NaOH and KOH are produced by electrolysis of the respective chlorides on an industrial scale. The caustic solutions obtained in these processes regularly contain certain amounts of chlorate, which adversely affect the processing equipment, particularly when made from nickel. In NaOH and KOH flaking plants, equipment at elevated concentration and temperature, especially the supply lines and flaking drums, are made from nickel.

Sugar is used in NaOH and KOH flaking and prilling plants as a corrosion inhibitor. Nickel surfaces in contact with NaOH or KOH solution are prone to corrosion. The higher the concentration and the higher the temperature of the solution, the stronger will be the corrosive attack.

NaOH and KOH solutions regularly contain certain amounts of chlorate (NaCI03 and KCIO3), which originates from the electrolytic production process. The chlorate also has an impact on the corrosion rate of nickel and adds up to caustic corrosion. The presence of oxygen, also in the form of air, further contributes to the corrosion.

As sugar readily reacts with chlorate, feeding a sugar solution to a caustic stream is suitable to reduce the chlorate content and accordingly the nickel corrosion. However, correct dosing is a problem, since the chlorate content in a caustic stream may vary. On the other hand, precise sugar dosing has a considerable impact on the lifetime of all equipment made from nickel, like for example tubing and flaker drums. An automatic sugar dosing system therefore has a high potential to save costs for plant operators of NaOH and KOH flaking and prilling plants.

It is therefore desirable to have an automatic sugar dosing system, which provides sugar in dissolved form to a caustic stream that reliably removes the chlorate contained therein by reaction.

Accordingly, the invention provides an automatic sugar dosing system for feeding sugar solution into a caustic stream containing chlorate, comprising at least one tank for preparing and supplying sugar solution, at least one pump for each tank for supplying sugar solution to the caustic stream, sensors for measuring the chlorate content of the caustic stream and the caustic volume flow, sensors for measuring the sugar concentration, in the sugar solution and the sugar solution volume flow, a local operating panel receiving the measured data from the sensors and calculating the correct dosing rate for the sugar solution, the dosing rate being communicated to the dosing pump.

In the following, caustic means a caustic solution from NaOH or KOH and chlorate NaCI03 or KCIO3, depending from the respective production plant.

The sugar dosing system of the invention is designed to work automatically which allows adaptation of the sugar dosing according to the actual chlorate content in a caustic stream. The sugar solution is provided in at least one tank and has a concentration in the range of 5% to 10% and a temperature in the range of 25°C to 70°C. A typical flow rate is about 50 l/h. The sugar solution normally is hand made from crystal sugar and tap water.

It is preferred to have two tanks with sugar solution, each tank provided with a stirrer for preparing a homogeneous solution, a low level detection and a dosing pump. This allows switching automatically between the two tanks in case the level in one tank is run down to a lower limit. While sugar solution is provided from the second tank, the first tank can be refilled with sugar solution. Of course, each tank has its own dosing pump, which is activated with the activation of the tank.

In order to provide the right amount of sugar solution to the caustic stream, the system comprises sensors for measuring the chlorate content in the caustic stream and the caustic stream volume flow. Further, there are sensors for measuring the sugar concentration and the sugar solution volume flow. These data are transmitted to a local operating panel for calculation of the correct dosing rate, the dosing rate being communicated to the respective dosing pump.

The local operating panel receives all data which are necessary to calculate the correct dosing rate. In addition, the local operation panel controls the functions of the system, being able to run down the system in case of failure of a component or in case of an alarm. In particular, the switching from a first tank to a second tank and from a first pump to a second pump in case the first tank runs empty is preferably controlled by the panel. Analysis of the chlorate content takes place by conventional means, e.g. by using the oxidation potential of chlorates, conductivity or ion exchange chromatography.

Preferably, the present sugar dosing system comprises a reactor, where a sample from the caustic stream is prepared for analysis of its chlorate content. Preparation takes place by chemical reaction of the sample in a conventional way including a neutralisation step, using the oxidation potential of chlorate. The reacted sample is then transferred to an analyser for determination of the chlorate content.

In the analyser, the chlorate content is determined and the data transferred to the local operating panel, where it is used to calculate the sugar dosing. It is preferred that all measurements take place simultaneously in order to adapt the injection of the sugar solution into the caustic stream to the actual chlorate content. Measurements must not take place continuously; measurements in regular intervals are sufficient. It is preferred to have the measurements automated. The sensors used for the measurements are conventional ones, for example usual flowmeters, for the caustic stream, which must be made from a corrosion resistant material. The concentration of the sugar solution is best measured with a Coriolis device. The present sugar dosing system may be installed at a place, where it is regularly used. However, it is preferred to mount the system on a skid, which facilitates maintenance by exchanging one skid against another one.

The invention is explained by means of attached Fig. 1 , which shows the system by means of a Pl-diagram. Figure 1 is a Pl-diagram showing the functioning of the automatic dosing system of the invention. Two sugar tanks 1A and 1B are each equipped with a stirrer 2 and a level switch 3 and connected via line 8 with a dosing pump 4. Fresh water and sugar in solid form are provided via way 5 manually.

Sugar solution is provided by dosing pumps 4 via line 8 to the caustic stream (not shown) to be treated with sugar, sensors 6 for measuring the sugar concentration and 7 for determining the volume flow being connected thereto.

Caustic solution is provided via line 10 and a dosing valve to reactor 9, where it is neutralised, diluted and reacted with chemicals for preparation for chlorate analysis. The prepared solution is transferred via line 14 to analyser 15 for chlorate analysis. The analyser also controls the chemicals and acid used to react the caustic solution in reactor 9. The data then transferred from analyser 15 to the local operating panel 16 for calculation. Local operating panel 16 also receives the data from sensors 6 and 7 and the measured data for the caustic stream volume flow. The calculated data for sugar dosing are then transmitted to the active pump 4.

The chemicals used in reactor 9 are analysed in analyser 15 before being transferred to the reactor 9 through lines 11, 12 and 13. All data are collected in local operating panel 16 for calculation of the sugar feed into the caustic stream.