Worldwide the preferred leaching chemical is cyanide at around 0.1 gram per liter of solution at fairly neutral pH and oxygenated too. That concentration will give you a fairly short residence time.
SaltwaterServr
In this first post you got the cyanide concentration (0.1 gram per liter of solution) wrong - which I see you later in the thread "somewhat" corrected
But what I really wanted to point out is what I underlined in the above quote --- you do NOT want to run your cyanide leach solution "at fairly
neutral pH"
The pH NEEDS to be "at least" 10.5 - & 11 is recommended - & in "some" cases it "may" need to run as high as pH 13 --- the reason for the high pH (10.5 - 11) is that is the optimum pH level for the leach to do its work - AND - more importantly "
do it safely" --- at a Ph below 10.5 you start to run a risk of exposure to the deadly cyanide gas --- pH 10.5 is the bottom pH level to prevent risk & still optimize the work of the leach - pH 11 is the "recommended" pH which still optimizes the leach work - but also "insures" no risk
Then when talking about growing sorghum to deal with your cyanide waste in an environmentally safe way (which by the way I find very interesting) you said the following
How's it work? Basically cyanide is HCN in solution. Hydrogen, carbon, and nitrogen. Any plant loves carbon and nitrogen. Sorghum and a few other species (I'm still researching wheat) can take the HCN and bust it apart to use the carbon and nitrogen.
Cyanide is the chemical compound CN which is a carbon atom that is "triple bonded" to a nitrogen atom --- it is NOT an H (hydrogen) atom bonded to a C (carbon) atom bonded to a N (nitrogen) atom --- so HCN is NOT a reference to a chemical compound of hydrogen, carbon & nitrogen
that stays in solution - at least not at the concentration of CN used for leaching AND at a pH below 10.5
HCN is the reference to the "
deadly GAS" Hydrogen - Cyanide (H-CN) when proper concentration &/or pH is not maintained/controlled
HCN - being a "gas" does not stay in solution - it "gases off" - at which point it becomes deadly due to breathing it - which is why you want to maintain the high pH - in order to prevent the formation of the of the HCN gas
You later in the thread posted
Alright. Not so minor update. Looks like most cyanide circuits run at .25g/L to .5g/L for leaching
The .5g/L is the correct cyanide concentration for optimum leaching run at the recommended (for safety concerns) pH of 11
If there is a lot of copper in the ore the leach can be made more selective to the gold by lowering the concentration of the cyanide - lowering the concentration of the cyanide (when a lot of copper is present) reduces consumption of the cyanide by the copper thereby making the lower concentration of cyanide more selective to the gold - and even though the cyanide concentration is lower - you still want to run at pH 11
The common cyanides used for leaching are potassium cyanide (KCN) sodium cyanide (NaCN) & some times calcium cyanide Ca(CN)2
The .5g/L concentration is achieved by dissolving 1 gram of any one of those cyanides in 1 liter water --- that level of cyanide will, without adding lime, give a protective alkalinity of about pH 10.5. - lime is added to increase the pH safety level to around 11. (NOTE - at pH 10.5 there is "some" HCN generated but the outgassing is slow/low enough that it gives you time to adjust your pH up to the safer 11 pH)
There is a smell of "bitter almonds" associated with HCN - the problem is that only about 50% of people can smell the bitter almond smell - its a genetic treat as to whether or not you can smell it - therefore when working with cyanide it should AWAYS be done with GOOD ventilation - if being done indoors you MUST have GOOD ventilation control systems in place
Acid of ANY kind added to a CN solution is the greatest danger of producing the "deadly HCN gas" --- NEVER work with acids & CN in the same work space &/or use the same equipment used with acid for also working with CN --- one mistake with acid & CN is likely to be the last mistake you will ever make & you likely will not live to tell about it
The point (reason) for posting this is that though CN leaching is a very effective way of leaching (some) ores - & for the most part also a safe way of leaching - "when done right" --- in fact - "when done right" - it is actually safer - & - more environmentally friendly then acid leaching --- the problem with CN leaching is the danger when not done right - one mistake can well be the last anything you do
Working with CN should ONLY be done after you have an absolute, full & complete understanding of working with it !!!! --- its not a "lets experiment" kind of thing - it doesn't give you any second chances when something goes wrong because of misinformation &/or incomplete information
Now then - concerning your idea of growing sorghum as an Eco friendly way to destroy the spent CN leach - I guess the question is - can you grow the sorghum in the high pH (high alkalinity) conditions that "need" to be maintained to prevent the CN from gassing off as HCN until the CN is actually destroyed ?
That's all I have time for today - there is a lot more that needs to be discussed (if you are really interested in CN leaching) including method to recover your gold in a way to reuse the leach (rather then destroy it) &/or method to destroy it when spent
Kurt
