Ferric arsenate

As seen in the above equations, the aqueous oxidation processes convert sulfur in the feed to dissolved sulfate, while arsenic is oxidized and precipitated as ferric arsenate compounds. So, problems of the emission of sulfur and arsenic oxides caused by roasting are avoided in the aqueous oxidation processes. The two different industrial methods which achieve the oxidation reactions are pressure oxidation and biological oxidation. 

Fig. 1. Composition of ferric arsenate-sulfate compounds forming in the autoclaves treating refractory gold ores.

Akg kaganeite, Apy=arsenopyrite, Ca-FA=amorphous C a ferric arsenate, Gt=goethite, HFA=hydrous ferric arsenate, HFO=hydrous ferric oxyhydroxide, Hm=hematite,... 

The precipitated hexahydrate gradually undergoes oxidation on exposure to moist air, yielding ferric arsenate and ferric oxide.3 It is sparingly soluble in aqueous ammonia,4 but is insoluble in the presence of ammonium salts. 

The monohydrate, FeAs04.H20, is formed when anhydrous ferric arsenate, arsenic acid, hydrogen peroxide and water are heated in a sealed tube for 14 days at 170° C. a hemihydrate may be obtained in a similar manner by substituting precipitated ferric dihydrogen arsenate for the normal salt. 

Langmuir, D Mahoney, J. and Rowson, J. (2006) Solubility products of amorphous ferric arsenate and crystalline scorodite (FeAsC>4 2H2O) and their application to arsenic behavior in buried mine tailings. Geochimica et Cos-mochimica Acta, 70(12), 2942-56. 

McCreadie, H., Jambor, J.L., Blowes, D.W. (1998) Geochemical behavior of autoclave-produced ferric arsenates jarosite in a gold-mine tailings impoundment, in Waste Characterization and Treatment (ed. W., Petruk), Society for Mining, Metallurgy, and Exploration, Littleton, Colorado, pp. 61-78. 

Similar rate equations can be developed for the parallel leaching of other minerals (see Eq. 3). The oxidation of intermediate sulfur compounds to sulfates such as Eq. 2 is a consecutive reaction to mineral leaching. Other reactions are virtually instantaneous, e.g., precipitation of insoluble ferric arsenates from soluble NH4H2ASO4 which is consecutive to Eq. 3. On the other hand, the instantaneous shift in the amonia -ammonium equilibrium,... 

Reductive Dissolution. Many substances in nature contain the same metal or metalloid, but under different oxidation states. For example, the metalloid arsenic may exist as arsenite (AsIII, As03) or arsenate (AsIV, As04) in the forms of ferrous-arsenite or ferric-arsenate, respectively. Ferrous-arsenite is more soluble than ferric-arsenate for this reason, one may be interested in studying the kinetics of arsenate reduction to arsenite. Similar chemistry applies to all elements present in soil-water systems with more than one oxidation state (e.g., iron, manganese, selenium, and chromium). 

The solubility product of ferric arsenate is so low that residual levels of 50 ppb arsenic in solution can be achieved.259 Hydrogen peroxide is also used to produce arsenic acid from arsenious oxide,260 where the cleanliness of the reaction offers advantages over other oxidants (Figure 6.13). 

Scorodite Scorodite is the mineral ferric arsenate, FeAs04.2H20. Because it is very sparingly soluble in water, the synthetic material has been used from 1997 in several laboratory processes for removing arsenic from hydrometallurgical solutions. 

Ferric ortho-arsenate occurs in nature as the mineral scorodite, FeAs04.2H20, and may be produced artificially by heating iron to 150° C. with a solution of arsenic acid,8 or by heating ferric arsenate in a similar manner with arsenic acid.9... 

Colloidal ferric arsenate is prepared by the action of ammonium hydroxide on the insoluble salt.10... 

The double arsenates, ammonium ferric ortho-arsenate, (NH4)H2As04.FeAs04, and potassium ferric arsenate,3 possibly represented by the formula KH2As04.FeAs04, have been obtained. 

SYNS ARSENATE of IRON, FERRIC FERRIC ARSENATE, soUd (DOT)... 

FERRATE(4-), HEX.YKIS(CYANO-C)-, TETRAPOTASSIUM, (OC-6-11)- seeTECSOO FERRIAMICIDE see MQW500 FERRIC ARSENATE, soUd (DOT) see IGNOOO FERRIC ARSENITE, soUd (DOT) see IGOOOO FERRIC ARSENITE, BASIC see IGOOOO FERRIC CHLORIDE see FAUOOO FERRIC CHLORIDE (UN 1733) (DOT) see FAUOOO FERRIC CHLORIDE, soludon (UN 2582) (DOT) see FAUOOO... 

Brown L. J., Gutsa E. H., Cashion J. D., Fraser J. R., and Coller B. A. W. (1990) Mossbauer effect study of ferric arsenates from an abandoned gold mine. Hyperfine Interact. 57, 2159-2166. 

Figure 4. Normalized, kl-weighted As-EXAFS spectra (a) and uncorrected Fourier transforms (FTs) (b) of scorodite (a crystalline ferric arsenate), an x-ray amorphous analog, and As(V) sorbed to amorphous hydrous ferric oxide (HFO), The EXAFS spectra can clearly be used to distinguish the coordination environment of arsenic in each of the materials. The highly symmetric local environment of arsenic in scorodite is shown in (c) each arsenic is surrounded by 4 Fe neighbors at a distance of 3.34 A. (see Table 2 for details of precipitate fits, and Table 3 for details of sorption sample fit). Reprinted from Foster (1999). The arrow highlights the region of particular difference among the three spectra. Peak positions in FTs are not corrected for phase-shift effects, and are therefore approximately 0.5 A shorter than the true distance.

Foster (1999) compared the local structure of arsenate in scorodite with that of an x-ray amorphous synthetic ferric arsenate. The synthetic compound had the characteristic pale-green/yellow color characteristic of scorodite, and not the rust-brown or orange color characteristic of Fe oxyhydroxides (which could have formed during the synthesis), but its measured stoichiometry was not reported. XAFS analysis indicates that the x-ray amorphous compound has fewer Fe atomic neighbors than scorodite, and the number and position of Fe shells is more similar to As(V) sorbed on Fe oxyhydroxides than to scorodite. However, the EXAFS spectrum of scorodite is clearly distinguished from that of the x-ray amorphous ferric arsenate, which is itself different from As(V) adsorbed to Fe oxyhydroxide (Fig. 4). 

Fig. 5.4. A scheme briefly presenting the refining processes of metals from Kuroko. Although the real manufacturing processes are much more complicated, the scheme insists that zinc ion is separable from ferric arsenate after ferrous ion has been oxidized by the action of the acidophilic iron-oxidizing bacterium (or bacteria)...

Robins, R.G., 1990. The stability and solubility of ferric arsenate an update. Paper presented at the Metallurgical Society Annual Meeting, Feb. 18-22, Anaheim, Ca., The Metallurgical Society, Warrendale, PA. 

The arsenic and iron in solution did not reflect the full extent to which the arsenopyrite had been oxidized. Acidiflcation of the culture medium in each flask with 1 ml of concentrated HCl at the end of the experiment increased the arsenic concentration in solution 1.6-fold and the iron concentration 4.4-fold in uninoculated flasks and 1.6- and 7.2-fold, respectively, in inoculated flasks. The increase in dissolved As and Fe on acidification suggests that a portion of the mobilized iron and arsenic was precipitated as iron arsenate and arsenite in inoculated as well as uninoculated flasks. The weight ratios of Fe/As were always higher over 21 days in uninoculated flasks than in inoculated flasks, and in both types of flasks dropped in the first few days of incubation and then increased again. Precipitation of ferric arsenate (scorodite) as well as potassium jarosite [KFcs (804)2(011)6] in bacterial arsenical pyrite oxidation was reported by Carlson et al.(35). 

Cassity and Pesic (36) found that arsenate but not arsenite stimulated dissolved Fe + oxidation by T. ferrooxidans through precipitation of Fe + as ferric arsenate. 

Ferric iron solution is now continuously added to both acid and neutral leaching. This has resulted in higher iron utilization and enhanced impurity precipitation efficiency. The addition of soluble iron to the acid leach promotes the precipitation of ferric arsenate, enhancing arsenic rejection to the residues and dramatically lowering the arsenic levels in the plant electrolytes. Previously, the arsenic concentration in the acid leach electrolyte was as high as 5 g/L. The arsenic concentration is now below one gram per liter. 

The solubility product of ferric arsenate is so low that residual levels of 50 ppb As in solution can be achieved [380]. 

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