<P> Economically, pressure is an expensive commodity . Pipes, valves, and reaction vessels need to be strengthened, and there are safety considerations of working at 20 MPa . In addition, running pumps and compressors takes considerable energy . Thus the compromise used gives a single pass yield of around 15% . </P> <P> Another way to increase the yield of the reaction would be to remove the product (i.e. ammonia gas) from the system . In practice, gaseous ammonia is not removed from the reactor itself, since the temperature is too high; it is removed from the equilibrium mixture of gases leaving the reaction vessel . The hot gases are cooled enough, whilst maintaining a high pressure, for the ammonia to condense and be removed as liquid . Unreacted hydrogen and nitrogen gases are then returned to the reaction vessel to undergo further reaction . </P> <P> The most popular catalysts are based on iron promoted with K O, CaO, SiO, and Al O . The original Haber--Bosch reaction chambers used osmium as the catalyst, but it was available in extremely small quantities . Haber noted uranium was almost as effective and easier to obtain than osmium . Under Bosch's direction in 1909, the BASF researcher Alwin Mittasch discovered a much less expensive iron - based catalyst, which is still used today . Some ammonia production utilizes ruthenium - based catalysts (the KAAP process). Ruthenium forms more active catalysts that allows milder operating pressures . Such catalysts are prepared by decomposition of triruthenium dodecacarbonyl on graphite . </P> <P> In industrial practice, the iron catalyst is obtained from finely ground iron powder, which is usually obtained by reduction of high purity magnetite (Fe O). The pulverized iron metal is burnt (oxidized) to give magnetite of a defined particle size . The magnetite particles are then partially reduced, removing some of the oxygen in the process . The resulting catalyst particles consist of a core of magnetite, encased in a shell of wüstite (FeO), which in turn is surrounded by an outer shell of iron metal . The catalyst maintains most of its bulk volume during the reduction, resulting in a highly porous high surface area material, which enhances its effectiveness as a catalyst . Other minor components of the catalyst include calcium and aluminium oxides, which support the iron catalyst and help it maintain its surface area . These oxides of Ca, Al, K, and Si are unreactive to reduction by the hydrogen . </P>

What element is used as a catalyst in the haber process
find me the text answering this question