Recycling of Waste Catalyst for Ammonia Synthesis

The spent and discharged catalyst from converter in ammonia plant is called as waste catalyst. The waste catalyst is mainly composed of 65% 80% of metallic iron, 10%-20% of iron oxide and 8%-10% of promoter oxides such as AI2O3, K2O, CaO, CoO etc. At present, they are generally abandoned or are interred in the scrapheap. This is not only a waste of resources, but also leads to environmental pollution, especially for the catalyst containing precious metals such as cobalt which has great recycling value. Example, the content of CoO and promoters in Fe-Co catalysts is illustrated in Table 4.13. 

Technically, it is difficult to recover a single component in the waste catalyst and the total amount of waste catalyst is limited. It may not be rational in an economic sense. Probably, it is the best way to recycle the catalyst as a whole and then use the recycled catalysts for production of a fresh catalyst, so that all the components of waste catalyst can be used effectively. However, it is not an easy thing to transform the waste catalyst to new catalyst because the main component of the waste catalyst is metallic iron (Fe) with the content of more than 70% while the iron in the new catalyst should be iron oxide. Therefore, the metallic iron should be first transformed into iron oxide. The method for the transformation is varied according to the required state of iron oxide in the fresh catalyst. 

That is, the 70%-80% of metallic iron of the waste catalyst can be used as a reducing agent for the reduction of magnetite to FeO, and Fej xO-based catalysts can be obtained. At the same time, all the promoters such as AI2O3, K2O, CaO, CoO, etc in the waste catalyst enter into the fresh catalyst (the amount of promoters in fresh catalyst should include that from the waste catalyst), so that all of the components of the waste catalyst are utihzed effectively. Based on this idea, the 

Ullmanns Enzyclopadie der technischen Chemie, Urban u. Schwarzenberg, Miinchen-Berlin III, 525, 1953. 

Slark AV, James GR. Amm,onia Synthesis (Part III), Translated by Staff Room of Inorganic Chemical Engineering, Dianlian Institute of Technology, Beijing Chemical Industry Press, 1980, 88. 

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