Dissolution kinetics of chemically pure quicklime in the presence of various anions

1 Introduction

Pure quicklime (CaO) is a white, solid material at room temperature, prepared by the thermal decomposition of limestone (CaCO₃). Of the heavy industrial chemicals, lime is second only to sulphuric acid in the amount produced. It is commonly used as an industrial base and plays an important role in many practical applications such as flue gas desulphurization, potable water treatment, paper manufacturing, cement production and steel and ferro-alloy production processes, where it can be used as a flux to remove impurities in refining steel. Another well-known use is to improve the qualities of mortar in the building and construction industries.  

The optimum temperature for calcining limestone in order to produce the most reactive quicklime is ~900 °C [1]. Limestone calcined at this temperature produces quicklime with the largest specific surface area available for reaction. When reacted with water, quicklime is converted to calcium hydroxide in a highly exothermic reaction.

Previous authors have reported studies that involve measuring the time it takes for a fixed mass of lime to raise the temperature of a fixed volume of the slaking water, for example from 20 °C to 60 °C, in order to investigate the effect that various parameters have on the reactivity of the lime. It has been established that this test is a reliable way to measure the rate of slaking/dissolution or reactivity of lime when it is added to water [2]. Investigations considered to date are, amongst others, the effects of production and storage conditions of the lime [3], lime calcined under N2 atmosphere compared to CO2 atmosphere [4], lime produced in the laboratory compared to industrial limes [4], calcination temperature of the limestone [1], microstructure and texture of the limestone [1], lime constituents [5], lime particle size [1,8], temperature of the slaking water [6], stirring of the slaking water [7], the concentration of Ca2+ and OH- ions in the slaking water [8] and the composition of the slaking water [5,9] on the reactivity or slaking rate of lime.

It was established that during periods of prolonged storage and under humid atmospheric conditions, lime often undergoes air-slaking and recarbonation. These processes have been found to decrease the reactivity of the lime [10]. Lime calcined under N2 atmosphere in the laboratory is more reactive than lime calcined under CO2 conditions. Both limes produced in this way are more reactive than industrial limes [4]. Quicklime is more reactive if it is calcined from limestone that exhibits small to large size grains which are distributed inhomogeneously throughout the mass, compared to limestone that exhibits fine grains with a homogeneous texture [1]. The rate of lime hydration increases with specific surface area exposed to water (i.e. smaller particle size) and with continuous stirring [8]. The slaking rate decreased with increasing Ca2+ and OH- ions in the slaking solution and increased with increasing temperature. Chlorides and nitrates present in the slaking water increase the rate of hydration whereas carbonates and sulphates decrease the rate [5].  

It is considered that the reaction of lime with water proceeds via the three following steps:

The conversion of calcium oxide to calcium hydroxide.

The dissolution of calcium hydroxide to give calcium ions and hydroxide ions in solution.

­The diffusion of calcium and hydroxide ions into the bulk of the solution.

The third step is the rate determining step as it is the slowest process. The slaking rate of pure lime powders can be described by the shrinking sphere model, which models the lime particles as spheres and takes into account the surface area and volume of the particles [8]. From studies with rotating disks of lime, it was found that a linear relationship exists between the amount of lime dissolved in water and the reaction time. From this Ritchie and Xu [11] concluded that the dissolution of lime reaction is essentially zero order and during this investigation the value of the rate constant for the reaction was found to be 1.2 x 10-4. These authors also found that the more alkaline the slaking solution the slower the reaction rate.  

Although the amount of lime that will finally dissolve in solution can be predicted from thermodynamic principles, it is not possible to predict the kinetics of the reaction. Due to the differing origins and production conditions of limes, this must be done experimentally. Because the slaking of lime releases Ca2+ and OH- ions into the slaking solution, it is possible to follow the slaking or dissolution reaction of lime using conductance measurements. As the reaction proceeds, more Ca2+ and OH- ions will be released into solution, and therefore conductance will increase with time.

Despite the fact that the reactivity of lime has been studied previously, the kinetics of the slaking reaction has not been studied in much detail. The aim of this study was to investigate what effect different anions in the slaking solution will have on the rate of dissolution of quicklime, by using conductance measurements.

2 Lime slaking with different additives

Laboratory grade calcium carbonate (CaCO3) was heated to 1000 °C in a furnace for 3 hours. This was immediately transferred from the furnace to a desiccator to prevent air-slaking and recarbonation. The resulting quicklime (CaO) had an average particle size of 3.6 μm, as determined from analysis using a Malvern Zetasizer particle size analyser.  

A 100 ml of the slaking solution was heated in a 100 ml beaker to the desired temperature and maintained constant throughout the reaction using a water bath and set to the desired stirring rate using a magnetic stirrer. Once the desired temperature and stirring rate were set, the conductivity meter probe was inserted into the beaker and the initial conductance recorded. The quicklime was then quickly added to the slaking solution and the time recorded with a stopwatch. A conductance reading was taken at various time intervals until the reading remained constant for at least fifteen minutes (to ensure the reaction had reached completion).

The measurements were carried out at 25 °C, and at a stirring rate of 200 r.p.m in deionised water. The slaking solutions to which the lime was added consisted of 0.1 M sodium chloride, 0.1 M sodium nitrate, 0.1 M sodium sulphate, 0.1 M sodium acetate, 0.1 M tri-sodium citrate, 0.1 M sodium fluoride, 0.1 M sodium carbonate and 0.1 M sodium bicarbonate respectively. This was done with different initial masses of 0.1±0.0020 g, 0.2±0.0020 g and 0.5±0.0020 g of quicklime. The effect of increased stirring rate from 200 r.p.m to 600 r.p.m, changing the concentration of the anion in the slaking solution and increasing the temperature were investigated.

3 Experimental determination of the slaking reaction

In order to ensure the validity of the measurements and kinetics approach adopted, a conductance probe was calibrated against various initial masses of quicklime added to water and stirred for 30 minutes before the final conductance reading was recorded. Figure 1 indicates that there is a linear relationship between the mass of quicklime added to the water and the equilibrium conductance measurement. This indicates that conductance measurements could be substituted for concentration values in the kinetic rate equations and calculations. 

When quicklime is added to deionised water, the conductance of the resulting solution increases rapidly in the initial stage (Fig. 2). This increase slows down after approximately 60 s, before eventually remaining constant. At this point either all of the quicklime has dissolved or the solution has become saturated. This is to be expected due to more Ca2+ and OH--ions being released into solution as the reaction proceeds.

This reaction seems to display typical first order kinetic characteristics. We can rule out a zero order reaction as in this case the graph would display a constant linear relationship between conductance and time and the rate of reaction would be constant as the reaction progresses.

The rate equation for a first order reaction is usually written as r = k[reactant]. To describe the influence of the anion on the reaction, the equation can be extended to: r = k[reactant][anion]. In order for pseudo first order conditions to apply, it is required that the concentration of the anion influencing the lime dissolution must be at least 10 times larger than the concentration of the dissolving lime. This would then yield the following equation: r = kobs[reactant], where kobs= k[anion]. For comparison of the various rate constants, the concentration of the anion was kept constant in each case.  

To confirm first order kinetics, the equation ln(l∞ - lt) = -kobst was applied to the data, where l∞ = final conductance, lt = conductance at time t, t = time and k = the rate constant. A graph of ln(l∞ - lt) vs. t should give a straight line plot with gradient –kobs if the reaction is first order. This graph is indeed linear with a gradient of -7.2x10-3 (Fig. 3). Therefore the slaking of lime in deionised water displays first order kinetics with a rate constant for the reaction of 7.2x10-3 s-1.

The rate constants for the dissolution of quicklime in a number of other solutions were found in the same way (Fig. 4). The results show that the rate of dissolution of quicklime is increased by sodium chloride, sodium nitrate, sodium carbonate and sodium acetate compared to deionised water only. The rate is within experimental error virtually unchanged by sodium tri-citrate and sodium hydrogen carbonate. The rate is decreased by sodium sulphate and soduim flouride. From solubility principles it may be expected that calcium compounds such as CaCl2, which are more soluble in water than Ca(OH)2, should accelerate the dissolution of lime and consequently accelerate the rate of the slaking reaction. Carbonates and sulphates, which form more insoluble compounds than CaO should retard the slaking process [5]. This is in agreement with most of the results obtained, with chloride ions present in the slaking water having the greatest effect on increasing the rate. However there is one anomalous result in the case of Na2CO3. One would expect that the rate constant for the dissolution of quicklime in 0.1 M Na2CO3 would be less than that for deionised water. However, for all three masses of quicklime added, there was a slight increase in the rate. This is surprising as previous work has found that carbonates in the slaking water do in fact decrease the reactivity of lime and to a greater extent than sulphates [5,11]. The decrease in rate in the presence of sulphate ions can be explained by assuming that insoluble layers of CaSO4 partially or completely coat the lime particles and prevent them from further dissolution or slow down the dissolution process. An observation when carrying out the investigation was that sodium sulphate was difficult to dissolve in water and took considerably more effort to dissolve than any other of the reagents used. The 1.0599 g of Na2CO3 added to 100 ml of deionised water to make up the 0.1 M solution dissolved relatively easily. The result emphasised that there is clearly a difference in the rate of dissolution compared to the total amount of CaO dissolved, as is measured by the available lime test.

An observation from the graphs plotted is that the initial rate (up to approx. 105 seconds) seems to be increased quite significantly when different amounts of lime are used, but then the rate decreases, consequently there is not an increase in the rate of the whole overall reaction. For example, there is no difference in the rate constants for the dissolution in deionised water with 0.2019 g and 0.5015 g of quicklime.

From the results in Table 1 it can be concluded that the rate of dissolution is enhanced in the presence of chloride ions up to 0.1 M sodium chloride. However, in the higher concentrations of 0.5 M and 1.0 M NaCl, the overall rate is decreased, compared to deionised water. The decrease in rate is more significant in 1.0 M than in 0.5 M solution.

Increasing the stirring rate does indeed increase the rate of dissolution (Table 2). This is in agreement with the observations of Ritchie and Xu [8,11], who concluded that the diffusion of the dissolved lime is the rate limiting step in the process. The reason for the increase in rate with increasing stirring rate is that stirring brings fresh particles of the quicklime in contact with the slaking solution hence increasing the rate of solubility.

The rate of the slaking reaction increases with increasing temperature. Carrying out the reaction at 32 °C, 40 °C, 47 °C and 55 °C all increase the rate. This can be explained from thermodynamic principles. According to Le Chatallier’s principle, the effect of increasing the temperature on an exothermic reaction is to shift the equilibrium to the reactants in order to counteract the heat given off by the reaction. When increasing the temperature in this investigation, the reaction seems not to obey this prediction. This is possibly because of the fast heat dissipation in the large volume of solution compared to a very small mass of lime being dissolved. An Arrhenius plot shown in Figure 5 indicated that the activation energy of the reaction in NaCl is

22.80 kJmol-1.

4 Conclusions

The following conclusions can be drawn from this investigation:

­­­The rate of the dissolution of quicklime is increased in the presence of Cl-, NO3-, and CO32-. Apart from in the case of CO32-, this can be attributed to the fact that the presence of these anions means that the calcium compounds formed are more soluble than calcium hydroxide and therefore facilitate dissolution.

Addition of NaCl to the slaking water seems to have the greatest enhancement on rate. The order of anion addition enhancement is Cl- > NO3- > CO32-.

­­The rate of dissolution of quicklime remains unchan­ged in the presence of CH3COO- and C2H5O73-.

The rate of the dissolution of quicklime is decreased in the presence of SO42- and F-, with the SO42- anion having the greatest effect. This finding can be attributed to the fact that the presence of these anions means that the calcium compounds formed are less soluble than calcium hydroxide and therefore slow down dissolution.

Addition of more quicklime to the reaction increases the initial rate of reaction.

The rate of the dissolution of quicklime is enhanced in solutions containing up to 0.1 M NaCl, but is significantly retarded in solutions containing 0.5 M and 1.0 M NaCl.

­­The rate of dissolution of lime generally increases with increased stirring rate.

­­The rate of dissolution increases with increasing temperature.

The activation energy in 0.1 M NaCl was calculated as 22.80 kJmol-1 from the Arrhenius plot.