Quicklime quality

The limestone, fuel and refractory linings used for lime burning should be selected with care in the context of the kiln design and the required quicklime quality. For a new installation, the kiln supplier should be able to test and assess the suitability of the raw materials. For an existing lime works, the potential effects of proposed changes in the limestone, fuel or refractories should be assessed with care. 

The following sections outline the principles underlying the production of quicklime, and describe briefly the main types of kiln being operated and available from suppliers. They also consider how the choice of kiln, feedstone and fuel affects quicklime quality, as well as discussing a number of related subjects. A number of references for further reading is given for most sections. The closely related topics of processing and despatch of quicklime are addressed in chapter 17. 

A recent paper [27.11] indicates that at least one steelmaking company is considering the use of mean apparent density and particle size to characterise quicklime quality for the BOS process. Increasing solution rates may become increasingly important as blow times are reduced to maximise production rates. 

Luminescence. Limestone possesses only limited luminescent qualities, ranging from very faint or none with the impure types. However, quicklime is very luminescent at calcining temperatures, hence the term limelight. 

Chemical quality lime, 15 27 Chemical quality quicklimes, sales of, 25 60 Chemical rates, variation with temperature and pressure,... 

Groundnut oil, in soap making, 22 735 Ground quicklime, 75 28 production of, 75 54-56 quality control for, 75 70 uses for, 75 61-62... 

In addition to the different shaft kiln types, rotary kilns are often used for lime burning. This enables good quality quicklime to be obtained with cheap fuels. Smaller limestone particles (10 to 60 mm) are utilized. 

In dry slaking calcium hydroxide is obtained as a dry powder. Use is made of the fact that so much heat is produced by the reaction of calcium oxide with water that the amount of water bound is approximately equal to that which can be evaporated. The so-called reactivity of the quicklime influences the product quality. Highly reactive lime leads, upon slaking, to a granular material, which has to be ground again. 

Following Black, Anderson held that pure limestone was made up of calcareous earth united with fixed air. In his words, calcareous earth was a general term denoting all those substances that consist of the matter which lime may be made, in whatever state it may be found - whether alone - or mixed with other substances, that prevent it from being reduced to powder after calcination. 33 For reasons of simplicity, he often used lime synonymously with calcareous earth. 34 Lime, as he explained, existed in three states. First, mild calcareous earth was limestone in its pre-calcined state. Second, Caustic calcareous earth was exactly synonymous with quicklime, in its strict and philosophical acceptation. 35 Finally, Effete calcareous earth was the hardened cement that formed from the quicklime which meant that it was a post-calcination form of limestone. As Anderson explained it Lime is no sooner slaked, than it immediately begins to absorb its air, and return to its former mild state or, in other words, it becomes effete, in which state it possesses the same chemical qualities, in every respect, as limestone. 36... 

In-process quality control chemical analysis, as well as that conducted on magnesium oxide, can be performed using either an X-ray fluorescence spectrometer according to ASTM C1271-94a Standard Test Method for X-ray Spectrometric Analysis of Lime and Limestone or using ASTM C25-95b Standard Test Methods for the Chemical Analysis of Limestone, Quicklime, and Hydrated Lime. Details of chemical analysis are outside the scope of this text however, there are numerous texts on analytical chemistry that should serve as useful references (Jeffery et al., 1989 Dean, 1995). 

To Find the Percentage of Morphia in Opium. An excellent process for ascertaining the quality of opium is to boil an indision of 100 grains opium with 25 grains quicklime, made mto a milk with wa-... 

Quicklime is produced by the thermal dissociation of limestone. Its principal component is ealcium oxide. Its quality depends on many factors including physical properties, reaetivity to water and chemical composition. As the most readily available and cost-effective alkali, quicklime plays an essential part in a wide range of industrial processes. 

The requirements of the quicklime market generally dictate the quality of limestone required. Although a high CaCOs content of > 97 % is an advantage in some markets, it is by no means essential, as shown by the following two examples. 

The choice of lime kiln is of paramount importance to a lime producer. It must be suitable for burning the selected feedstone and for producing the required quality of quicklime. It must have sufficiently low capital and operating costs to produce quicklime at a competitive price. Its capacity must also be appropriate for the market requirements. 

The next step is to establish, as clearly as possible, the quality of the quicklime required, in terms of % CaCOa, sulfur, reactivity and other key parameters, as well as the lead grade in terms of particle size. The impact of the design of the quicklime processing plant on product quality should not be overlooked (see chapter 17). 

The demand for various grades and qualities of ground quicklime has grown rapidly ever since the 1950s. Particle size requirements vary from relatively coarse products used for soil stabilisation (e.g. 99 % less than 2 mm with 50 % less than 75 pm) to very finely divided products for specialist applications (e.g. more than 99 % less than 50 pm). 

While sampling and sample preparation may appear, at first sight, to be an unskilled and low technology operation, they are probably the most common causes of disagreement between laboratories, and are critical to the characterisation of product quality. Problems arise as a result of the failure to obtain representative samples. They also arise, particularly with quicklime, as a result of inadequate precautions when storing or handling the sample. It is, therefore, important that samples are taken, prepared and stored in accordance with specified procedures. 

Because of the above complexities, before deciding to invest in a hydrating plant, the hydration characteristics of the available grades of quicklime should be evaluated. The quality of the water supply should also be assessed, bearing in mind any seasonal variations. 

The reactivity influences both the rate of reaction and the particle size distribution. Good quality hydrated limes can be made from quicklimes with a wide range of reactivities, providing the plant is correctly designed. 

On a pragmatic basis, the acceptability of a proposed water supply may be assessed by using a representative sample in a standard reactivity test and comparing the result with that obtained using distilled water. If the difference is within the normal variability of the quicklime reactivity, the water supply should be acceptable. If otherwise, hydration tests using the water should be carried out to ensure that the required quality of hydrated lime can be produced consistently. 

Quicklime was traditionally hydrated on the small scale by sprinkling water on to a pile of lime until no more water was absorbed. The lime was left for some time to mature and was then passed over fine sieves to remove grit. However, such a product was of poor and variable quality and required a high labour input. 

The quality of building limes is specified in [22.6]. It requires minimum values for standard mortars produced from lime putties — namely penetration and air content — and lists water demand and water retention as parameters that customers may request. It also specifies a minimum volume yield of putty per kg of quicklime. Building limes must also be sound (i.e., pass the popping, pitting and expansion tests). A workability test, based on the spread of a standard mortar on a jolting table, was specified in the 1972 edition of BS 890 [22.11], but is no longer included. 

The size and relative importance of the various market segments for quicklime and slaked lime (including products based on calcined dolomite) vary widely from one country to another. They depend on many factors, including the degree of industrialisation in general and on the specific industries which have become established, the quality/availability of limestone, and traditional building methods. 

Originally only low reactivity quicklime was used and many producers specify that quality. In the UK, however, suitable quicklimes were not generally available and many of the processes were initially modified to use cement as the sole binder. Subsequently, part of the cement has been replaced, with advantage, by moderate and high reactivity high-calcium quicklimes. 

Milk of lime is produced either by slaking quicklime, or by dispersing hydrated lime in water. The particle size of the slaked lime is an important parameter in the PCC process. Where quicklime is slaked, its reactivity and the quality of the water should be controlled. Where hydrated lime is used, its particle size distribution, and the quality of water used to disperse it, should be controlled. Milk of lime should be screened to remove coarse particles before use. 

ASTM C911 [31.5] (formerly ASTM C 258) specifies the quality of quicklime required by the process. The particle size requirements are described in section 31.4.1. From the chemical viewpoint, a high calcium lime is required with a MgO content of less than 2 % as it volatilises and can produce troublesome deposits. The level of minor impurities should be limited (Si02 < 2 %, Fe203 + AI2O3 < 1 %) and phosphorous should be less than 0.02 %. The loss on ignition (combined CO2 and H2O) should not exceed 4 %. 

Because quicklime is relatively sensitive to abrasion and impact, it is an inherently dusty product. Run of kiln lime typically contains 5 to 10 % of material finer than 6 mm, a significant proportion of which is sufficiently fine (less than 75 pm) to become airborne at transfer points. Such material is generally of marketable quality and, after collection, can usually be blended back into the product. 

Portland cement, invented in 1892 by Joseph Aspdin of Leeds, was so called because it resembled expensive Portland stone (at least to the eye of the inventor). It is made from about 80 % limestone and about 20 % clay. It was widely adopted because it possessed superior qualities to the older quicklime-based material, including the especially important property of being able to harden in damp conditions. This latter property was especially valuable at a time when tunnel constmction was widespread, including amongst other projects, the London underground system. 

QUALITY - Weight fraction of the vapor in a vapor-liquid mixture. QUICKLIME - Unslaked lime (calcium oxide). 

One very important raw material for the production of PCC is a suitable deposit of natural CaCOs. Only rather few limestone deposits meet the stringent demands for the production of high quality PCC. Carefully selected limestone is calcined at 800-900 °C to obtain calcium oxide (CaO or quicklime), requiring energy and releasing CO2. The quality and uniformity in quality of the lime used has an immense influence on the quality of the final PCC product The addition of water (exothermic reaction) produces calcium hydroxide (Ca(OH)2 or slaked milk of... 

Purer quicklime is obtained from vertical kilns. In vertical kilns limestone is fed into the top of the kiln while fuel is introduced at the bottom. Either fuel oil or natural gas is used to cook the limestone. Fuel oil usually contains enough sulfur to generate SO2 and SO3 that are trapped in the quicklime, in much the way these gases are removed from stack gases in powerplants. Natural gas is the highest grade fuel and produces high quality lime. In my opinion it is the only lime that should be used as a raw material in chemical processes where it is to become a part of a consumer product. 

Effects of moisture contents on sinter quality were studied when basicity was 1.7, quicklime was 3% and return fine was 30%. Results are given in Figure 1. 

---