In our research work we need to minimize the carbonate content of our quartz sand (containing quartz, calcite, feldspar, dolomite). We are going to wash the sand with acidic water (distilled water + HCl). Which pH is more suitable considering maximum removing of carbonate content and minimum influence on physical and chemical properties of other minerals, especially quartz?
Dear Towe, thaks fot your answer.
I want to compare physico-mechanical properties of the sand with and without carbonate content. You said dolomite may give problem, what problem?
Hamed, you may consider performing some simple geochemical modelling using Visual MINTEQ or PHREEQC to understand the solubility of carbonates as a function of pH.
You may also consider using organic acids instead of inorganic, as carboxylic acids (e.g. oxalic, citric, formic, acetic, etc.) are know to be good Ca and Mg chelators and the acids are milder, so maybe better to keep the rest of the material inert.
Another point, consider that leaching from solid phases is a surface based reaction, so depending on the particle size of your sand, ir may be difficult to remove the carbonates contained in the interior of particle grains (mineral liberation issue).
Lastly, remember that thermal decomposition of carbonates is possible, you simply need to heat in a furnace to 800degC. Of course in this case only the CO2 leaves, while the CaO and MgO will stay behind.
Dear Kenneth Towe,
the endothermic transformtion of quartz around 600°C is a reversible process. Above 600°C the quartz lattice is hexagonal, but if it cools below this transition temperature, it will be trigonal, just like before heating up. Anyway, the heating may cause fractures in the grains, thus changes the mechanical properties.
Dear Hamed Khodadadi,
Here is a text about acidic dissolution of carbonate minerals: http://www.most.gov.mm/techuni/media/PE_04026_7.pdf
HCl is used in general, 20+ % HCl solution dissolves dolomite as well. You may try dilute (10-15%) nitric acid as well, since it is thought to be less agressive to silicate minerals than HCl.
Kenneth, I've worked with organic acids for leaching fresh and carbonated olivine (magnesite rich), and they work quite well (methodology is 24 hour shaking test at 25degC).
I work mostly with thermal residues, which have been to high temperatures before, that is why in my case heating is a suitable method to decompose carbonates (I regularly do this to "refresh" the minerals). For natural minerals that have no thermal history, like you say, it is possible that heating will cause irreversible mineralogical or morphological effects, so something to be careful of.
Kenneth... using 20% HCl will dissolve dolomite, and then there's no need for heating. Of course, an "acidic water" will not work.
You may to delete ALL carbonates (calcite, dolomite, magnesite, siderite, rhodochrosite, huntite, malachite and many others) together with Fe and Mn hydroxides in NEUTRAL (pH=7) environment, if you'll use 5% solution of Trilon-B (EDTA, C10H14O8N2Na2*2H2O). The solution works better in warm state - 40-50 C.
Hi Kenneth, I am mainly investigate rare-metal minerals (Nb, Ta, Zr, REE, Y etc.).
So I had clean pyrochlore crystals from dense crusts of 'limonite', siderite and Mn-hydroxides. With great results.
Besides that, many times I had try to concentrate metamict mineral of chevkinite group from carbonatite matrix (calcite, ferrodolomite, aragonite-strontianite, strontianite, witherite, cordylite-(Ce), ancylite-(Ce) etc.). But in acidic solutions the mineral was etched and even completely dissolved. The problem was solved with EDTA using. I'd obtain necessary mineral in completely unaltered state and had observe many other interesting things, which before were completely damaged by acid treatment.
Simple calcite/aragonite dissolves in even cold EDTA solution very fast. Much more fast then dolomite.
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