4. Mooring of Adsorbent in the Ocean

The required number of basic adsorption units comprising stacked adsorption beds are moored in an ocean region that has a warm current. Since the buoy method and the floating body method use 1 unit constructed from 100 individual adsorption beds, the weight of adsorbent becomes 12.5 tons. However, the overall weight of a single unit becomes roughly 100 tons when the connecting rope and cage (adsorption bed) holding the adsorbent are included. The weight in seawater, about 80 tons, is supported by buoyant force. The weight when pulled out of seawater, due to the inclusion of seawater in the adsorbent, reaches over 120 tons.

Design conditions for the mooring equipment at the ocean site assume a current velocity of 1.4 m/s and an ability to withstand a wave height of 10 m and a wind velocity of 55 m/s during abnormal conditions (storms, etc.).

(1) Buoy Method

The buoy method shown in Figure 5 (a) moors 1 unit (100 adsorption beds) per 1 buoy. These buoys are spaced at 200 m intervals and are each connected to 4 anchors sunk into the seabed. After 60 days mooring, the units are pulled up in order one at a time for operational efficiency. Therefore the crane ship is has a cargo capacity on the order of 1,000 tons. In consideration of the crane ship operational time period, 59 ships are required, and the optimum annual frequency of recovery (pulling up) becomes 3.3 times. However, in this case, the number of buoys increases from 3,200 to 4,530, and the quantity of utilized adsorbent increases 1.4-fold.

Figure 5. Adsorbent mooring methods

Table 3 shows test calculations of costs required for recovery. Although a large percentage is equipment cost, operating cost including personnel cost is a relatively high proportion in comparison to other methods.

(2) Floating Body Method

The dimensions of the floating body used by the floating body method of Figure 5 (b) are set in consideration of production of the float body structure. 27 rows 20 long of adsorption units are arrayed such that 540 adsorption units are used per 1 floating body. Therefore in order to recover the prescribed uranium, a total of 3,200 adsorption bed units are divided between 6 floating bodies and are moored. The adsorption bed unit is winced up by a crane placed upon the floating body, and the units one at a time are loaded into a recovery ship. If the recovery ship is taken as operating 300 days per year, in consideration of the time required for the recovery-exchange operation and the round tip from the harbor to the placement ocean site, 20 recovery ships of a scale of 5,000 tons cargo are required.

Figure 3 shows a test calculation of the cost of mooring. Float construction and maintenance cost (assumed to be 3% of construction cost) occupy the major portion.

(3) Chain-Binding Method

In consideration of drooping of the rope in an ocean region of 50 m water depth, 1 unit is constructed from 10 adsorption beds. Therefore 32,000 units become necessary. If these units are placed continuously at 10 m intervals, total length of the long line (rope) becomes 320 km. The chain-binding equipment is established as 14 segments, taking each 1 segment as 23 km in length. The number of adsorption bed units becomes 2,300 for each 1 segment. The rope that connects these segments is attached to 1 float (buoy) for every 15 units (150 m intervals). This float holds the adsorption beds at a fixed water depth. Recovery of the adsorption bed units is carried out by moving the long line in the same manner as a ski lift, pulling the adsorption bed unit up on land, removing the adsorption bed unit, and attaching a new adsorption bed unit. If a cycle takes 60 days, then 134 adsorption bed units are treated per day.

Mooring cost and details thereof are shown in Table 3. Equipment construction cost occupies a large fraction of total cost also for this method. Recovery cost per 1 kg uranium is the lowest among the 3 methods. However, for the present test, 3 mooring support devices are provided upon the sea per 1 segment in order to prevent mutual interference due to rope drooping, and the loop of rope becomes 1 km long.

Note:

• Adsorption bed counts for each of the methods include a required margin for transport, exchange, etc.
  
  
• 15-year at 3% interest for equipment amortization (i.e., 0.00967 times equipment total price)
  
  
• Maintenance cost is 3% per year of the equipment cost.
  
  
• Personnel cost is 10 million yen per year per person.
  
  

5. Desorption-Purification Step

Figure 6 shows the process of desorption of heavy metals adsorbed upon the adsorbent and purification of uranium from various types of metals contained in the desorption solution. After seawater jet cleaning to remove marine animals and marine plants attached to the adsorbent pulled up from the mooring in seawater, the adsorbent is immersed in a 0.01 N hydrochloric acid solution, and sodium (Na) and magnesium (Mg) are desorbed and separated out. Thereafter the adsorbent is immersed in 0.1 N nitric acid solution, and this solution is circulated to desorb heavy metals for recovery in the form of desorption solution. The recovered heavy metal solution undergoes separation-purification at a refinery, and various types of metal products are removed such as uranium, etc. After desorption of heavy metals, the adsorbent undergoes alkali treatment using sodium hydroxide, and the adsorbent is reused. Adsorbent that has been replaced due to a drop in performance undergoes incineration.

Table 4 shows cost values by assuming that 10,000 tons per year of adsorbent undergoes desorption and purification. Here since the test calculation is for the case of uranium recovery alone, purification costs for other simultaneously-recovered heavy metals are not included.

Figure 6. Desorption-purification step

30 year amortization of buildings, 15 year amortization of equipment, 3% interest.

IV. Considerations

    Respective adsorbent cost, mooring cost, and desorption-purification cost were found for 3 methods of mooring adsorbent at an ocean site, and these are summarized in Table 5. The required sea surface area and problems uncovered by this investigation of systems are presented within this table. The major object of these cost estimates is to uncover items that may become key to improvement of economics. Profits derived from recovery of useful metals, such as V, etc., fishing industry compensation for occupation of the ocean, and harbor construction costs, etc. are not included.

Although desorption-purification cost is identical whatever the utilized mooring method, different values are obtained since the adsorbent cost varies according to the mooring method. The chain-binding method is found to have the lowest cost upon comparison between the three mooring methods. Investigation is required for this method to test a mechanism for dependable prevention of rope breakage at the mooring support device during high wave conditions.

However, during high wave conditions, the major problem for the buoy system is thought to be violent movement of the buoy rather than strength of the buoy itself (e.g. limitation of the adsorbent exchange operation by sea conditions). Therefore problems include investigation of dynamic properties of the adsorption bed unit during high waves as a design condition and speeding up of the rate of recovery. Moreover, since the floating body method hangs a large number of adsorption bed units beneath the float, investigation is requires as to whether the adsorption bed units will contact one another, collide, etc.

Each of the recovery systems investigated here is based upon using a layered adsorbent in the form of polymeric nonwoven and mooring in seawater after insertion of such adsorbent into a stainless steel cage. As a result, about 80% of cost is for mooring even though costs very according to the mooring method. This is due to construction spending for mooring the large mass of adsorption beds. This adsorbent has a specific gravity equivalent to that of seawater and has no net weight within seawater itself. However, weight of the metal cage occupies the majority of the weight of the adsorption bead of Figure 4, so weight is particularly imparted in seawater only by the metal. For example, in the case of the chain-binding method after pulling up, it is estimated that mooring cost declines to 62% if weight of this adsorption bed can be lowered by 50%, and mooring cost declines to 42% when weight can be lowered to 25%. Therefore uranium recovery cost may possibly be greatly lowered if a light cage material is used in place of the metal mesh of stainless steel. Also since this adsorbent was obtained in a length-wise continuous cloth-like form, if a method of mounting other than the assumed insertion into a cage as shown in Figure 4 is used (e.g., if a method is adopted of supporting the multiple adsorbent sheets streaming in the current), then an entirely different method would be is used for mooring than that utilized here.

Performance of the adsorbent directly affects cost. According to this relationship, cost roughly halves if performance (recovery rate) becomes 2-fold higher. Although the during this cost evaluation the present performance value from actual marine site testing was assumed, this results in roughly 5 to 10 times the cost of mined uranium (about 5,600 yen/kg-U). As mentioned in section II, if the present performance goal (10 kg-U/kg-adsorbent) can be attained, it can be anticipated that cost becomes about 3 to 6 times that of mined uranium even for the same adsorption system. Increased working life of the adsorbent is also an important problem. Cost reduction is tied to an increase in the number of reuses.

However, desorption-purification cost is a small portion of total cost. Although purification technology is mostly established, research is progressing to develop technology for separatory desorption capable of suppressing this performance decline. This can improve adsorbent performance and effectively lower cost.

V. Conclusions

    Three types of mooring methods were investigated for seawater uranium recovery using polyethylene fibrous nonwoven that has amidoxime groups and is synthesized by radiation-induced graft polymerization technology. Test calculations for these mooring methods were carried out. As a result, seawater uranium cost using existing technology becomes 5 to 10 times the present cost of mined uranium. These cost estimates had the object of identifying technical problems for practical use. Detailed system planning and equipment design were not performed. The following items were identified as research and development problems to be dealt with in the future in order to make possible the economical use of seawater uranium. Based upon these research results with respect to these problems, more detailed design research is needed in order to increase the precision of cost evaluation.

(1) Performance of the adsorbent directly impacts adsorbent cost. Research and development are needed for improving the morphology of the adsorbent such that recovery efficiency can be raised and for prolonging working life to increase the number of reuses.

(2) Each of the 3 mooring systems investigated here had a mooring cost that occupies about 80% of overall adsorbent cost. The major reason for this is the metal construction of the adsorption bed used for holding the adsorbent and the resultant weight. Research and development are needed to lower mooring cost by lightening using means such as reinforced plastic, etc.

(3) In addition to lightening, the buoy method needs design of adsorbent bed placement in order to reduce occupied area.

Research and development have just begun for the utilization of seawater uranium.

If improvement of performance of the adsorbent, development of a desorption method that doesn't lower adsorbent performance, and lightening of the adsorption bed for adsorbent mooring are anticipated as a result of future technical development, it is possible that the recovery cost of seawater uranium will drop further. Continuation of steady research and development is necessary from this standpoint.

Technical Report. Recovery System for Uranium from Seawater with Fibrous Adsorbent and Its Preliminary Cost Estimation. Takanobu Sugo , Masao Tamada, Tadao Seguchi, Takao Shimizu, Masaki Uotani, Ryoichi Kashima. Nihon Genshiryoku Gakkaishi, Vol. 43, No. 10 (2001).