The following is theory, constructed from readily available public sources. This theory has not been tested in practice. Anthrax spores disperse in neutral water and most nutrient broths, settling to the bottom so slowly that for all practical purposes they do not settle. To separate them one would need to add a suitable agent to the solution -- probably acetic acid would do it. Acidify the water to PH 4 or so, and let them settle out. I do not know if this would harm the spores, but spores are tough. Take some sodium bentonite clay, let it swell in water to maximum volume, and add water till it is nice and liquid. The clay should be moderately alkaline. Because the clay is alkaline, the spores will disperse in it, as they do in clean water. Mix spores very thoroughly into the clay. Acidify the mixture with acetic acid till the PH ensures the clay precipitates from the slurry The clay should precipitate out to form a crumbly precipitate which when dried can be readily pulverized to very fine dust. Different clays will yield precipitates that have different physical properties. Have no idea what PH would ensure the best properties of the precipitate, but one can test this without any need for bacteria. Now comes the tricky bit. One could have a well equipped laboratory to grind the precipitate without exposing oneself to too much of the dust, or one could take several people from some unwanted group, for example Tajiks or Kurds, put the precipitate in a barrel containing granite rocks, and have them roll the barrel around until the precipitate is well powdered. It seems to me that it is not very difficult to produce weaponized anthrax with primitive means, except that such means would liberally spread anthrax spores around the plant. If the plant was located in the frozen wastes of afghanistan, this would not be a serious problem.