Potassium hydroxide is a strong alkali, can be widely used for electroplating or as chemical raw materials for use, but we will find that the color of some products is blackened when we use it. This is because there are some mistakes in the steps when the products are made. Let's have a detailed understanding of the reasons for the black color of the products.

1、开车投料后需要使用氮气密封设备

1. Nitrogen sealing equipment is needed after start-up and feeding

当设备处于微正压状态的时候，可以有效的避免空气进入到系统当中。所以制作的时候开车投料后用氮气密封设备，保持通入的氮气处装置始终处于高温强碱状态，氧气进入从而会加剧镍材腐蚀，进而增加其中的镍含量，从而导致颜色发黑。

When the equipment is in micro positive pressure state, it can effectively avoid air entering into the system. Therefore, the equipment is sealed with nitrogen after the start-up and feeding, and the device is always in the high temperature and strong alkali state at the nitrogen inlet. The oxygen entering will aggravate the corrosion of nickel material, and then increase the nickel content in it, resulting in black color.

2、能力控制

2. Capability control

能力需要控制在额定产量的60%~100%的范围内。在蒸发器中，如果进料量太小，蒸发器中液膜就会因为太薄以致断裂，从而出现干壁区，这样就会使管子承受更大的的热应力，从而加剧应力腐蚀。当液体的进料量太大时，液体就无法正常成膜或者膜太厚，在升温过程中，液体经常不能得以充分的热量传递，从而达不到沸腾所需要的温度，因此不能全部的进行蒸发，就会导致浓缩过程不能很好地进行，甚会发生无法进行的情况。流量太大或太小都会损坏设备，腐蚀镍材，这样就会增加氢氧化钾中的镍含量，使其发黑。

The capacity should be controlled in the range of 60% ~ 100% of the rated output. In the evaporator, if the feed rate is too small, the liquid film in the evaporator will break because of its thin thickness, resulting in dry wall area, which will make the pipe bear greater thermal stress, thus aggravating stress corrosion. When the amount of liquid feed is too large, the liquid can not be normal film-forming or the film is too thick. In the process of heating, the liquid can not be fully heat transferred, which can not reach the required temperature for boiling. Therefore, it can not be fully evaporated, which will lead to the concentration process can not be carried out well, and even can not be carried out. If the flow rate is too large or too small, it will damage the equipment and corrode the nickel material, which will increase the nickel content in potassium hydroxide and make it blackened.

3、减少开、停车次数

3. Reduce the number of driving and stopping

尽量减少设备开停车的次数，因为每一次开车，进料碱液都会冲掉一层镍材表面起保护作用的氧化镍层，然后继续冲刷腐蚀下层镍材，所以开车过程中难免产生黑碱。可以进行延长蒸气吹扫系统的时间等措施，尽量的降低开车黑碱量。

The number of start-up and shut-down of the equipment should be reduced as much as possible, because each time the feeding alkali liquor will wash away a layer of nickel oxide layer which plays a protective role on the surface of nickel material, and then continue to wash and corrode the nickel material at the lower layer, so it is inevitable to produce black alkali during the start-up process. Measures such as prolonging the time of steam purging system can be taken to reduce the black alkali content as much as possible.

4、开车前用蒸气吹扫设备

4. Purge the equipment with steam before start-up

开车前用蒸气吹扫干净系统，保证有足够的压力和吹扫时间。吹扫既可以预热设备，降低投料时的热应力，又能在吹扫过程中把管路中的残留碱吹扫干净；还可以通过吹扫把系统中的空气排出，尽量减少氧气含量；同时可以减少投料时对镍材设备的冲刷和腐蚀，降低其中的镍含量，防止发黑。

Clean the system with steam before start-up to ensure sufficient pressure and purging time. Purging can not only preheat the equipment, reduce the thermal stress when feeding, but also purge the residual alkali in the pipeline during the purging process; it can also exhaust the air in the system by blowing to reduce the oxygen content as far as possible; at the same time, it can reduce the scouring and corrosion of nickel equipment during feeding, reduce the nickel content in it and prevent blackening.

上面介绍的就是氢氧化钾在制作时颜色发黑的原因，主要是因为制作时候的产量的控制不好，以及设备可以没有清理干净，导致的产品在制作时颜色发黑。

The above is the reason why the color of potassium hydroxide turns black in the production. The main reason is that the production is not well controlled, and the equipment can not be cleaned up, resulting in the black color of the product during production.