2.7.4. Synthesis of Acid chloroauric trihydrate (HAuCl4.3H2O)

This protocol allows to calculate the life cycle inventory data to produce 1 kg of acid chloroauric trihydrate (HAuCl₄.3H₂O) based on the assumptions established by the Ecoinvent framework and the data collected from literature. 

Input material is calculated based on stoichiometric reactions

1. The reactions that were involved are:

Au + 3HNO₃ + 4HCl à HAuCl₄ + 3NO₂ + 3H₂O

3HCl + HNO₃ à Cl₂ + 2H₂O + NOCl

2. The mass balance was obtained from the protocol described in the Handbook of preparative inorganic chemistry (S. Gross, 2008), as follows: 

• Preparation of aqua regia: Aqua regia is produced by mixing 75% HCl and 25% HNO₃. Density of HNO₃ is 1.42 g/mL (16 M) and the density of HCl is 1.18 g/mL (12 M). Therefore, aqua regia composition is 20 wt.% HNO₃, 26.5 wt.% HCl, and 53.2 wt.% H₂O.
• Production of HAuCl₄: 70 mL of aqua regia is mixed with 0.7 g of metallic gold. This mixture is stirred at 50 °C. When metallic gold is completely dissolved, HCl is slowly added until HNO₃ is completely removed. This reaction produces a gas stream with the following composition: 1.4 wt.% NO₂, 51.0 wt.% Cl₂, 47.1 wt.% of NOCl, and 0.5 wt.% of HCl. The flow rate of the gas stream is 35.59 g
• Decomposition of NOCl in the atmosphere results in the production of Cl₂ and NO. Therefore, 16.76 g of NOCl produces 9.09 g Cl₂ and 7.68 g NO. 
• The composition of the liquor obtained after the reaction is: 0.4 wt.% HNO₃, 26.1 wt.% HCl, 73.0 wt.% H₂O, and 0.5 wt.% HAuCl₄. 
• Purification of HAuCl₄: To purify HAuCl₄, the aforecited solution is heated to concentrate the solution. Distilled water is added to control the temperature between 70 and 80 °C. During evaporation HCl, HNO₃, and water is removed and a solution of HAuCl₄ is discarded. 

3. Based on the above the following quantities were included in the life cycle inventory to produce 1 kg of HAuCl₄.3H₂O:

• 0.54 kg metallic gold
• 13.57 kg HNO₃
• 68.07 kg HCl

4. We employed the Ecoinvent V 3.4 to calculate the providers. According to Ecoinvent the following providers were included.

• Gold production |gold| APOS, S – Row: This data set described the beneficiation, concentration, and refinement of gold. It includes energy and water consumption, land use, transportation, transports, waste disposal as well as emissions to air and water. 
• Nitric acid production, product in 50% solution state |Nitric acid, without water, in 50% solution| APOS, S – RER:
• Market for Hydrochloric acid, without water, in 30% solid state, APOS – S, RER: 

Detailed information of the laboratory protocol is found:

• Gross, S. (2008). Colloidal Dispersion of Gold Nanoparticles. In: Schubert, U., Hüsing, N., Laine, R.M. (eds) Materials Syntheses. Springer, Vienna. https://doi.org/10.1007/978-3-211-75125-1_21

Detailed information of the mass balance performed to calculate the production of chloroauric acid trihydrate is found: 

• Sanchez, N., Ruiz, R., Rödl, A., & Cobo, M. (2021). Life cycle inventory data for power production from sugarcane press-mud. Data in Brief, 37, 107194.