Reaction Of Lime Research Articles

Influence of fallow, wheat and subterranean clover on pH within an initially mixed surface soil in the field

To ascertain the cause of the decrease in pH with depth through the surface 15 cm of moderately acidic soils, pH was monitored in layers of an initially mixed surface soil (to a nominal depth of 10 cm) during two consecutive seasons under fallow, wheat, and subterranean-clover plots. Variation of pH-influencing processes within soil layers to 15 cm depth was measured during the first season. Initially, soil pH was relatively uniform within the surface 7.5 cm, although there was an average 0.53 unit decrease of pH from 0–2.5 cm to 10–15 cm depth. Under all plots, residual lime reaction, net organic anion association and oxidation, net manganese oxidation and reduction, and particularly net N mineralisation and subsequent nitrification, tended to decrease with depth through the surface 15 cm of soil. In wheat and subterranean-clover plots, the alkalinity added with the return of 3.9–4.7 t ha–1 of plant residue dry matter was predominantly released within the surface 2.5 cm of soil. The dominant pH-influencing processes were net N mineralisation and subsequent nitrification, and the return of alkaline plant residues. In the fallow plots, the surface 10 cm of soil tended to acidify due to nitrification. However in wheat and clover plots, alkalinity added to the surface 2.5 cm of soil from plant residues exceeded acidification resulting from nitrification at this depth. The magnitude of the pH gradient through 0–15 cm depth was therefore maintained under wheat, increased under clover, and decreased under fallow.

Strength behaviour of lime-treated soils in the presence of sulphate

Lime has been used extensively to improve the shear strength of fine-grained soils. It has been recently reported that the presence of sulphate causes abnormal volume changes in lime-stabilized soil. The paper presents the strength behaviour of lime-treated montmorillonitic natural black cotton soil in the presence of varying sulphate contents after curing for periods of up to 365 days. Alteration of soil–lime reactions in the presence of sulphate affects the strength development by cementation. Consequently, the stress–strain behaviour effective stress paths of soil cured with sulphate are similar to those of normally consolidated soil rather than cemented soils. The reduction in shear strength due to a reduction in effective cohesion intercept occurs for lime-treated soil cured with sulphate for long periods.Key words: clays, cohesion, fabric, friction, shear strength.

Behaviour of selected minerals in an improved ash fusion test: quartz, potassium feldspar, sodium feldspar, kaolinite, illite, calcite, dolomite, siderite, pyrite and apatite

A suite of minerals found in coals were analysed using an improved ash fusion test. These minerals were quartz, potassium feldspar, sodium feldspar, calcite, dolomite, siderite, pyrite, kaolinite, illite and apatite. Each specimen was also analysed by XRD as an “as-received” specimen, an 800°C product and a 1600°C product. These results allowed evaluation of the applicability of the improved ash fusion test for determination of chemical changes occurring in the ash fusion furnace. Phase changes were observed in the improved ash fusion test for the Na feldspar, siderite and pyrite specimens. K feldspar movement could, in part, be related to phase changes. The complex reactions occurring in the illite and kaolinite specimens limited the interpretation. The calcite and dolomite specimens movement was dominated by reactions of calcite, lime and portlandite. The quartz and apatite specimens were relatively inert. These results indicate that the improved ash fusion test can provide a link between the various standard ash fusion tests and analytical tools such as XRD to provide enhanced understanding of the processes occurring in the ash fusion furnace.

Kinetic Modeling of the Reaction of HCl and Solid Lime at Low Temperatures

Calcium hydroxide is a commonly used sorbent in the dry-scrubbing of hydrogen chloride from flue gases. In this study the kinetic parameters for the reaction between gaseous HCl and solid Ca(OH)2 have been obtained at low temperatures in a fixed-bed laboratory reactor. The influence of the operating temperature (323−400 K), HCl concentration (150−1000 ppm), and humidity (0−11% M) was studied. The experimental results show that in the first few seconds a very fast chemical reaction occurs, with a reaction rate constant per unit surface area of solid larger than 10-3 ms-1. This reaction was found to be first order relative to HCl concentration and its mechanism is apparently independent of the presence of moisture. However, the relative humidity of the gas has a major impact on the progress of the reaction: when no moisture is present the reaction stops after a short period of time (2−3 min), with very low maximum solid reactant conversions (<5%). For the experiments with humidified gas an almost complete conversion of Ca(OH)2 was obtained after about 40 min of reaction time. For this case the grain model with product layer diffusion limitations is in very good agreement with the experimental results. The diffusion coefficient in the product layer obtained through this model varied from 10-13 to 10-11 m2 s-1 and the activation energy for this parameter was estimated at ≈19 kJ mol-1 for the range of temperatures studied. A simple linear relationship describes well the effect of relative humidity of the gas on the diffusion coefficient in the solid product layer. In the presence of humidity, the very high conversions of the solid reactant show a good potential applicability to continuous dry-scrubbing of HCl at low temperatures.

Reactions of lime with carbonate-containing solutions

In the Bayer process for refining alumina, lime (either quicklime or slaked lime) is added to the process liquor to precipitate calcium carbonate and restore alkalinity to the solution. The work described in this paper was undertaken to elucidate the mechanism of this precipitation reaction about which surprisingly little is known. In the first part of the study, the composition of calcium oxide disks after reacting with sodium carbonate solutions was examined by XRD and TGA. TGA results show that the amount of calcium carbonate formed on the disks increases with sodium carbonate concentration but the amount of calcium hydroxide reaches a minimum at a solution concentration of about 0.05 M carbonate and then increases at higher carbonate concentrations. The kinetics of the formation of calcium carbonate were also followed using HPLC to monitor the change in bulk carbonate concentration due to the reaction of powdered lime in sodium carbonate solutions. The effect of the agitation speed and temperature on the reaction rate indicates that the formation of calcium carbonate is a diffusion-controlled process. To account for these findings it is suggested that at concentrations above 0.05 M carbonate a more porous heterogeneous CaCO 3–Ca(OH) 2 layer appears owing to direct attack on the surface by carbonate before a layer of calcium hydroxide can form. Scanning electron micrographs of disks after reaction in carbonate solutions support this suggestion. The second part of this study deals with the dissolution of lime in the presence of sodium carbonate. The concentration of Ca 2+ from dissolution of calcium hydroxide disks was determined by AAS. It was found that the dissolution process is slowed by the formation of calcium carbonate, and is diffusion controlled. The reaction of powdered calcium oxide in the sodium carbonate solution was followed by calorimetry. It was found that the relationship between the extent of dissolution and the carbonate concentration is similar to that obtained by TGA for the formation of calcium carbonate, suggesting the same mechanism.

Formation of lime-containing desilication product (DSP) in the Bayer process: factors influencing the laboratory modelling of DSP formation

This paper examines the effect of certain reaction variables on the mineralogy of desilication product (DSP) formed during the laboratory modelling of the reaction of kaolin and lime in synthetic Bayer liquors. In particular, the effect of the desilication (100°C) to digestion (250°C) heating rate, kaolin source and liquor composition are discussed. At a low desilication to digestion heating rate (1.6°C/min, sample cooled once the temperature reaches 250°C) the DSP mineralogy is generally unaffected by the kaolin source used or by the form in which the reactants are added (e.g.: adding the CaO, extra gibbsite and kaolin to a sodium aluminate liquor at 90°C versus adding the CaO, gibbsite and kaolin to an aluminate-free liquor at room temperature). Hydrogarnets (HG, Ca 3Al 2(SiO 4) n(OH) (12-4n) with a silicon incorporation “n” ≈ 0.65 and a calcium-containing cancrinite (Ca-CAN) form under these conditions and the reaction pathway appears to reflect the thermodynamics of the system. By contrast, the mineralogy of the DSP formed at the higher heating rate (30°C/min, sample cooled once the temperature reaches 250°C) depends on the specific reaction conditions employed and more care must be given to the choice of initial modelling conditions. The reactant addition procedure still influences the DSP mineralogy when the time at 250°C is extended from 0 to 10 min. The implications of these results for the modelling of DSP formation using synthetic reaction mixtures are discussed. A possible mechanism for the formation of Ca-CAN is also discussed.

Reaction of Ca(OH)2 with SO2 at Low Temperature

Lime is widely utilized as a sorbent in dry and semidry sulfur removal processes. In this study limestone was calcined at high temperature and lime was produced by water vapor steam treatment at low temperature. Experiments were performed at conditions that also cover those usually present in coal-fired power plants. The investigation of the reaction of lime with synthetic flue gas showed that the relative humidity of the gas has a major impact on the reactivity. Also the presence of oxygen and/or carbon dioxide does not show a significant influence on the sulfur dioxide removal reaction though some intermediate formation of calcium carbonate cannot be excluded. Mainly calcium sulfite hemihydrate was found to be the reaction product, but some calcium sulfate hemihydrate was also formed when oxygen was present. Four regions with different prevailing reaction mechanisms can be defined: In the beginning the formation of a monolayer is mainly determined by the SO2 concentration and to some smaller extent als...

Reactions of lime with aluminate-containing solutions

In the Bayer process for refining alumina, lime (either quicklime or slaked lime) is added to the process liquor to precipitate calcium carbonate and restore alkalinity to the solution. Lime also precipitates calcium aluminate in an unwanted side reaction which consumes both lime and aluminate. The work described in this paper was undertaken to elucidate the mechanism of this precipitation reaction about which little is known. The kinetics of the reaction between quicklime powder and aluminate in solution were studied as a function of stirring, reaction temperature, lime charge and sodium hydroxide concentration. It was concluded that the reaction was diffusion controlled, at least in the early stages. The second part of this study was directed towards assessing the effect of the calcium aluminate deposit on the lime slaking reaction. The kinetics of this reaction were investigated as a function of aluminate concentration, stirring, temperature and lime charge. The kinetic measurements were complemented by X-ray diffraction and scanning electron microscopy. It was found that the calcium aluminate deposit progressively shuts down the lime slaking reaction. If the aluminate concentration in solution is high enough, the lime particles are encapsulated by calcium aluminate and dissolution ceases.

Hydrothermal transformation of tobermorite gel to 10 Å tobermorite

The tobermorite gel, formed by hydrothermal reaction of silica and lime, is transformed either into “Z” phase if the quantity of lime is less than 37 % or into 10 Å tobermorite if the quantity of lime is between 37 % and 42 %. The sheets of tobermorite gel are progressively ordered in a direction perpendicular to their plane in a turbostratic way to form “Z” phase. The order is correct in the plane but there is rotations with different angles between planes (large variations of a and b parameters occur).On the contrary, the sheets of tobermorite gel are ordered in a perfect way during the formation of the 10 Å tobermorite and without noticeable modification of parameters a and b. The 10 Å tobermorite is stable in the presence of gyrolite, whereas “Z” phase is metastable.

Alkali activation of reactive silicas in cements: in situ 29Si MAS NMR studies of the kinetics of silicate polymerization

In the highly alkaline environment of the cement paste of a concrete, a source of silica can potentially react in two ways. In the pozzolanic reaction, it can combine with free lime to generate additional calcium silicate hydrate binding phase. Alternatively, reaction with alkali to form a gel can occur; this gel may swell and degrade the concrete. 29Si magic angle spinning (MAS) and cross-polarization (CP) MAS nuclear magnetic resonance (NMR) studies have been performed to determine the silicate connectivity in some model cement systems; 29Si enrichment was utilized to enable a series of spectra to be acquired in situ from a single sample. The hydrate from pozzolanic reaction of lime with silica was similar to the hydrate formed around silica in blended pozzolanic cements, with a relatively high crystallinity and long silicate chains. In the absence of lime, silica reacted with an alkaline solution to produce a gel having a high degree of cross linking, and a range of silicate mobilities. Tricalcium silicate hydration was found to be accelerated significantly by high levels of alkali (KOH) in solution; the hydrate formed had shorter silicate chains and was more crystalline than that produced by reaction in pure water. Hydration in alkali solution of a model blended cement, comprising a mixture of tricalcium silicate and silica, gave rise to two products, a long chain calcium silicate hydrate (C-S-H) and an alkali silicate of low rigidity. The alkali silicate phase gradually polymerized; at later ages it underwent a phase change, although no crystalline phase appeared to be formed. Silicate exchange took place between the C-S-H and the alkali silicate phase at a slow rate.

Ab initio theoretical investigation on the reactivity as bases of mixed LiMe/KOMe complexes. A model for schlosser LICKOR superbase

Abstract The metal-hydrogen exchange reaction of LiMe·KOMe 1 : 1 complex with CH4 is first examined, and its reactivity is found to be quite similar to that displayed by LiMe alone. On this basis, other possible aggregations are studied. The 1 : 3 complex, in which Me− and Li+ are kept ca. 3.5 A apart by three K+ ions and three negative oxygen atoms, respectively, is particularly interesting. Its formation is computed to be easier than that of the 3 : 1 tetramer. On the basis of its structural and electronic features, this complex can be considered a good candidate as a reactive metalating species.

Pozzolanic properties of flash-calcined kaolinite: A comparative study with soak-calcined products

Flash-calcination enables the dehydroxylation of powdered kaolinite clay within several tenths of a second, when traditional soak-calcinations require minutes at least. The pozzolanic properties of the metakolin produced from two different kaolinites, using two different flash calciners, are shown to increase with the dehydroxylation rate, and rapidly to decrease at the occurence of recrystallisation for temperatures above 900 °C. Flash-calcined products also revealed structural properties different from soak-calcined products. Two different lime reactivity tests were thus necessary to asses the pozzolanic properties of products: the compressive strengths of minicylinders of metakaolin + lime + water after solidification, and the Chapelle test. Processing flash-calcination in a temperature range that was determined, with a sufficient residence time, lead to metakaolins with lime reactivities similar or better than reactivities of standard metakaolin obtained by soak-calcination.

Dez anos de sucessivas adubações com boro no algodoeiro

Instalou-se um ensaio de adubação boratada do algodoeiro, de longa duração, pela primeira vez, em 1983, em Guaíra (SP), visando avaliar os efeitos de sucessivas aplicações de boro. Aplicaram-se doses anuais de 0; 0,2; 0,4; 0,8; 1,6 e 3,2 kg/ha de B, como bórax, na mistura de adubos de semeadura, em esquema estatístico de quadrado latino. Utilizou-se uma gleba de latossolo roxo, distrófico, argiloso, anteriormente cultivado, corrigido no aspecto de acidez e adubado com NPK. As parcelas foram calcariadas no quarto, sexto e nono ano de estudo, enquanto, no sétimo ano, cultivou-se guandu, em rotação. O efeito de boro sobre a produção de algodão aumentou com o passar dos anos e com a realização das calagens; na fase inicial, obteve-se a maior produção, com a dose de 0,4 kg/ha de B e, nas etapas posteriores, com a de 0,8 kg/ha. A concentração de boro no limbo foliar mostrou-se muito sensível à aplicação do micronutriente, destacando-se as diferenças com as adubações sucessivas e após as aplicações de calcário. As doses de 1,6 e 3,2 kg/ha de B proporcionaram decréscimo de produtividade das plantas em relação à produção máxima mesmo nos primeiros anos. Após as calagens, os níveis de boro no limbo foliar, associados a essas doses, mostraram-se superiores a 50 mg/kg de B. Análise química, efetuada durante o nono ano de estudo, indicou acúmulo de boro na superfície do solo e uma lixiviação do micronutriente para camadas até 60 cm de profundidade, proporcionais às doses usadas.

The Influence of Particle Size on the Agronomic Effectiveness of Agricultural Liming Materials

Periodic liming to maintain soil pH (the measure of acidity or alkalinity) is essential to achieve and sustain high crop yields on non-calcareous soils. Liming materials must meet certain specifications regarding their ability to neutralize soil acidity if they are to be sold within the EC. At present, the effectiveness of liming materials is assessed by analysis of their chemical reactivity or Neutralizing Value (NV). However, EC legislation is likely soon to require that methods of analysis also take into account the physical properties of liming materials: particle size, mineralogy and hardness are all physical factors that can Influence reactivity. In particular, particle size can have a substantial effect on lime reactivity in the short term. The significance of particle size as a factor in the overall effectiveness with which agricultural liming materials neutralize soil acidity is reappraised below.

Expansive mechanisms in concrete

The purpose of this paper is to remind readers that past experience with cement and concrete has useful information to offer for clarification of current problems in technology and research. Early experience with expansive reactions of free lime, CaO, and magnesia, MgO, in cement and concrete are referred to and compared with later observations of non-expansive recrystallisation of secondary calcium hydroxide, Ca(OH) 2, and magnesium hydroxide, Mg(OH) 2, in field concrete. Mechanisms of expansive vis a vis non-expansive reactions in cement paste, mortars, and concrete in field concrete and laboratory test specimens are explained with reference to the described observations.

Nature of the immediate reaction of lime in treating soils for road construction : Verhasselt, A F In: Physico-Chemcial Aspects of Soil and Related Materials (Papers to a Symposium presented at St Louis, 29 June 1989)P7–17. Publ Philadelphia: ASTM, 1990 (ASTM Special Technical Publication N1095)

Nature of the immediate reaction of lime in treating soils for road construction : Verhasselt, A F In: Physico-Chemcial Aspects of Soil and Related Materials (Papers to a Symposium presented at St Louis, 29 June 1989)P7–17. Publ Philadelphia: ASTM, 1990 (ASTM Special Technical Publication N1095)

Effects of long‐chain alkyl peroxydicarbonates on the properties of PVC produced by suspension polymerization

AbstractCommercially available diethyl, dicetyl, and dimyristyl peroxydicarbonate initiators are investigated in a commercial PVC suspension polymerization receipe. Diethyl peroxydicarbonate, which is formed in situ by the reaction of hydrogen peroxide, lime, and ethylchloroformate, produced PVC with less “fish‐eyes”, whereas it has the disadvantage of being toxic and gives inconsistent polymerization times, therefore the reactor efficiency decreases. Dicetyl and dimyristyl peroxydicarbonates are solid, less toxic materials easy to handle, therefore, when these initiators are used, consistency between batches is obtained. However, PVC with coarse particle sizes containing considerable amounts of “fish‐eyes” is obtained, which eventually causes processing problems. It is found that particle coarsening occurs as a result of water soluble impurities of initiators, and “fish‐eyes” occur as a result of morphological changes. When dicetyl and dimyristyl type initiators are used, the number of non‐porous PVC particles increases. This structure heterogeneity causes formation of ungellified spots, so‐called “fish‐eyes”, during processing. It is found that the non‐porous particles are formed due to accumulation of an excessive amount of initiator in a single vinyl chloride monomer droplet.When the distribution of these solid initiators is homogenized by adding them in solution, the formation of non‐porous particles is inhibited. However, the coarsening effect still remains, which could be eliminated by extraction of initiator solutions with water just before they are fed to the reactor. Extraction also decreases the amount of crust formation.

Studies on the lime reactivity with thermally activated clay from Jammu region (India)

Clay from Purmandal area of Jammu (J&K State, India) was calcined at various temperatures. The calcined products were subjected to lime reactivity test. The results obtained were correlated with X-ray diffractograms and DTA curves of the raw and calcined samples respectively to explain the pozzolanic activity of the calcined clay.

Investigation of Pozzolanic Binders Containing Calcined Laterites

ABSTRACTWhen heated at 750°C for five hours, African laterites show good pozzolanic activity. The kaolin content of the materials affects their pozzolanicity as indicated by lime reactivity. Blended cements containing 20, 30, 40, and 50% calcined laterite admixture were examined.The rate of hydration was studied from compressive strength up to 90 days of curing and the hydrates formed as measured by DTA and XRD. The burnt laterite addition considerably reduced calcium hydroxide content and contributed to the formation of hydrated gehlenite (C2 ASH8) and gismondine (CASH).The best compressive streng hs were obtained with blended cements containing 30% calcined laterite. In some cases, there was no more portlandite in binders where the replacement level of cement by burnt laterite was 50% by mass.

The influence of gypsum content on microstructural development, strength and expansion of cured PFA-lime mixes

The results ol the current investigation show that the presence of small amounts ol gypsum has a profound effect on the reaction of lime with PFA and on the strength development and expansion of cured PFA-lime mixes. PFA-lime-gypsum-water mixes were compressed into cylinders of dry density 1·52 g/cm3 and bulk density 1·81 g/cm3, and were cured for up to 28 dars in ]00% relative humidity at 95°C. The linear expansion during curing and the compressive strength after curing were determined, and the cured specimens were analysed using X-ray powder diffraction, thermogravimetric analysis and scanning and transmission electron microscopy with energy dispersive X-ray analysis. Gypsum additions were found to delay strength development and expansion, hut produced significant increases in the long-term strength and expansion up to an optimum gypsum addition of 4 wt%. High gypsum levels also resulted in a severe retardation in the rate of lime consumption. The analytical results show that these changes are controlled hy the nature and behaviour of the layer of surface reaction products which are observed to form on the surface of the PFA particles. The products consist of a sulphate containing fibrous and foil like C-A-S-H gel, a sulphate containing C-A-S-H globular colloidal product and crystalline ettringite and hydrogarnet. It is the development of and the relative amounts of these different products at the different gypsum concentrations which determine the physical hehaviour of the cured material.