LIME SLAKING I Slakers and movers by Roman Stoiber, STT Enviro Corp Systems & Solutions, Canada Poorly-slaked lime can have a major negative impact on lime consumption and the operating cost at an end-user’s site. The amount of lime used in the process is determined by a combination of slaking parameters. The optimisation of the slaking process will help drive operating costs down and also reduce CO2 emissions. W hen the slaking of lime (ie, the conversion to calcium hydroxide – Ca(OH)2) takes place on site it is often carried out by operators who don’t have specialised slaking training. Optimising the slaking step can result in significant savings in lime consumption. Studies have shown that anywhere between 10-40 per cent can be saved in chemical consumption by ensuring lime slaking is carried out under optimum slaking conditions. This is true whether the process application is water treatment, exhaust gas scrubbing, mining beneficiation or downstream chemical processing. Quicklime is provided by calcining limestone. Limestone properties can vary greatly depending on the precise location from which it originates and its mineralisation. Thus, quicklime properties produced from limestone of different qualities can vary greatly. Most quicklime suppliers understand the importance of tightly controlling the calcining parameters in the conversion of their particular limestone (CaCO3) to quicklime (CaO) Lime production, conversion and what slaking efficiency really means to the end user to produce the best-quality quicklime possible for the end-user. In many cases quicklime received by an end-user is slaked prior to application in the process. Dual silo make down system for hydrated lime and magnesium oxide INTERNATIONAL CEMENT REVIEW JULY 2015 Importance of optimum conditions The importance of optimum conditions during slaking taking place at the point of use are not always apparent to most quicklime users. The ‘quality’ of calcium hydroxide produced on site from quicklime is often measured by the following key parameters: • 30 seconds, three minutes, and final temperature rise during lime slaking process • total active slaking time (time taken to complete the slaking reaction) • total and available lime (in per cent terms). Key slaking parameters The quality of the hydrate produced is determined by the following slaking parameters: • final slaking temperature achieved • water-to-quicklime ratio used (by weight) • grit production and composition, generally reports to a waste stream • hydrated lime particle size (as it relates to specific surface area) • slaking water composition • neutralisation capacity of calcium hydroxide produced • neutralisation kinetics (how fast does it react?). Final slaking temperature achieved In general, it is best to target a slaking temperature as high as practical without allowing the lime slurry to boil over. Different types of slaking equipment can have slightly different maximum ©Copyright Tradeship Publications Ltd 2015 LIME SLAKING Lime slaking apparatus as per ASTM C-110-09a Section 11 Acid neutralisation capacity and kinetics apparatus losses. Some sites use a vertical ball mill as a slaker to alleviate some of this problem. Slaking water composition If high-quality quicklime was slaked with cold winter temperature ground- or lake water as is often the case (thus not allowing the final slaking temperature to reach the target value), the amount of lime residue or grit production multiplies. The amount of quicklime wasted therefore multiplies. If a site is using recycled process water containing impurities such as sulphates/ sulphites, carbonates and phosphates/ phosphites, research has shown that the slaking efficiency can suffer dramatically. Hydrated lime particle size The overall goal of efficient slaking is to make the smallest-possible calcium hydroxide particles which yield the highest specific surface area (SSA) and will be the most chemically reactive. SSAs of greater than 200,000cm2/g are considered to Sulphates: Sulphate/sulphite anions can coat quicklime particles when they are mixed together and dramatically slow the slaking reaction. This coating blocks the pores of the quicklime particle. It effectively prevents water from getting inside the particles and inhibits the recommended operating temperatures. For detention slakers a target temperature of 85˚C ±3˚C (185˚F ±5˚F) is typically recommended and for paste slakers a target temperature of 88˚C ±3˚C (190˚F ±5˚F) is often recommended. Water-to-quicklime ratio used In the case of detention slakers, it is normal to operate at a water:lime ratio of between 3.5-6.0:1 by weight. This ratio keeps the lime slurry in a manageable viscosity range as well as allows for higher final slaking temperatures. In the case of paste slakers it is normal to operate in the range of 2.0-2.5:1 by weight. Grit production and composition Grit is considered a byproduct or waste stream of the slaking process. It is often thought of as being an inert waste material that comes delivered with any quicklime, but this is only a part of the story. A very high-quality quicklime might have 96 per cent total CaO and only 1-2 per cent inert grit which can be comprised of silica, alumina or ferric oxide. If this quicklime was slaked with warm, potable water under ideal conditions such that the target slaking temperature is achieved, grit produced might be in the 2-5 per cent range. (The ASTM standards report lime residue (grit) as being over 30 mesh or 600µm in size). If the same quicklime was slaked with cold recycled or process water containing impurities research has shown that lime residue production can be as high as 42 per cent. If a grit screen or grit screw was used, this would often contribute to waste resulting in unnecessary quicklime be very high quality. This corresponds to an average d50 particle size of 4-6µm (surface area weighted mean diameter). Keep in mind that SSA is a combination of particle size and porosity of the material. Slaking at a lower target temperature or with cold initial water will shift the particle size curve towards larger particles. Larger particles will have a lower combined SSA and will be less reactive. More of this lessreactive calcium hydroxide will be needed to do the same job. It is not uncommon that an end user consumes 20 per cent more quicklime in the winter when their slaking water is cold than in the summer when the incoming water temperature is higher. If a site is using recycled process water containing impurities such as sulphates/sulphites, carbonates and phosphates/phosphites, research has shown that the slaking efficiency can suffer dramatically. Pebbled quicklime sampling – cone and quartering ©Copyright Tradeship Publications Ltd 2015 JULY 2015 INTERNATIONAL CEMENT REVIEW LIME SLAKING test the efficiency of what an end user is currently producing with their lime, their water, at their site and under their operating conditions and compare that result to what they could produce at their site (see Figure 1). Figure 1: neutralisation capacity of equivalent dry weights of calcium hydroxide reaction with quicklime which is critical to efficient slaking. Slaking efficiency will be progressively worsened by elevated sulphate levels. It is generally accepted that concentrations of <100ppm SO42- have a negligible effect on slaking while concentrations between 200-500ppm SO42- have a moderate negative effect on slaking. A severe negative impact on slaking lime is observed at concentrations >500ppm SO42-. It is accepted by some that ball mills used as slakers can typically handle up to 500ppm SO42- because the inherent grinding action wears away this inhibiting coating. Carbonates: Recycled plant or process water typically has elevated carbonate hardness which is reported as CaCO3 concentration in common water analyses. The level of CO32- has a direct impact on scale formation in the slaking equipment as well as in the pumps and slurry pipelines often associated with these systems. Concentrations higher than 1000ppm CO32- can cause catastrophic scaling issues. Various strategies are used to try to combat scale formation including lime slurry additives, running with very high lime slurry per cent solids and treating the water hardness or better yet switching the slaking water source. Saving lime reagents costs, although significant on an annual basis, are only part of the complete story. Those same savings of 7050tpa of quicklime also led to a reduction in CO2 emissions of 9150t, offering a benefit to the environment. The various standards that exist to test the efficiency of hydrated lime slurry, whether from ASTM, EPRI, AWWA or another organisation, compare the efficiencies of laboratory-produced samples only. Although these can be somewhat useful benchmarks, it is far more important to Neutralisation capacity Ultimately the majority of quicklime is made into hydrated lime slurry which is used to vary the pH of a process. INTERNATIONAL CEMENT REVIEW JULY 2015 Neutralisation kinetics End-users vary wildly in terms of how long of a retention time their process has to effect this pH change. It can vary from a low of just a few seconds to several hours in some cases. As a result, the neutralisation kinetics trend is very important but often overlooked. In Figure 2 a properly-slaked site sample (blue line) is fully reacted in 7-9min. Even adding an extra 17 per cent mass of poorly-slaked lime (red line) did not compensate for the reaction kinetics. In this case, the larger amount of poorly-slaked lime did not complete the neutralisation reaction until 22 minutes had elapsed. Slaking optimisation in practice In one example alone, where a single flue gas desulphurisation end-user had been using 44,000tpa of quicklime, minor process optimisation has saved 16 per cent or 7050tpa to remove the same amount of SO2. In financial terms, this offered cost savings of US$700,000 per year. Saving lime reagents costs, although significant on an annual basis, are only part of the complete story. Those same savings of 7050tpa of quicklime also led to a reduction in CO2 emissions of 9150t, offering a benefit to the environment. _______________________________I Figure 2: equivalent neutralisation capacity for two different milk of limes ©Copyright Tradeship Publications Ltd 2015
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