Besides the recovery of copper otherwise lost in mine
      waters, the cement copper process has several additional
      advantages.  It is simple in principle and operation,
      usually requiring a minimum of human attention and no
      machinery.  Fluctuations in copper content of the solutions
      do not affect the efficiency of the process. Also it uses
      relatively inexpensive scrap metals and produces a high-
      grade concentrate.
           Precipitation of cement copper requires only a source
      of copper-rich water, suitable pipe to convey it and pre-
      cipitation launders containing scrap iron.  Usually less

than an hour of contact with the scrap iron suffices to
precipitate virtually all dissolved copper. Periodically
more scrap metal is added to the launders to replace that
eaten away.  The precipitated copper sinks to the bottom
of the launders as a sludgy mass of fine particles.  At
intervals it is removed, dried, and shipped to the smelter.
Concentrates produced vary from about 65 to 95 per cent.
copper according to the procedures used.
  Through years of practice and experiment several
principles have been recognised which lead to cheaper and
more efficient copper precipitation:-
       (1)  As much scrap as possible should be kept
            crowded into the precipitation launders at all
            times.                                        

       (2)  Rapid movement of solution through the tanks
            is more efficient than slow motion and prolonged
            contact.
       (3)  The greatest surface area possible is desirable
            in the scrap iron, thin sheet metal being best.
       (4)  Tinned sheet metal, such as tin cans, causes
            the copper to stick too tightly unless burned
            or otherwise detinned.
       (5)  Any oil, grease, or paint on the scrap will slow
            the process and should be removed.
       (6)  Aeration and agitation of the solutions in the
            tanks is helpful to loosen hydrogen gas bubbles
            and precipitated copper to bring fresh solution
            against the bare metal surface.
     Cement copper is produced at.some mines where the
 copper in dumps or in low-grade bodies is systematically
 leached.  Ore of too low a grade to mine or mill is treated
 by causing water to percolate through it dissolving the
 copper which is then recovered as cement copper.  After
 precipitation of the copper the water may be returned to
 leach again through the dumps or the underground ore
 bodies.  Thus a continuous cycle of leaching and precipi-
 tation may be kept as long as sufficient copper is leached
 into solution.  As little as 0.02 per cent. of copper in
 solution may, in some instances, be treated profitably.
 Sulphuric acid may be added artificially to the waters to
 increase their leaching power and increase the life of the
 operation.
         For many years mining men have known that copper
      in mine waters will precipitate as metallic copper on any
      piece of iron placed in the water, while the iron is dis-       
      solved.  Most of the exceedingly fine particles of metallic
      copper are usually dislodged from the iron and washed
      away soon after being precipitated.  Therefore, the most
      noticeable feature of the process is disappearance of the
      iron.  Mine track., pipe., pumps. tools, even nails in the
      miners' shoes might be eaten away in periods of only a
      few weeks.
         About :1860 this destructive chemical activity was
      turned to advantage by placing pig iron and iron scrap
      in copper-bearing mine waters and collecting the precipi-
      tated copper commonly called "cement copper" by miners.
      This practice has become common and profitable in mines
      with appreciable dissolved copper, to supplement regular
      mine production.
         Several cement copper operations are active in
      California and expansion of this phase of copper mining
      in that State appears promising.  It is estimated that 300
      to 500 tons of cement copper were produced in California
      in 1951.  Total copper produced in California was about
      830 short tons in 1951.
          The formation of cement copper hinges on the relative
      positions of copper and iron in the Electromotive Force
      series of elements.  This is a list of metals in the order of
      their natural electromotive activity.  A given metal, placed
      in a solution of another metal below it on the electromotive
      list, will go into solution causing the previously dissolved
      metal to precipitate in metallic form.  Copper is below zinc
      and iron in this series, so these metals go into solution while
      metallic copper is deposited from copper solutions.  Lead.
      on the other hand., is below copper and is not affected in
      copper solutions.
          Iron is used to precipitate cement copper because it is
      abundant and inexpensive as scrap.  Galvanised iron is
      just as effective as common iron or steel because the zinc
      coating also replaces copper in solution.
          Because of this electromotive activity other materials
      must be substituted for iron and steel in all equipment
      used in the cement copper processes.  Wooded, concrete,
      or lead-lined tanks are used.  Bronze pump fittings, copper-
      coated nails, copper wire, and plastic, rubber, or lead
      pipe are commonly used.
          Although dissolved zinc is common in many mine
      waters, it is never produced by this replacement method.
      Those few metals which are above zinc in the electromotive
      series are comparatively expensive and not available as
      scrap.  Zinc cannot be precipitated by this method in the
      presence of dissolved iron or copper.

 

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