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Dissolution Behavior of Environmentally Regulated Elements from…
Dissolution Behavior of Environmentally Regulated Elements
from Steelmaking Slag into Seawater
Introduction :
Japan
Produced more than 13 million tons of steelmaking slag
Around 0.5 million tons of steelmaking slags are landfilled each year
Can threaten the global environment because large amount of carbon dioxide was produced from steelmaking industries
By using Electron Probe Micro Analyzer (EPMA) to investigated the morphology of fluorine in steelmaking slags
Steelmaking slags contain nutrition such as Si, P and Fe which can dissolve into sea water and help the phytoplankton growth
Dissolution of Ca, Mg, Mn and F from Steelmak-
ing Slag into Artificial Seawater
Dissolution behavior for each element from steelmaking slag are differs.
Calcium content increase from 400 to 1400 mg/L.
Magnesium concentrations in converter slag reduced somewhat, whereas those in hot metal pretreatment slag remained same.
Rates of manganese dissolution very slow & maximum manganese content is 2 mg/L.
Dissolution behavior of fluorine
Rates of fluorine elution positive
Rates of fluorine elution negative
Morphology of Fluorine in Steelmaking Slag
In slag P9, fluorine coexisted with calcium and phosphorus which had a low calcium content
Fluorine, calcium and phosphorus are type of example mapping images was observed using Electron Probe Micro Analyzer (EPMA) for slag P9 and P11
In slag P11, no fluorine was detected in the phosphorus concentrated phase, but it presence with calcium which had high calcium content
As a result, fluorine in this slag was assumed to exist primarily as apatite phase
Stability Diagram in Seawater
Stability diagrams of Ca, Mg, Co2, Cr, Pb, Cd, As, and Fe were calculated.
Dissolution behavior of environmentally restricted elements from steelmaking slag
Mechanism of pH increase with calcium dissolution.
It was calculated at 298K in the present work because most of the standard potentials of dissolved species were unknown at room temperature and applicable to ocean conditions.
Variable: the contents of objective element in seawater and pH were chosen.
Fixed: concentrations of all the other elements except for stability diagram of fluorine.
Stability diagram of calcium
In the present work, CaCO3, Ca(OH)2, CaO and CaSO4 were considered as possible solid substances.
Calcium content that equilibrates with CaSO4 extremely high, thus the calcium solubility to CaSO4 saturation is not shown.
concentration of Ca2+ equilibrates with Ca(OH)2 and CaO approximatey 10^8 times larger than that equilibrating with CaCO3
CaCO3 would participate before Ca(OH)2 when calcium content of seawater increases.
The solubility of calcium decreased with increasing pH up to 10 and it becomes constant at more basic region.
when steelmaking slag is added to the ocean, and if CaO phase exists on the surface, it will easily dissolve into seawater.
As the result, both pH and Ca2+ concentration will increase.
Calcium content in equilibrium with 4CaO · P2O5 is extremely high, and thus the calcium solubility to 4CaO · P2O5 saturation is not shown
Dissolution of these phase into seawater results in the increase of Ca2+ concentration and pH as well as CaO dissolution
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Stability of Magnesium
Ion such as Mg2+, MgSO4, MgCl+, MgF+, MgHPO4, MgHCO3+, MgCO3 were also taken into account.
There are a huge amount of Mg2+ ion in seawater and the buffering action will take place to prevent more increase of pH
Concentration of Mg2+ that equilibrates with MgO and Mg(OH)2 are always 1000 times bigger that that equilibrating MgCO3
Magnesium solubility in seawater is governed by MgCO3
Stability diagram of Carbon dioxide
Partial pressure of CO2 in air was set at 0.0003/10^5 (Pa).
The solubility of carbon is constant when pH is lower than 6 and it increases with pH at higher pH condition.
Stability diagram of Chromium
Cr(OH)3, Cr2O3, Cr(OH)3.nH2O were considered as possible solid substances that could equilibrate with seawater.
Predominant trivalent chromium species are Cr^3+, Cr(OH)^2+, Cr(OH)2^+, Cr)2^-, pH is lower than 4.0, between 4.0 and 6.8, between 6.85 and 8.5, and higher than 8.5 respectively.
Solubility of chromium increases when the equilibrium with solid substances change from Cr(OH)3, CrO3 to Cr(OH)3.nH2O
Hexavalent chromium ions are more
stable than trivalent ions in air.
HCrO4^2- and CrO4^2- considered as hexavalent chromium ions.
Stability diagram of lead, cadmium and arsenic
Lead and cadmium, solubility has minimum value caused by change of predominant ionic species with pH
Concentration of these elements equilibrating with pure oxides in seawater exceed the environmental regulation limit in the whole pH region.
it will be satisfied when activity of PbO, CdO and As2O3 lower than 10^-5, 10^-7 and 5x10^-6 respectively
Steelmaking slag contain certain amount of chromium, lead, cadmium and arsenic should not be added into seawater without immobilizing these elements.
Stability diagram of fluorine
Increase of pH with Ca dissolution, and buffering action to pH increase by Mg ions
The pH and Ca^2+ content of seawater would increase when CaO containing steelmaking slag is added.
Fluorine solubility at CaF2 saturation would decrease with increase of pH due to the solubility product of CaF2 is constant.
Mg(OH)2 precipitates when pH is higher than 9.4, and the equilibrium content of MgF^+ ions decreases with pH increase, and resulting in the decrease of total fluorine concentration in seawater.
Relation among fluorine elution behavior from steelmaking into seawater and CaO and P2O5 content of steelmaking slag.
It can be said that fluorine does not dissolve when CaO content of steelmaking slag is low and P2O5 content is high
Conclusion
When concentrations are low, environmentally prohibited elements found in steelmaking slag do not dissolve & it is unstable in seawater.
Steelmaking slag that contain prohibited element must be immobilized.
Mechanism of buttering of magnesium ion in seawater can be shown by means of stability diagram.
The addition of steelmaking slag would not significantly change the pH or fluorine concentration of seawater.
The composition of the steelmaking slag has a significant impact on how fluorine dissolves.
One option to immobilized fluorine is by control the slag composition.
