Ammonia recovery from calciner off-gas

Abstract

Ammonia has been recognized as an effective leaching agent in hydro-metallurgical processes for the recovery of metals\cite{AmmLeach2013}. These processes result in ammonia-metal crystals, whereby in the last step, water and ammonia are removed from the crystals by a calcination process. In the past, the vaporized ammonia is considered a waste stream. However, regulations in the last few decades make it impossible for companies to dispose large amounts of ammonia into the air\cite{LRTAP2005}\cite{SOER2020}. Therefore, expensive installations, like catalytic oxidizers (CATOX) or membrane separators, need to be installed for ammonia reduction in the effluent gas stream. Climax Molybdenum b.v. installation in the Botlek Rotterdam produces pure molybdenum oxide according to the process described above. With the production, molybdenum oxide is leached in an ammonia solution to remove all contaminants, then crystallized and calcined. For the calcination process, the crystals are heated by a gas burner to vaporize the ammonia and water. The off-gas contains air, \ce{NH3}, \ce{CO2}, dust particles, and water vapor. The objective is to recycle the ammonia from the off-gasses of the calciner. Reclaiming the ammonia from the off-gas stream could reduce the costs up to \euro2000 per day. In this thesis, insights are gathered related to the chemical and physical reactions that occur when off-gasses of the calciner enter the process. Additionally, both the beneficial effects and potential complications of reclaiming ammonia from the off-gas are explained. This research is conducted by making mass balances on three different options for off-gas handling. These options include absorption into the leaching fluid (ARS:Ammonia Reduction System), absorption by water through a scrubbing system (AER:Ammonia Emission Reduction), and absorption by water through a scrubbing system with additional carbon dioxide removal with sodium hydroxide (AER+NaOH). In order to understand what happens when inorganic carbon (IC) comes into the system, experiments are conducted on both the leaching procedure of molybdenum and the stripping and condensing process of ammonia. Results show that inorganic carbon reaches its solubility in the leaching fluid at 2.12 mol/kg. Also, no effect on the solubility of ammonium molybdate was found, and no additional precipitation occurs on metals like zinc, calcium, magnesium, and iron. In the leaching fluid, the solubility of ammonium bicarbonate is reached at 183 grams per liter. Additionally, the influence of inorganic carbon on the stripping process showed that the strip rate of ammonia was reduced by 1.4. Also, could be concluded that stripping of ammonia was done efficiently at 80 degrees until the total ammonia nitrogen (TAN) mole concentration equals the total inorganic carbon (TIC) mole concentration. As a result, additional water treatment is needed for the discharge water to ensure the amount of ammonia does not exceed 25kg per day. Furthermore, another complication found was scaling. Scaling happens in both ARS and AER-system in the form of ammonium bicarbonate. In contrast, AER+NaOH system does not have the complication of scaling or ammonia in the discharge water, but the pH of the solution needs to be neutralized before discharge. Moreover, higher pH in the recycling water of the scrubber causes lower absorption efficiency of ammonia. ARS, AER, and AER+NaOH-system have an ammonia reduction of 41\%w, 33\%w, and 73\%w per day, respectively. For the ARS and AER-system, this means a cost-saving of \euro{1.160,-} and \euro{920,-} per day, respectively. The AER+NaOH-system has a direct cost of \euro{10.130,-} per day, due to the expensive sodium hydroxide. The cost of sodium hydroxide as a water treatment agent is higher than the cost savings of ammonia recovery from the off-gasses of the calciner. Therefore can be concluded that, ammonia recovery from off-gasses has large potential, but other methods are needed to treat ammonia in the discharge water to make it economically feasible. Further research can elaborate on alternatives for ammonia treatment in discharge water. Examples include the use of lime (CaO)\cite{AdPhysicochem2006}, increasing the stripping temperature\cite{Kim2013}, and leaching in sulfuric acid (\ce{H2SO4})\cite{EICforEngin2017}. However, this does not solve the scaling problem in the process. Therefore, further research should shine a light on the reduction of carbon dioxide that enters the chemical plant system. Examples proposed in this research, include condensing ammonia from the calciner off-gas stream. Multiple studies \cite{Xu1999}\cite{Mirl2020} show that, the condensing recovery is more effective for ammonia than carbon dioxide. Furthermore, this research also gives an economically feasible example of using an alternative calcination process without natural gas.