|
|
An acid solution below a pH of 2 will readily dissolve both ferrous (Fe++) and ferric (Fe+++) iron scale and rust. Upon spending of the acid, that iron present in the ferric (Fe+++) form will begin to precipitate from solution at a pH of about 2.2. Upon reaching a pH of about 3.2, all of the dissolved Fe+++ iron will have essentially precipitated as gelatinous ferric hydroxide unless a stabilizer has been added to the solution. On the other hand, ferrous hydroxide will not precipitate below a pH of 7.7. Since acid washing solutions rarely exceed a pH of 5.3, the primary concern here is to stabilize the Fe+++ while the pH is below 2.0. However, if the system becomes well aerated or some other electron acceptor other than oxygen is present, the ferrous iron could be oxidized to the ferric state and add to the ferric hydroxide precipitation problem. |
