .. Reaction 6
Na2S(s) + 2 O2(g) Na2SO4(s) .. Reaction 7
Thus, as the mud moves further inside the kiln, water-
soluble sodium becomes a mixture of Na2CO3 and Na2SO4,
which, in the presence of other impurities in the mud, meltsat about 800oC (1470oF). This melting temperature is
approximately the same as the calcination temperature of
the lime mud in the kiln.
The water-soluble sodium content in the mud generally
increases with:
• decreased mud solids content;
• inadequate mud washing;
• increased dust recycling load
Water-insoluble Sodium
This is the type of sodium that is chemically bound in thelattice structure of silicates and consequently does not
dissolve readily in water. Water-insoluble sodium is formed
mainly as a result of reactions between the water-solublesodium and silica or silicate minerals in the mud and
refractory bricks in the high temperature zone of the kiln. It
may be also derived from impurities in the make-up lime.
The water-insoluble sodium content in the mud generally
increases with:
• increased SiO2 content of the make-up lime;
• increased kiln front end temperature;
• increased dregs carryover;
• increased use of make purchased lime (increased
number of passes of the reburned lime in the recovery
cycle.
Due to the high melting temperature of the silicates,typically >1200oC (2190oF), water-insoluble sodium is
expected to be solid, relatively inactive, and unlikely tocause problems in the kiln environment.
Guarded Sodium
This type of sodium is the least known of the three types. It
is not soluble in water at room temperature but becomes
water-soluble after the mud has been heated at high
temperatures. It is believed to form during the causticizing
process where a few Na+ ions precipitate along with Ca2+
Figure 7. Cross section of nodules from a lime kiln [4].
In mills where only small nodules are tested, it is likely that
a large amount of carbonate “deadload” is unknowingly
circulating within the lime cycle. This would result in a burden to the causticizing equipment and lime kiln. It is,therefore, important to develop/adopt an improved
procedure for determining the residual CaCO3 content in the
product lime which must include large nodules. One
example of this is to analyze lime samples after the limecrusher, and not before. However, depending on the amount
of oversize nodules that come out of the kiln, the residual
CaCO3 target may have to be raised to avoid over cookingthe smaller nodules.
The quality of reburned lime is generally judged by its
residual CaCO3 content, availability and reactivity [5]. The
residual CaCO3 content is typically controlled between 1.5to 2.5 wt% by adjusting the front end temperature of the
kiln. This control target is necessary in order to avoid
overburning, and to ensure the production of reactive lime.Lime availability is the amount of CaO present in the
reburned lime that is available for slaking. It is typically
about 90% ranging from 85 to 95%, depending on theamounts of impurities, and residual CaCO3 in the reburned
lime. Lime availability is also be used to indicate the extent
of inert materials accumulated in the lime cycle.
Lime reactivity refers to the speed at which the reburned
lime can be slaked in the slaker. A highly reactive lime has
a porous structure and will slake within 5 minutes. A low-
reactivity lime has a low availability and a low specific
surface area, and may take 15 to 20 minutes to slake,causing problems in slakers and causticizers [6].
RING FORMATION
Ring formation is the most troublesome problem in lime
kiln operation. In severe cases, ringing results inunscheduled kiln shutdowns for ring removal (Figure 8).
Figure 8. Ring formation in a lime kiln
There are three main types of rings which occur in lime
kilns [7]. Mud rings, which are located within 30 meters (90feet) from the chain section, are believed to form when the
mud has a higher than normal moisture content, and when
the feed end temperature is low. Mud rings are soft, but theymay form rapidly. Mid-kiln rings occur in the middle of the
kiln, starting near the beginning of the calcination zone and
ending at about 30 meters (90 feet) from the front end. This
is the most common and also the most troublesome type ofring. It is believed to form as a result of recarbonation of
CaO particles, as will be discussed later. Front-end rings
occur near the burner. They are presumably formed as a
result of the sulphation of CaO at high temperatures.
Other types of rings occur less often; these include rings
that are formed due to the dripping of liquid fuel at the frontend, followed by agglomeration of lime particles, and rings
that are formed by the agglomeration of large mud balls.
In order for a ring to form, lime mud or product lime particles must first adhere to the kiln wall. The ability of
the particles to adhere is a function of particle size and the
amount of liquid phase that covers the particle surface. In
general, small, wet particles tend to adhere more readily
than large, dry particles. The stickiness of lime mud isdictated by the presence of a liquid phase, which is either
water at low temperatures or a molten material at high
temperatures. Lime mud with low solids content may not