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# Solid-Liquid Reaction Doubts

leaching solid-liquid reactio

5 replies to this topic
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### #1 Europio

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Posted 29 October 2020 - 03:46 PM

Hello everyone,

I have to make several calculations  about solid-liquid reactions, but I have a lot of doubts. The more I look at information, the more doubts I have.

For example, the reaction Fe2O3 + 6 HCl --> 2 FeCl3 + 3 H2O

Can I write a rate equation depending on concentration of HCl? Or the volume will change during the reaction?

I have donde the following reasoning:

At time=0; 1 liter of HCl 37% = 1190 g = 440 grams HCl (12.22 mol) + 750 grams H2O

At infinite time all HCl is consumed, we have the previos 750 grams of water + 110 new grams of H2O (6.11 mol) = 840 mL

Is this correct?

Thank you.

Edited by guillermocp89, 29 October 2020 - 03:49 PM.

### #2 MrShorty

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Posted 29 October 2020 - 05:08 PM

That looks good for the HCl and H2O material balance. What are you expecting for the Fe2O3 and FeCl3 material balance? If initial and final aqueous volumes are part of the question, what assumptions are you making for the solubilities of Fe2O3 and FeCl3 in aqueous solution?

### #3 Europio

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Posted 29 October 2020 - 06:04 PM

Fe2O3 is solid and FeCl3 a dissolved salt. Supossing stechiometric reaction, as we have 12.22 mol HCl, we have 2.03 mol of Fe2O3 (325 g) that will give 4.06 mol FeCl3 (658 g). Solubility of FeCl3 is 92g/100 mL, so all the FeCl3 should be dissolved.

At the end of reaction, we will have 840 mL of water and 658 g of FeCl3, about 1500 g in total. If density of 90% FeCl3 dissolution is about 1.9 (linear regression take into account comercial dissolutions), the final volume will be 790 mL.

Edited by guillermocp89, 29 October 2020 - 06:28 PM.

### #4 MrShorty

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Posted 30 October 2020 - 11:10 AM

If density of 90% FeCl3 dissolution is about 1.9 (linear regression take into account comercial dissolutions), the final volume will be 790 mL.
. I see you are assuming that the solubility of 92g/100mL means 92 g FeCl3 in 100 mL of solution rather than 100 mL of water. I find this way of expressing solubility can be ambiguous, and it makes a big difference for salts like this with a high solubility. Of course, the actual concentration of the salt figures prominently in how you estimate the density of the solution (whether you actual concentration or the concentration at saturation). Double check your concentrations (is it really 90 wt% FeCl3 at saturation? what is your actual concentration after reaction?) and the densities you are using. Once you are comfortable with those, then your overall analysis of liquid volume looks fine.

### #5 Europio

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Posted 01 November 2020 - 05:42 PM

I am sorry, I made a mistake as you said. At the end of reaction, we will have 840 mL of water and 658 g of FeCl3, about 1500 g in total. If density of 78% FeCl3 dissolution is about 1.78 (linear regression taking into account comercial dissolutions), the final volume will be 842 mL. Thus, volumen variation and density is appreciable.
So I don't understand why in most leaching papers kinetics of leaching are written depending on concentration, if volumen and density is not constant.

### #6 MrShorty

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Posted 03 November 2020 - 05:41 PM

My colleagues on the forum are probably grimacing, because we haven't been very clear as to what our concentration units mean. When I see a number like 78% -- I will usually assume that it means mass fraction (or mole fraction). x(FeCl3)=m(FeCl3)/m(solution), but that number should be no where near 80%, based on the numbers given. When you say your concentration is almost 80%, what do you mean? What is your source data for FeCl3 solution densities using for concentration? Your overall thought process looks correct to me, but something in the details (your concentration unit specifically) seems incorrect to me.

So I don't understand why in most leaching papers kinetics of leaching are written depending on concentration, if volumen and density is not constant.

I'm not sure I know the answer to this, since I cannot see the leaching papers that you are referring to. I've seen it suggested that one of the differences between chemists and chemical engineers is that chemists tend to prefer concentrations using volumetric units (like molarity), where chemical engineers tend to prefer compositions in mass or mole bases (like mass fraction). One of the reasons I personally prefer mass/mole based units is because the result is independent of temperature or density or volume. But there is no question that there is a lot of information in the chemical literature based on volumetric units, and we need to be able to work with them.