Health and Safety - HF Training - Neutralisation

Clean Suite HF Procedures:

Neutralisation & Disposal

Hydrofluoric acid is essential to much of our work as an etchant for silicon. It presents particular hazards due to the reactivity of the fluorine ions and requires neutralising before disposal.

Calcium Chloride has a high solubility and is available in the easily handled form of pellets or flakes. When reacted with HF it produces insoluble CaF_2, the other product being HCl. The process takes place in aqueous solution and the acceptable means of disposing of HCl is high dilution and waste to drain - i.e. “down the sink with lots of water”.

Note that CaCl₂ comes in anhydrous and hydrated variants and the required weight will vary accordingly; currently we use the bihydrate but do check labels. Accurate weighing will ensure complete neutralisation, economic use of reagents and minimal impact on the environment. Don’t just use “plenty”.

CaCl₂.2H₂O	+	2(HF)	==>	CaF₂	+	2(HCl)
129 g	+	40 g	==>	78 g	+	73 g

But we use ~50% HF solution, s.g. 1.15 g/mL 40 /(0.5 x 1.15) = 70 so,

129 g	+	70 ml	==>	78 g	+	73 g
129	:	70	=	1.84	:	1

Therefore, approximating and allowing an excess of neutraliser:

2 g CaCl₂.2H₂O per mL HF(50%)

To prepare a solution of CaCl₂

Maximum solubility of CaCl₂ (anhydrous) is 600 g / litre , » 900g CaCl_2..2H₂O or 1.1L/kg

Allow excess of water say 1.5 mL/g

How many ml of HF (50%) must be neutralised?	....................	ml HF	A
This will require	x 2 = ...........	gms CaCl_2..2H₂O	B
This will dissolve in	x 1.5 = .........	mL water	C
Which will require a minimum vessel of	x 2 = ............	mL capacity	D
Plus volume of other etch chemicals, V	+ V = ...........	mL capacity	E

Fill the vessel E with at least C mL of water. Slowly add B g chloride while stirring. It is exothermic going into solution so you should allow time for it to cool between preparing the neutralising solution and adding the HF or add extra water.
Carefully add the HF solution to the chloride, a little at a time, and stirring.
When complete, add more water into the vessel to cool and dilute.
Pour the solution slowly to drain, using more water to dilute.
Rinse all equipment and the work surface with plenty of water.
Run washdown for a further time to flush all traces.

Example: ”Mesa” etch is 75 parts nitric, 17 parts acetic, 8 parts HF
If you use 25 ml of etch, that’s 2 ml of HF

A=2, B= 4, C= 6, D=12, V=(75+17)/4=23, E= 35

so a 250 mL beaker would be a suitable choice - shallow reaction area and adequate volume for dilution.

Sept 2007