Optimization of the spark plasma sintering conditions for the consolidation of hydroxyapatite powders and characterization of the obtained products

A comparative investigation regarding the consolidation behavior displayed by three commercially available hydroxyapatite powders during Spark Plasma Sintering (SPS) is performed in this work. Starting powders are different in terms of purity, particle size, morphology and thermochemical stability. A completely dense product without secondary species is produced by SPS at 900 °C, when starting from highly pure powders with relatively small sized particles and grains. The resulting consolidated material, consisting of sub-micrometer sized hydroxyapatite grains, exhibits optical transparency and good mechanical properties. On the other hand, temperature levels up to 1,200 °C are needed to sinter powders with larger particles. This holds also true when relatively finer powders are used, also containing CaHPO4, are used. In both the latter cases products with coarser microstructures and/or significant amount of β-TCP, as a result of hydroxyapatite decomposition, are obtained. Optical, chemical resistance and mechanical properties of the resulting dense materials are correspondingly deteriorated

Mechanochemical Treatment of Soils Contaminated by Heavy Metals in Attritor and Impact Mills: Experiments and Modeling

An integrative approach was developed to support the scale-up from lab-into pilot-scale mechano-chemical reactors for immobilize heavy metals in contaminated mining soil

Rationale of heavy metals immobilization by ball milling in synthetic soils and remediation of heavy metals contaminated tailings

In thiswork, the use of mechanical milling for the remediation of heavy metals in synthetic soils and tailings sampled fromthe mining area of “Barraxiutta”, SW of Sardinia, Italy is investigated. Specifically, Pb(II) contaminated synthetic soils of sandy, bentonitic and kaolinitic type are taken into account following the results obtained in previous works. Suitable sequential extraction procedures have been performed on both untreated and treated synthetic soils. It is found that mechanical loads which occur during collisions amongmillingmedia and soils are able to modify the distribution of Pb(II) onto the different solid fractions of contaminated synthetic soils. Such phenomena may contribute to Pb(II) immobilization efficiency since the heavy metal trans-speciation occurs in favor of fractions characterized by stronger bonds and lower solubility. As for the heavy metals contaminated tailings, their immobilization is obtained using both ball and attrition milling devices under specific ball to powder ratio values. The degree of metals immobilization is evaluated by analyzing the corresponding leachable fraction obtained through the Synthetic Precipitation Leaching Procedure (SPLP) proposed by EPA

Immobilization of heavy metals in contaminated soils through ball milling with and without additives

In the present work, the use of ball milling for remediation of heavy metals from contaminated soils of sandy, bentonitic and kaolinitic type is investigated. Immobilization of heavy metals (Cd(II), Pb(II), Zn(II)) is achieved by mechanically treating the contaminated soil with or without additives, by taking advantage of weak transformations induced on the soil through mechanical loads occurring during collisions in the milling process. When hydroxylapatite (HA) is added to the heavy metal contaminated soil, the corresponding immobilization efficiency of the mechanochemical treatment increases. The degree of metal immobilization is evaluated by analyzing the leachable fraction of heavy metal from the treated soil as obtained through the “synthetic precipitation leaching procedure”. In particular, for soils contaminated by heavy metals displaying concentration levels similar to field contaminated soils, specific ball-milling treatments without the use of hydroxylapatite were able to reduce the leachable fraction of heavy metals to levels lower than the USEPA regulatory thresholds. XRD, SEM/EDS and granulometric analyses reveal no significant alterations of the intrinsic character of sandy and bentonitic soils after milling except for a partial amorphization of the treated soil. On the other hand, the mechanical treatment causes the total amorphization of kaolinitic soil. The increase of immobilization efficiency when soils are mechanically treated is hypothesised to be due to the specific phenomena induced during ball milling such as entrapment of heavy metals into aggregates, solid diffusion into the crystalline reticulum of soil particles as well as the formation of new fresh surfaces (through particle breakage) onto which heavy metals may be irreversibly adsorbed. In addition, when HA is added to the heavy metal contaminated soils, the mechanical treatment is hypothesised to be able to provide an increase of the specific surface and reactivity of hydroxylapatite, whose metal immobilization properties are well known

BONGRID: tecnologie di bonifica, analisi dei costi e metodologie di analisi per il monitoraggio dei siti inquinati

2006-11-28Sardegna Ricerche, Edificio 2, Località Piscinamanna 09010 Pula (CA) - ItaliaKick-off Meeting del Progetto GRIDA

Remediation of heavy metals contaminated soils by ball milling

In the present work, the use of ball milling reactors for the remediation of lead contaminated soils was investigated. Lead immobilization was achieved without the use of additional reactants but only through the exploitation of weak transformations induced on the treated soil by mechanical loads taking place during collisions among milling media. The degree of metal immobilization was evaluated by. analyzing the leachable fraction of Pb(II) obtained through the "synthetic precipitation leaching procedure". The reduction of leachable Pb(II) from certain synthetic soils, i.e., bentonitic, sandy and kaolinitc ones, was obtained under specific milling regimes. For example, for the case of bentonitic soils characterized by a Pb(II) concentration in the solid phase equal to 954.4 mg kg(-1), leachable Pb(II) was reduced, after 7 h of mechanical treatment, from 1.3 mg l(-1) to a concentration lower than the USEPA regulatory threshold (i.e., 0.015 mg l(-1) for drinkable water). Similar results were obtained for sandy and kaolinitic soils. X-ray diffraction, scanning electron microscopy, electron dispersive spectroscopy and granulometric analyses revealed no significant,alterations of the intrinsic character of sandy and bentonitic soils after milling except for a relatively small increase of particles size and a partial amorphization of the treated soil. On the other hand, the mechanical treatment caused the total amorphization of kaolinitic soil. The increase of immobilization efficiency can be probably ascribed to specific phenomena induced by mechanical treatment such as entrapment of Pb(II) into aggregates due to aggregation, solid diffusion of Pb(II) into crystalline reticulum of soil particles as well as the formation of new fresh surfaces (through particle breakage) onto which Pb(II) may be irreversibly adsorbed. (c) 2006 Elsevier Ltd. All rights reserved

Rationale of lead immobilization by ball milling in synthetic soils and remediation of heavy metals contaminated tailings

In thiswork, the use of mechanical milling for the remediation of heavy metals in synthetic soils and tailings sampled fromthe mining area of “Barraxiutta”, SWof Sardinia, Italy is investigated. Specifically, Pb(II) contaminated synthetic soils of sandy, bentonitic and kaolinitic type are taken into account following the results obtained in previous works. Suitable sequential extraction procedures have been performed on both untreated and treated synthetic soils. It is found that mechanical loads which occur during collisions amongmillingmedia and soils are able to modify the distribution of Pb(II) onto the different solid fractions of contaminated synthetic soils. Specifically, for sandy soils the milling treatment induces a significant increase in Pb(II) content in the Fe–Mn oxides fraction. On the other hand, for bentonitic and kaolinitic soils, Pb(II) content in both carbonate and Fe–Mn oxide fractions is augmented after treatment. Such phenomena may contribute to Pb(II) immobilization efficiency since the heavy metal trans-speciation occurs in favor of fractions characterized by stronger bonds and lower solubility. As for the heavy metals contaminated tailings, their immobilization is obtained using both ball and attrition milling devices under specific ball to powder ratio values. The degree of metals immobilization is evaluated by analyzing the corresponding leachable fraction obtained through the Synthetic Precipitation Leaching Procedure (SPLP) proposed by EPA. X-ray diffraction and granulometric analyses revealed no significant alterations of the intrinsic character of the tailings after milling except for a relatively small increase in particles size. The increase in immobilization efficiency when tailings are mechanically treated may be due to specific phenomena induced during milling such as entrapment of heavy metals into aggregates, solid diffusion of metals into the crystalline reticulum of soil particles, the formation of newfresh surfaces onto whichheavy metalsmay be irreversibly adsorbed aswell asmetal trans-speciation onto tailings fractions characterized by stronger bonds and lower solubility

Tantalum carbide products from chemically-activated combustion synthesis reactions

A rapid route based on the use of small amounts of polytetrafluoroethylene to chemically activate and sustain the combustion synthesis reaction for the formation of TaC from its elements is successfully exploited in this work. Other than a reaction booster, the polymer is found to play also a role as a carbon source, so that part of graphite can be replaced by Teflon to produce a single phase material. A relevant importance in the activation of the synthesis reaction is provided by the intermediate phase TaF3, whose formation is clearly evidenced, along with that of Ta2C, by combustion front quenching experiments. Additive free TaC products with relative density up to about 98% and grains size less than 5 µm are finally obtained when combustion synthesized powders are processed for 20 min at 1800 °C by Spark Plasma Sintering. A further increase in the sintering temperature to 2050 °C and/or the dwell time to 30 min is found to negatively affect product densification. This outcome is mainly ascribed to the significant grains coarsening (above 20 µm) correspondingly observed as well as to other vapor-phase generating events, which could be more easily induced when powders are exposed to higher temperature conditions