Dissolution rates of limestones of different sources

Study on the process of mass transfer and deterioration of limestone under dynamic dissolution of CO2 solution

The long-term erosion of rock by solution can induce a series of karst problems. Therefore, this study focused on limestone and conducted dynamic dissolution experiments under deionized water and CO2 solution conditions to study the deterioration mechanism of limestone under nonequilibrium conditions. The results showed that the degree of degradation of the mechanical properties of the samples in a CO2 solution was obviously greater. In a deionized water environment, the degradation of the mechanical properties of the sample is mainly controlled by the physical softening action of the solution. In the CO2 solution environment, the degradation process can be divided into two stages. In the early stage of the experiment (10 days to 20 days), the degradation of mechanical properties of the sample is also controlled by the physical softening action of the solution. With increasing soaking time, the main rock-forming minerals of limestone gradually react with the CO2 solution, the degradation of the sample is controlled mainly by the chemical corrosion of the CO2 solution, and its degradation rate is much greater than that of physical softening. The results can be used as a reference for assessing the long-term stability of underground engineering in limestone karst development areas.

Development and Application of Carbonate Dissolution Test Equipment under Thermal-Hydraulic-Chemical Coupling Condition

The latest continuous flow micro reaction technology was adopted to independently develop carbonate rock dissolution test equipment. Carbonate rock dissolution tests were conducted under different temperatures, flow rates, and dynamic water pressure conditions to study the dissolution process of carbonate rocks under the coupling of heat-water-chemistry. The dissolution effect and development law of carbonate rocks were explored by quantitatively studying carbonate rock dissolution rate and chemical composition of karst water. The results showed that the self-designed dissolution test equipment has obvious advantages. After dissolution, carbonate rock specimens were damaged to varying degrees. The dissolution rate was proportional to water velocity and hydrodynamic pressure, with the velocity effect being greater than the hydrodynamic pressure effect. The pH value, conductivity, and Ca²⁺ ion content of the reaction solution gradually increased after dissolution. The development and application of the equipment have proved that, at low dynamic water pressures (2 MPa), the water flow velocity effect on the dissolution velocity was 1.5 times that when the dynamic water pressure was high (6 MPa); at a low water flow velocity of 15 mL/min, the dynamic water pressure effect on the dissolution velocity was three times that when the water flow velocity was high (75 mL/min). The development process is gradually becoming strong and stable. Its research has important theoretical significance and engineering application value to provide technical means and guarantee for the early identification, karst development, and safety evaluation of karst geological disasters.

Reactivity of calcium carbonate prepared from flue gas desulfurization gypsum

Reactivity of various calcium carbonate samples for flue gas desulfurization was tested. Two groups of CaCO3 samples were considered; natural limestone containing calcite phase dominantly and samples prepared by the conversion of gypsum with ammonium and carbon dioxide (precipitated CaCO3) containing different amounts of calcite, aragonite and vaterite.

Reactivity of precipitated calcium carbonate depends primarily on the particle size, similarly as in case of industrial samples. The initial reaction rate was comparable with the industrial limestones for samples with the average particle size lower than 15 μm. However, the conversion of laboratory samples was significantly higher after 5 min of the reaction.

Phase composition of the precipitated calcium carbonate has a minor but noticeable impact on the reactivity. The presence of vaterite slightly increased the reactivity, which is in accordance with its lower compact structure in comparison with calcite and aragonite. Unexpected effect of the increased content of aragonite, which is the most compact phase in comparison with calcite and vaterite, was observed. If calcium carbonate contains up to approximately 30 % of aragonite the reactivity increases, which can be explained by the SEM pictures showing agglomerate composition with relatively high specific surface. At higher contents of aragonite, the reactivity decreases. All the obtained results proved the suitability of precipitated CaCO3 prepared from flue gas desulfurization gypsum to be recycled in the flue gas desulfurization process.

Effect of ultrasound on the dissolution of magnesium hydroxide: pH-stat and nanoscale observation

It has been previously confirmed that the dissolution of magnesium hydroxide was a crucial procedure in its application process. Note that the ultrasound is an effective method for enhancing solid dissolution. In this study, the enhancement of ultrasound on the dissolution of magnesium hydroxide was investigated by the pH-stat method. The titrating results indicated that the promotion effect of ultrasound was only be observed at some certain cases, such as lower pH value. Meanwhile, the activation energy data obtained based on Shrinking Core Model for sonication case ranged from 6.56 to 49.13 kJ/mol, implying that magnesium hydroxide dissolution proceeds at sonication case cannot be explained well by this model. Nanoscale observation was conducted to identify the crystal surface variations during the dissolution process by using SEM and AFM. The analysis results indicated that the dissolution behaviors of magnesium hydroxide in the absence and presence of ultrasound were quite different. In the silent case, the dissolution followed the famous Stepwave model. As for the sonication case, the larger particle was broken into flake by the ultrasound firstly. Then, the heterogeneous global dissolution accompanied by the Stepwave model drove the dissolution process. The analysis results of nanoscale observation provided a reasonable explain for the kinetics research results on micro-level.

Influence of bone meal degelatinisation and calcium source and particle size on broiler performance, bone characteristics and digestive and plasma alkaline phosphatase activity

1. The current experiment was performed to elucidate the effects of degelatinised bone meal (DBM) in combination with different particle sizes of limestone or oyster shell on broiler performance, bone characteristics and digestive and plasma alkaline phosphatase (ALP) activity. 2. Treatments were applied as a 3 × 3 factorial arrangement with three sources of P (DCP, bone meal and DBM) and three particle sizes (50, 100 and 200 µm) of limestone. Chickens were given either DCP or DBM with oyster shell (523 µm), resulting in a total of 11 treatments with 5 replicates of 8 chicks. 3. Performance criteria were measured weekly. Tibia strength, ash, calcium (Ca) and phosphorus (P) content and plasma P and Ca concentration along with plasma and intestinal alkaline phosphatase (ALP) activity and P digestibility were measured on d 14 and 28. 4. Body weight and FCR were improved in chicks which were fed DBM or oyster shell in comparison to the DCP and limestone respectively (P ≤ 0.05). Performance was influenced (P ≤ 0.05) by particle size; with coarser particles BW and feed intake were increased (P ≤ 0.05). Tibia shear force and P content were reduced (P ≤ 0.001), whereas tibia shear energy, length, ash and Ca content were increased by substitution of DCP with DBM or bone meal (P ≤ 0.001; P ≤ 0.05). A significant difference was observed in the tibia length between the chicks fed oyster shell or limestone with different particles (P ≤ 0.05). Plasma P concentration was reduced in chicks were fed with DBM, bone meal and lower limestone particle size. Intestinal ALP activity was increased (P ≤ 0.001) in chicks which were fed DBM, bone meal, oyster shell or coarse particles of limestone. The P digestibility in chicks fed bone meal was lower than that of those fed DBM or DCP (P ≤ 0.01). Overall, gelatin removal from bone meal improved broiler bone characteristics through the P digestibility and intestinal ALP activity enhancement.

Effects of limestone particle size and dietary Ca concentration on apparent P and Ca digestibility in the presence or absence of phytase

The present study evaluated the effects of limestone particle size and Ca concentration on apparent ileal digestibility (AID) of P and Ca in the presence or absence of a 6-phytase derived from Buttiauxella sp., expressed in Trichoderma. Treatment diets were corn-soybean meal (SBM) based with no added inorganic Ca or P. The design consisted of a factorial arrangement of 2 particle sizes of limestone from the same source (mean particle size: pulverized, PUV < 75 μm; particulate, PAR = 402 μm); 3 Ca (0.6, 0.8, and 1.0%) and 2 phytase levels (0 and 1,000 (FTU)/kg) plus the corn-SBM basal diet with no added Ca (0.14% Ca), resulting a total of 13 treatments (Trts, n = 9, 3 birds/n). Starting at 27 d of age, broilers were fed the mash diets for 32 h, then sampled for gizzard pH and distal ileal digestibility. Gizzard pH was 1.94 in birds fed diet without added Ca and was similar to those fed diet with PAR limestone regardless of phytase. Gizzard pH was increased in birds fed PUV limestone diets irrespectively of phytase inclusion compared to birds fed 0.14% Ca (P < 0.05), except in birds fed 0.60% Ca diet. In the absence of phytase, AID P was reduced as a result of increases in limestone inclusion (P < 0.05), with a greater effect seen in PUV as compared to PAR limestone. The AID Ca in birds fed PUV limestone diets was lower than that of those fed PAR limestone diets (22.1% vs. 28.2%; P < 0.05). In the presence of phytase, the AID P was not affected by increasing Ca concentration when PAR limestone was used as the Ca source (average = 64.3%), whereas AID P decreased from 66.9% (0.6% Ca) to 51.0% (1.0% Ca) when PUV limestone was used as the Ca source (P < 0.05). In summary, impact of Ca concentration on AID Ca and P was dependent on limestone particle size and phytase inclusion.

The dissolution kinetics of industrial brine sludge wastes from a chlor-alkali industry as a sorbent for wet flue gas desulfurization (FGD)

The disposal of industrial brine sludge waste (IBSW) in chlor-alkali plants can be avoided by utilization of IBSW as a sorbent in wet flue gas desulfurization (FGD). The shrinking core model was used to determine the dissolution kinetics of IBSW, which is a vital step in wet FGD. The effects of solid-to-liquid ratio (m/v), temperature, pH, particle size, and stirring speed on the conversion and dissolution rate constant are determined. The conversion and dissolution rate constant decreases as the pH, particle size, and solid-to-liquid ratio are increased and increases as the temperature, concentration of acid, and stirring speed are increased. The sorbents before and after dissolution were characterized using x-ray fluorescence (XRF), x-ray diffraction (XRD), and scanning electron microscopy (SEM). An activation energy of 7.195 kJ/mol was obtained and the product layer diffusion model was found to be the rate-controlling step.

IMPLICATIONS

The use of industrial brine sludge waste as an alternative sorbent in wet flue gas desulfurization can reduce the amounts of industrial wastes disposed of in landfills. This study has proved that the sorbent can contain up to 91% calcium carbonate and trace amounts of sulfate, magnesium, and so on. This can be used as new sorbent to reduce the amount of sulfur dioxide in the atmosphere and the by-product gypsum can be used in construction, as a plaster ingredient, as a fertilizer, and for soil conditioning. Therefore, the sorbent has both economic and environmental benefits.

Wet flue gas desulfurization using alkaline agents

The continued great dependency on fossil fuels entails increasing SO

Dissolution kinetics of sorbents and effect of additives in wet flue gas desulfurization

Flue gas desulfurization (FGD) technology has been adopted by a number of power stations for the removal of sulfur dioxide (SO

Dissolution kinetics of magnesium hydroxide for CO2 separation from coal-fired power plants

The dissolution of magnesium hydroxide in water for the release of magnesium and hydroxyl ions into the solution to maintain suitable alkalinity is a crucial step in the Mg(OH)2-based CO2 absorption process. In this study, the rate of dissolution of Mg(OH)2 was investigated under different operating conditions using a pH stat apparatus. The dissolution process was modeled using a shrinking core model and the overall Mg(OH)2 dissolution process was found to be controlled by the surface chemical reaction of Mg(OH)2 with H(+) ions. Under the chemical reaction control regime, the dissolution of Mg(OH)2 in alkaline conditions was found not to follow a first-order reaction, and the fractional order of reaction was estimated to lie between 0.20 and 0.31. This suggests that the dissolution reaction is a non-elementary reaction, consisting of a sequence of elementary reactions, via most likely forming a surface magnesium complex. The true activation energy value of 76 ± 11 kJ/gmol was found to be almost twice as much as the observed activation energy value of 42 ± 6 kJ/gmol determined at pH 8.6, and was comparable with the previously reported values. The particle sizes predicted from the intrinsic kinetics determined from the model were in good agreement with the experimentally measured particle sizes during the dissolution process.

Influence of limestone characteristics on mercury re-emission in WFGD systems

This work evaluates the influence of the effect of the properties of limestones on their reactivity and the re-emission of mercury under typical wet scrubber conditions. The influence of the composition, particle size, and porosity of limestones on their reactivity and the effect of sorbent concentration, pH, redox potential, and the sulphite and iron content of the slurry on Hg(0) re-emission was assessed. A small particle size, a high porosity and a low magnesium content increased the high reactivity of the limestones. Moreover, it was found that the higher the reactivity of the sample the greater the amount of mercury captured in the scrubber. Although sulphite ions did not cause the re-emission of mercury from the suspensions of the gypsums, the limestones enriched in iron increased Hg(0) re-emission under low oxygen conditions. It was observed that the low pH values of the gypsum suspensions favored the cocapture of mercury because Fe(2+) formation was avoided. The partitioning of the mercury in the byproducts of the scrubber depended on the impurities of the limestones rather than on their particle size. No leaching of mercury from the gypsum samples occurred suggesting that mercury was either tightly bound to the impurities of the limestone or was transformed into insoluble mercury species.

Characterization of calcium carbonates used in wet flue gas desulphurization processes

This paper presents an experimental characterization of two sources of calcium carbonate, limestone and calcium carbonate precipitate (CCP) used in wet flue gas desulphurization processes. Characterization of the two carbonate sources was carried out by chemical analysis, IR spectra, thermal behavior, particle size distribution for CCP, BET surface area and absorption capacity of SO2 in calcium carbonate suspensions. The absorption temperature, suspension concentration and carbonate grain size were found to be the most influential parameters in the absorption capacity measurements.

Effect of particle size in a limestone-hydrochloric acid reaction system

Experimental characterization of the wet flue gas desulfurization process is carried out using a model limestone-hydrochloric acid reaction system, with in-situ measurement of the dissolution rate and particle size distribution. The limestone source, initial particle size distribution, working temperature and pH value are varied in large ranges. The dissolution rate is found to be higher when the average particle size is smaller, the temperature is higher, or the pH is lower. An empirical equation is established to correlate the dissolution rate with the particle size and working conditions, which agrees well with measurements. The results may be useful for providing insights to improve the efficiency of the wet flue gas desulfurization process, as well as other solid particle-liquid solution reactions.

Effect of ammonium compounds on dissolution rate of South African calcium-based material

The rate at which limestone dissolves is very important in wet flue gas desulphurisation process (FGD). High dissolution rates provide better alkalinity that is important for sulphur dioxide (SO(2)) absorption. The dissolution characteristics have been studied by using a pH-Stat method at 60 degrees C, at pH value of 5, stirrer speed of 100 rpm and particle size of 44 microm. This paper examines the use of ammonium compound as the possible additives that will enhance the dissolution rate of limestone. The dissolution rates were measured according to the shrinking core model with surface control, i.e. (1-(1-X)(1/3))=k(r)t. It was found that the dissolution rate increases in the presence of ammonium compounds. Upon addition of 0.5 g of ammonium nitrate, the dissolution rate constants increased by 170%. As the pH is increased the dissolution rate decreases. The dissolution reaction follows a shrinking core model with the chemical reaction control as the rate-controlling step.

Dissolution rate of limestone for wet flue gas desulfurization in the presence of sulfite

Limestone dissolution rate was measured by a pH-stat method with CO(2) sparging and dissolved sulfite. The dissolution rate of limestone under these conditions was found to be controlled by mass transfer and surface kinetics. As can be seen from the results, in the presence of sulfite, limestone dissolution rate increases with increasing stirring speed, reaction temperature and CO(2) partial pressure. The crystallinity of limestone has a great impact on the dissolution rate: The lower the value of the crystallinity of limestone is, the higher the dissolution rate is. The presence of sulfite promotes the dissolution rate when pH value is below 5.5 but inhibits it when pH value is above 5.5.

Dissolution rate of South African calcium-based materials at constant pH

One of the most important steps in the wet limestone-gypsum flue gas desulphurization (WFGD) process is limestone dissolution, which provides the dissolved alkalinity necessary for SO(2) absorption. Accurately evaluating the limestone dissolution rate is important in the design and efficient operation of WFGD plants. In the present work, the dissolution of limestone from different sources in South Africa has been studied in a pH-Stat apparatus under conditions similar to those encountered in wet FGD processes. The influence of various parameters such as the reaction temperature (30<or=T<or=70 degrees C), sorbent particle size (25<or=dp<or=63 microm), solution acidity (4<or=pH<or=6), and chemical composition were studied in order to determine the kinetics of the sorbent dissolution. The results obtained indicate that the dissolution rate increased with a decrease in particle size, decrease in pH and an increase in temperature. Kinetic analysis of the results indicates that the dissolution of limestone is according to the shrinking core model with surface control, i.e. 1-(1-3)(1/3)=kt.