3.Methodology  and  model The model used  to analyze  the environmental impact of primary  aluminium  production  consists of  several layers  of flows,  sub-processes and  materials.Important conversion  ratios,  like the  mass of  bauxite required  for the production  of  1kg  of  alumina  and the  mass  of alumina  required  for 1kg  of  primary  aluminium, are  fixed  for every  country,  respectively  1.53kg/kg  and 1.935kg/kg (according  to global  data in  EcoInvent  v3.0 [10]).According to the USGS [16] about 10% of all  mined  bauxite and 10%  of  alumina  is  used  for other processes (chemicals, cement,  abrasives etc).Many  hundreds  of other  flows and processes are  included  in  the  total analysis,  and  the variables that  were actively  used  or altered  in  the different simulations  are shown  in  red  color.These are: i) heat energy mix,  ii) electricity  energy  mix  and  iii)  energy  density  (in kWh/kg)  for example in  the  smelting  and refining  process.For instance, the impact of  alumina  refining  is  computed for 1kg  of  refined  product, taking  into consideration  the heat mix  and bauxite production,  but not the  smelting  process.Aluminium  hydroxide is  produced  from  bauxite  ore,  which  was  taken  equal  for all countries throughout the study.In  the analysis  methods  where the three process steps are separately  treated, impact results  are obtained  by  only  considering  the  upstream  elements  of the given  flow  scheme  up  to the point  where the impact  is assessed.Alumina and bauxite inputs  are given in  kg  while electricity and  heat  inputs  in  kWh.The  required  energy  for alumina refining  is provided  by  heat generation, of  which a part can be electricity  (at  medium  voltage).Generally  speaking  the electricity  is not used  to generate heat, but is required  for auxiliary  processes  of  the refining  plant.This assumption  is  valid as  most  plants in  the  world operate with the same  basic technology,  mainly  the energy  requirements differ.Additionally,  the fact  that some  bauxite and alumina  is being  used  for other products than primary  aluminium  is  not taken into account.The last step  of  the simulations  regards the computation  of the impact,  which  is based  on  the production  of  one final kilogram  of  primary  aluminium.Similarly, the impact  of  bauxite production  is computed  for only  1kg  of  ore, leaving  out all other branches.Regarding  alumina, the process is  divided  into two  subprocesses: the production  of aluminium  hydroxide and the production  of  alumina.Figure 1 is a simplified illustration  of  the structure of  the process network implemented  for this  analysis.