Phosphate removal and recovery through crystallization of hydroxyapatite using xonotlite as seed crystal

Xonotlite was synthesized and tested for phosphate removal and recovery from synthetic solution in a batch mode. The effects of pH, initial calcium concentration, bicarbonate concentration on phosphate removal through crystallization were examined. The morphology and X-ray diffraction （XRD） pattern of xonotlite before and after crystallization confirmed the formation of crystalline hydroxyapatite. The results indicated that the crystallization product had a very high P content （〉 10%）, which is comparable to phosphate rock at the dosage of 50-200 mg xonotlite per liter, with a maximum P content of 16.7%. The kinetics of phosphate removal followed the second-order reaction equation. The phosphate removal ability increased with increasing pH. The precipitation of calcium phosphate took place when pH was higher than 7.2, whereas the crystallization occurred at pH 6.0. A high calcium concentration could promote the removal of phosphate via crystallization, while a high bicarbonate concentration also enhanced phosphate removal, through that the pH was increased and thus induced the precipitation process. When xonotlite was used to remove phosphate from wastewater, the removal efficiency could reach 91.3% after 24 h reaction, with removal capacity 137 mg/g. The results indicated that xonotlite might be used as an effective crystal seed for the removal and recovery of phosphate from aqueous solution.

Mechanism of low thermal conductivity of xonotlite-silica aerogel nanoporous super insulation material

In an effort to incorporate the low thermal conductivity of the silica aerogel and the superior structure strength of the xonotlite,a composite material of these two was produced. It was synthesized under vacuum condition and dried by supercritical drying technique. The thermal conductivity of the new material,which is at 298K with the gas pressure ranging from 1.01×10^5 to 1×10^-2 Pa,was measured using the transient hot-strip method. The mechanism of the low thermal conductivity was studied. The results indicate that the low thermal conductivity mainly results from the significant decrease of gaseous thermal conductivity of the new material due to the restriction of the motion of gas molecules in its fine structures. The formation of the fine structures is because the new material takes the pore structure of the silica aerogel which consists of mainly nanometer-sized pores.

Preparation of Ultra-light Xonotlite Thermal Insulation Material Using Carbide Slag

Using carbide slag as the calcareous materials, xonotlite thermal insulation material was successfully prepared via dynamic hydrothermal synthesis. The experimental results show that the xonotlite thermal insulation material is made up of large numbers of ＂chestnut bur shape＂ particles. Optimum conditions of calcination temperature of carbide slag, synthesis reaction temperature and time, stirring rate, CaO/SiO2 mol ratio, water/solid weight ratio, amount of fiberglass, molding pressures, dryness temperatures and the presence of dispersant （glycol and polyvinyl alcohol） favor the preparation of xonotlite thermal insulation material. The evaluation of xonotlite thermal insulation material reveals that the product is ultra-light and excellent in physical performances. Such a little amount of impurities in carbide slag has no effect on the phase, morphology, stability at high temperature and physical performances of products.

Promoting sludge granulation by putting xonotlite into the UASB reactors during starting-up stage

Four reactors of up-flow anaerobic sludge blanket (UASB) were concurrentlyoperated to examine the effects of the xonotlite secondary particles on promoting the sludgegranulation during the starting-up stage at room temperature. The results show that the putting ofthe xonotlite secondary particles into the UASB reactors can increase the basicity of the reactingliquid significantly. The particles can act as the media for biomass accumulation. Thus, thegranulation process of the sludge within the reactor can be largely promoted by the specialperformances of the particles both in physical and chemical aspects.

Effect of Na^+ on xonotlite crystals in hydrothermal synthesis

The effect of Na＋ ion concentration on the crystalline phase composition and morphology of xonotlite crystals pre- pared in a CaO-SiO2-H20 system via hydrothermal synthesis was analyzed. X-ray diffraction （XRD） and scanning electron mi- croscopy （SEM） results indicate that Na＋ ion concentration has a significant impact on the composition and morphology of crystalline phases in the products under the initial conditions of a molar ratio of CaO/SiO2 of 1.0 and a reactant concentration of 0.05 mol.L-1 at 225℃ for 15 h. The main crystalline phase in the products has a phase transition from xonotlite to pectolite, and the morphology changes from fibrous to broomlike shape with the Na＋ ion concentration increasing. Therefore, the content of Na^O in the raw material should be less than 5wt% for preparing pure xonotlite crystals via hydrothermal synthesis in a CaO-SiO2-H20 system.

Hydrothermal Synthesis of Xonotlite-type Calcium Silicate Insulation Material Using Industrial Zirconium Waste Residue

Xonotlite-type insulation material was prepared by hydrothermal synthesis technology using industrial zirconium waste residue in this paper,and the phase analysis together with the observation of micro–morphology were also carried out by XRD,SEM and TEM.The density and thermal conductivity were measured finally.The results indicate,chlorine ion impurity contained in zirconium waste residue can be removed effectively via water washed process,and the reactive activity of silicon dioxide is almost not affected,which make it be a good substitution of silicon material for the preparation of calcium silicate insulation material by hydrothermal synthesis technique.The density and thermal conductivity of xonotlite-type calcium silicate insulation material obtained by hydrothermal synthesis technique can reach 159 kg/m3,0.049 W/（m·° C）,respectively,meeting with National Standard well,when synthesis conditions are selected as follows：Ca/Si molar ratio equal to 1,synthesis temperature at 210°C and kept for 8 hrs.It provides a new approach to realize lightweight and low thermal conductivity of calcium silicate insulation material.

Investigation of the Transition Reaction of Tobermorite to Xonotlite under Influence of Additives

The objective of the present work was to investigate the transition reaction of the calcium silicate hydrate tobermorite into xonotlite under influence of additives. Tobermorite is the main binding agent in steam hardened building materials and the appearance of xonotlite indicates the progress of hardening and an overcuring of the material. Hydrothermal experiments under addition of sucrose, calcium formate and calcium chloride dihydrate to the main components quartz and lime were done using temperatures of 220°C and a reaction time of 40.5 h. All experiments were performed with powders as well as with pressed educts. The products of all syntheses were analyzed with XRD, SEM/EDX and FTIR. The references as well as the syntheses with calcium chloride dihydrate led to the formation of 11 &#197 tobermorite and xonotlite. The former showed the best results and even synthesis with pressed educts and calcium chloride dihydrate revealed an accelerating effect of the additive. In contrast syntheses with sucrose had the worst reactivity and led to the formation of calcite beside the CSH-phase scawtite. The additive calcium formate was only slightly oppressing the crystallization of tobermorite and favouring the formation of xonotlite. Syntheses with pressed pellets and sucrose or calcium formate showed generally worse results.

Low-cost oriented preparation of ultra fine xonotlite fibers

Nanometer-sized xonotlite fibers have great potential application in many fields. The traditional method of preparing ultra fine xonotlite fibers uses the ultra fine and highly active silica as the major raw materials, which is not only expensive but also difficult to prepare the xonoflite fibers with diameters around 100 nm. In this study, the ultra fine xonotlite fibers with diameters around 100 nm were prepared by an autoclaving method. The preparation was low-cost oriented by using natural powder quartz and lime as the major raw materials. The intergrowth of the fibers formed thin shell hollow balls or ellipsoids, namely the secondary particles. The length of the nanometer-sized xonotlite fibers was around several microns. The fibers and their secondary particles were produced at 216℃ for 6 h with a continuous stirring of 300-500 r/min. Zirconium oxychloride was used as an additive. The experiments show that zirconium oxychloride has an enormous effect on the growing habit of xonotlite crystals and plays an important role in controlling the diameter of xonotlite fibers.

Synthesis of Xonotlite and Effects of Ceramic Fiber Addition on Its Compressive Strength

In order to improve the compressive strength of xonotlite, the pretreated quartz powder, slaked CaO, additive, deionized water(water-solid ratio of 30) were placed in a magnetically stirred autoclave, and the corresponding products were obtained after incubating at 220 ℃ for 0, 1, 3 and 6 h, respectively to explore the synthetic process of xonotlite. In the prepared xonotlite powder, 0, 5%, 10%, 15% and 20%(by mass, the same hereinafter) of pre-treated ceramic staple fibers were added, and two sets of specimen with the specifications of φ50 mm×35 mm were molded by pressed filtration.One set of specimens were not sintered, and the other group was fired at 1 000 ℃ for 2 h to explore the effect of ceramic fiber addition on the compressive strength before and after sintering of xonotlite. The results show that C-S-H gel is first synthesized in the synthesis of xonotlite, and then the C-S-H gel is transformed to form tobermlite, at last tobermlite fully reacts to produce xonotlite. The addition of ceramic fiber enhances the compressive strength of the xonotlite before and after sintering. When the 15% ceramic fiber is added, the compressive strength is the highest. The specimens before and after the high temperature firing contain xonotlite phase and calcium silicate phase, respectively, and the compressive strength of the fired specimens is higher than that of the green ones.

Low-cost oriented preparation of ultra fine xonotlite fibers

Nanometer-sized xonotlite fibers have great potential application in manyfields. The traditional method of preparing ultra fine xonotlite fibers uses the ultra fine andhighly active silica as the major raw materials, which is not only expensive but also difficult toprepare the xonotlite fibers with diameters around 100 nm. In this study, the ultra fine xonotlitefibers with diameters around 100 nm were prepared by an autoclaving method. The preparation waslow-cost oriented by using natural powder quartz and lime as the major raw materials. Theintergrowth of the fibers formed thin shell hollow balls or ellipsoids, namely the secondaryparticles. The length of the nanometer-sized xonotlite fibers was around several microns. The fibersand their secondary particles were produced at 216 deg C for 6 h with a continuous stirring of300-500 r/min. Zirconium oxychloride was used as an additive. The experiments show that zirconiumoxychloride has an enormous effect on the growing habit of xonotlite crystals and plays an importantrole in controlling the diameter of the xonotlite fibers.

A composite thermal insulator based on xonotlite and perlite

A low cost thermal insulating material can be produced by compounding anactive xonotlite slurry, fired-perlite, HOMO PAN fibers and glass fibers. The maximum servicetemperature of the product is 800 deg C; linear shrinkage after 800 deg C X 16 h firing is 0.9percent; the cold crushing strength is 1.56 MPa; the flexural strength at ambient temperature is0.81 MPa; the thermal conductivity at ambient temperature (25 deg C) is 0.056 and 0.128 W/(m centredot K) at 800 deg C. The production cost of such a composite is only 1/3 of that of the normalxonotlite thermal insulators. It can substitute the normal xonotlite thermal insulators on mostoccasions with a similar cost to that of normal perlite products.

Synthesis and Structural Characterization of CSH-Phases in the Range of C/S = 0.41 - 1.66 at Temperatures of the Tobermorite Xonotlite Crossover

Calcium-Silicate-Hydrate-phases (CSH-phases) are important binding agents of building materials. The synthesis of CSH phases and their structural characterization was done to investigate the crystallization in dependence of an increasing CaO/SiO2 ratio (C/S ratios) from 0.41 up to 1.66 at temperatures in the crossover region of tobermorite to xonotlite (180℃ and 230℃). Parallel runs with the same C/S ratio but on the one hand with constant mass of quartz and variation of lime and on the other hand under reverse conditions (constant mass of lime but variable amounts of quartz) were performed at both temperatures. The aim was to clarify the connections of crystallization mechanism and kinetics of phase formation with structure, crystallinity and morphology of the CSH’s in the mentioned C/S ratio for both temperatures in the tobermorite-xonotlite crossover region. The parallel experiments with different mass ratios of the educts are important to study the influence of time evaluation of supersaturation within the solution under the peculiarities of the retrograde solubility of lime but accelerated solubility of quartz. The products were characterized by XRD, SEM/EDX, FTIR and 29Si MAS NMR spectroscopy (using the Q-site nomenclature [1]). The experiments could clarify some important connections of crystallization process and the reaction pathway.

FORMATION OF MICRO-POROUS SPHERICAL PARTICLES OF CALCIUM SILICATE(XONOTLITE) IN DYNAMIC HYDROTHERMAL PROCESS

Stirring during hydrothermal synthesis plays an important role in the formation of porous spherical xonotlite particles. The size of spherical particles formed during dynamic hydrothermal process is related to the size of minimum vortices in the reaction slurry, which is determined by stirring speed. The kinetics of growth of xonotlite particles is de-termined by measuring the apparent viscosity of the reactant slurry at various reaction time and reaction temperatures. It is found that the growth of particles follows the contracting-volume equation, and the activation energies for nucleation and growth are 104 and 123 kJ.mol-1, respectively.

Influence of Al^(3+) on Hydrothermally Synthesized Xonotlite Whiskers in CaO-SiO_2-H_2O System

Influences of Al3+ concentration on hydrothermally synthesized xonotlite whiskers were researched in this paper. The crystal phase composition and morphology of the products were analyzed and characterized using XRD and SEM techniques. The results indicate that Al3+ concentration have great impact on the crystal phase composition and morphology of the xonotlite whiskers in CaO-SiO2-H2O system under the condition of initial CaO/SiO2 mol ratio of 1.0, reactant concentration of 0.05 mol·L-1 , at 225 ℃ for 15 h. The main crystal phase of hydrothermally synthesized products had a phase transition from xonotlite to tobermorite, and the morphology changed from fibrous to tabular with increasing the concentration of Al3+ ion. At last, in order to prepare the pure xonotlite whiskers via hydrothermal synthesis in CaO-SiO2-H2O system, the Al2O3 contents in raw materials should be less than 1%.

Effect of Al^(3+) on Tobermorite Crystallinity

The difference between the systems of CaO-SiO2,-H2O and CaO-SiO2-Al(OH)3,-H2O, was studied, especially the effect of Al3+on the crystallinity of tobermorite was focussed. As a result, in the system of CaO-SiO2-H2O, tobermorite formed early, however, withreaction time forward, tobermorite was replaced by xonotlite. So far as CaO-SiO2-Al(OH)3,-H2O was concemed, hydrothermal reactionwas cdried out under the same condition as CaO-SiO2-H2O to study the morphological changes of tobermorite crystals. It was found thatAl3+ accelefated the crystal growth of tobermorite to some extent and was in faVor ofplaty crystals. Moreovef, with the Al3+ content in-creasing in the starting material, the morphology of tobermorite did not change magnificently, but platy crystals became more and moreeminent. As soon as the Al2O3 content was over 15.6%, synthetic mineral greatly changed in smicthe and haled into hibschite differentfrom 1 .1 nm tobermorite. Obviously, xonotlite was not apt to form in the presence of Al3+.