Overview of silicate scaling
Silicic acid compound is also a major impurity in natural water, which is often dissolved in water after water contacts with rocks containing silicate and aluminosilicate. Generally, the content of silicic acid compound in groundwater is more than that in surface water two Content<50mg/L.
In industrial wastewater reuse project (especially in coal chemical wastewater reuse system) two The content is generally high, which makes the reverse osmosis system prone to silicate scaling, thus seriously affecting the service life of the reverse osmosis system and the stable operation of the system.
In the current hot zero emission field in China, due to SiO two Silicate scaling is one of the most difficult problems to solve at present.
Silicate scaling mechanism
For SiO in RO system two The content of SiO2 is sensitive, because SiO2 can polymerize into colloidal silicon which is very difficult to dissolve and deposit on the membrane surface in a saturated state and is difficult to clean. SiO two The allowable concentration value in RO concentrated water section depends on the solubility product of SiO2, and is greatly affected by water temperature and pH.
SiO two The solubility of is proportional to the water temperature. For example, the solubility is 100mg/L at 25 ℃ and 160mg/L at 40 ℃.
SiO two Relationship between solubility of and pH:
SiO when pH=7 ～ 8 two As dissolved silicic acid, there is H in the water at the same time two SiO three And HSiO three - ； When the pH is low, it exists in the form of free acid solution or colloidal solution of calcium magnesium silicate; When the pH is high, if there are calcium and magnesium ions in the water, it will be in the colloidal state of calcium magnesium silicate.
Characteristics and harmfulness of silicate scaling
The Mohs hardness of calcium carbonate (diamond is 10) is 3, the Mohs hardness of calcium fluoride is 4, and the Mohs hardness of silicate is related to the water content, which is 4.5~7.5. The silicate scale is the hardest of many types of scale.
Once silicate scaling occurs in the reverse osmosis system, the desalination rate will decline rapidly, the water yield will decline rapidly, and the desalination rate of the system will decline significantly after chemical cleaning.
In case of severe scaling, the differential pressure will rise rapidly, and even the concentrated water grid will be washed out.
The membrane with serious scaling was observed under the microscope, and it was found that there were small scratches on the membrane surface, as shown in Figure 1. The membrane had irreversible physical scratches. See Fig. 2 for the electron microscope picture of silicate scaling.
SiO in water inlet two And other ion concentration requirements
SiO in RO inlet water two The concentration is determined according to the maximum solubility and concentration multiple at the concentrated water side, which is generally 20ppm. Prerequisite: when oxygen content (DO) in water is less than 0.5mg/L and pH is less than 6, iron ion and aluminum ion are less than 0.05mg/L, because the content of iron and aluminum ion has a greater impact on silicate scaling.
Effect of iron, aluminum, etc. on silicate scaling
Silicon scaling occurs mostly due to the presence of aluminum or iron in water. Iron and aluminum will react with silicon to form insoluble metal silicates (aluminum silicate and iron silicate), and the formed metal silicates will change SiO two Solubility, thus further quickly fouling and plugging membrane elements.
Even if the silicon concentration in the water is low (10ppm), the aluminum concentration of 50ppb will also cause the decline of the system performance.
If there is silicon, it should be ensured that there is no aluminum or iron in the water. It is recommended to use 1 μ m security filter element and take preventive acid cleaning measures.
Cleaning of silicate scaling
Conventional chemical cleaning is basically ineffective for silicate scale, while hydrofluoric acid can effectively clean silicate scale. Even at low temperature and concentration, hydrofluoric acid has good solubility for silicate scale.
0.1% HF+0.4% HCI can be used for chemical cleaning, or 0.1% NaF+0.4% HCI can be used.
be careful:
(1) The cleaning solution concentration must be adjusted according to the actual pollution and blockage situation. It is recommended to determine the appropriate cleaning concentration after experimental cleaning;
(2) Hydrofluoric acid is extremely corrosive. Inhalation of steam or contact with skin will cause burns that are difficult to cure. It is estimated that 1.5g hydrofluoric acid may cause immediate death. Inhalation of high concentration hydrofluoric acid mist may cause bronchitis and hemorrhagic pulmonary edema, and may also cause serious poisoning through skin absorption. Therefore, protective measures must be taken and professional personnel must operate during chemical cleaning.
Prevention of silicate scaling
(1) Control water inlet SiO two Concentration and control the recovery rate of reverse osmosis system to make SiO in concentrated water two The reduction of concentration to avoid exceeding the solubility product is to prevent SiO two Main scaling methods;
(2) Increase or strengthen pretreatment process, if lime is used to soften, 50% SiO in water supply can be reduced two Or adding magnesium oxide or sodium aluminate during lime soda softening pretreatment to reduce SiO in influent two Concentration;
(3) Properly increasing the water temperature (not more than 40 ℃) will help to improve SiO two Solubility of, slow down silicate scaling;
(4) Properly increasing the pH of inlet water helps to increase SiO two Solubility of, slow down silicate scaling;
(5) Add scale inhibitor dispersant targeted for silicon scaling in pretreatment, and allow maximum SiO at concentrated water side of different scale inhibitor dispersants two For different concentrations, please consult the scale inhibitor manufacturer for details;
(6) Colloidal silicon can be removed by strong basic anion exchange resin adsorption or ultrafiltration membrane with cutting molecular weight less than 10000.
（ Source: Bluestar Dongli ）
